Show the other computers, and apply the ten-minute rule

Three pieces on top of the link:

**The network status window**, on the entry window's Window menu. A row
per station, this computer included the way N1MM shows it: number,
address, operator, band, mode, run, transmit, pass frequency, the last
message type, how long ago it arrived, the counts each way and the echo
round trip. A station that broadcast the wrong version is named under the
table in red — that is the one fault where everything looks connected
and nothing arrives. The box at the bottom sends a line of chat, and
Echo asks every station whether it is there.

**Where this station is** goes out once a second when it has changed.
The frequency, the mode and run have a dozen places they can change from
— the radio moving, a band button, a QSY typed into the callsign box —
so it is read and compared rather than announced from each of them.

**The ten-minute rule.** `BandChangeRules` already counted changes and
the stay on a band, and the entry window already showed the countdown,
but only a user-defined contest carried a rule, so every built-in
contest allowed anything. `ForCategory` gives a multi-operator entry
with one or two transmitters a ten-minute stay, which is N1MM's fallback
in `ContestInstance.BandChangeTimerDuration` for every contest that does
not name its own. Its per-contest table is a few hundred cases in a
decompiled hash switch and is not repeated.

Config ▸ Edit Networked-Computer Names now covers both networks: the
12060 broadcast to other programs and the 12070 link, with the station
number, the port, the version to claim and stations named by address for
a network where a broadcast does not reach.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PdAYHcdRktqKry7nk414TU

.gitignore

@@ -6,3 +6,4 @@ obj/
 /bridge/nonemm-mmtty-bridge.exe
 /mmtty/
 /bridge/XMMT.ocx
+engine-probe.log

Nonemm.slnx

@@ -12,6 +12,9 @@
     <Project Path="src/Nonemm.Session/Nonemm.Session.csproj" />
     <Project Path="src/Nonemm.App/Nonemm.App.csproj" />
   </Folder>
+  <Folder Name="/tools/">
+    <Project Path="tools/Nonemm.EngineProbe/Nonemm.EngineProbe.csproj" />
+  </Folder>
   <Folder Name="/tests/">
     <Project Path="tests/Nonemm.Core.Tests/Nonemm.Core.Tests.csproj" />
     <Project Path="tests/Nonemm.Contests.Tests/Nonemm.Contests.Tests.csproj" />

bridge/XmmrControl.cpp

@@ -178,15 +178,25 @@ HRESULT XmmrControl::callLong(const wchar_t* name, long first, long second) {
 }
 
 long XmmrControl::getLong(const wchar_t* name) {
-    VARIANT value;
-    VariantInit(&value);
+    long value = 0;
+    tryGetLong(name, &value);
+    return value;
+}
+
+HRESULT XmmrControl::tryGetLong(const wchar_t* name, long* value) {
+    *value = 0;
+    VARIANT answer;
+    VariantInit(&answer);
     DISPPARAMS none = {nullptr, nullptr, 0, 0};
-    if (FAILED(invoke(name, DISPATCH_PROPERTYGET, &none, &value))) {
-        return 0;
+    HRESULT result = invoke(name, DISPATCH_PROPERTYGET, &none, &answer);
+    if (SUCCEEDED(result)) {
+        result = VariantChangeType(&answer, &answer, 0, VT_I4);
+        if (SUCCEEDED(result)) {
+            *value = answer.lVal;
         }
-    long found = SUCCEEDED(VariantChangeType(&value, &value, 0, VT_I4)) ? value.lVal : 0;
-    VariantClear(&value);
-    return found;
+    }
+    VariantClear(&answer);
+    return result;
 }
 
 std::string XmmrControl::getString(const wchar_t* name) {

bridge/XmmrControl.h

@@ -32,6 +32,11 @@ public:
     HRESULT callLong(const wchar_t* name, long first, long second);
 
     long getLong(const wchar_t* name);
+
+    // The same, saying whether the control answered. A property that is not
+    // there returns DISP_E_UNKNOWNNAME, which is how a name can be checked.
+    HRESULT tryGetLong(const wchar_t* name, long* value);
+
     std::string getString(const wchar_t* name);
 
 private:

bridge/main.cpp

@@ -145,6 +145,14 @@ void send(const std::string& text) {
     VariantClear(&argument);
 }
 
+// N1MM's {TX}: the control's own PTT property, which is what starts a
+// transmission. SetMmttyPTT is the other direction, and only that.
+void key(bool on) {
+    report("PTT", control->putBool(L"PTT", on));
+}
+
+// 1 stops when the buffer is empty, which is N1MM's XmitOff; 0 stops now,
+// which is its AbortXmit.
 void setPtt(long on) {
     VARIANT argument;
     VariantInit(&argument);
@@ -153,14 +161,36 @@ void setPtt(long on) {
     control->call(L"SetMmttyPTT", &argument, 1);
 }
 
+// How many characters MMTTY still has to transmit. It is asked for rather than
+// reported: the control has no event for it.
+void askBuffer(const std::string& name) {
+    const std::wstring wanted = toWide(name.empty() ? "TxBufLen" : name);
+    long left = 0;
+    HRESULT result = control->tryGetLong(wanted.c_str(), &left);
+    if (FAILED(result)) {
+        char message[128];
+        std::snprintf(message, sizeof(message), "%s failed: 0x%08lx", wanted.empty() ? "" : name.c_str(),
+                      static_cast<unsigned long>(result));
+        // not an "error": a property the control will not answer stops nothing
+        protocol::write("log", {message});
+        protocol::write("buffer", -1);
+        return;
+    }
+    protocol::write("buffer", left);
+}
+
 void act(const std::string& text) {
     protocol::Line line = protocol::read(text);
     if (line.verb == "open") {
         open(line);
     } else if (line.verb == "send") {
         send(line.field(0));
+    } else if (line.verb == "key") {
+        key(line.number(0) != 0);
     } else if (line.verb == "ptt") {
         setPtt(line.number(0));
+    } else if (line.verb == "buffer") {
+        askBuffer(line.field(0));
     } else if (line.verb == "post") {
         control->callLong(L"PostMmttyMessage", line.number(0), line.number(1));
     } else if (line.verb == "close") {

docs/digital-bridge.md

@@ -30,8 +30,10 @@ To the bridge:
 |---|---|
 | `open <title> <port> <command line>` | sets `Title`, `ComName` and `InvokeCommand`, sets `bActive`, then posts the host window handle |
 | `send <text>` | `SendString` |
-| `ptt <0\|1>` | `SetMmttyPTT` |
+| `key <0\|1>` | sets the control's `PTT` property, which is what starts a transmission |
+| `ptt <0\|1>` | `SetMmttyPTT`: 0 stops now, 1 stops once the buffer is empty |
 | `post <message> <parameter>` | `PostMmttyMessage`, for everything in `MmttyMessage` |
+| `buffer [property]` | reads `TxBufLen`, or the property named instead, and answers `buffer` |
 | `close` | shuts the engine down and leaves the bridge running |
 | `quit` | shuts the engine down and exits |
 
@@ -46,9 +48,17 @@ From the bridge:
 | `tx <0\|1>` | the engine started or stopped transmitting |
 | `mark <hz>`, `space <hz>` | the tone pair moved |
 | `switch <bits>`, `view <bits>` | AFC (4), net (8) and reverse (256), and the engine's view state |
+| `buffer <n>` | how many characters the engine still has to transmit, or -1 when the control would not say |
 | `log <text>`, `error <text>` | anything the bridge has to say |
 
-Each of those comes from an event of the control's own: `OnCharRcvd`,
+`buffer` is the one thing that is asked for rather than reported: the control
+has no event for the length of the transmit buffer, so it is polled. A property
+name in the request is only for finding out what the control answers to; a name
+it does not know fails with `DISP_E_UNKNOWNNAME`, which comes back as a `log`
+line and `buffer -1` rather than an `error`, because a property the control will
+not answer stops nothing.
+
+Each of the others comes from an event of the control's own: `OnCharRcvd`,
 `OnPttEvent`, `OnFreqChanged` (mark and space in one event), `OnSwitchChanged`
 and `OnViewChanged`. The control raises `OnTranslateMessage` only for the
 MMTTY messages it has no event for — width, resolution, thread and the like —

docs/unfinished.md

@@ -16,8 +16,9 @@ is still sitting there undiscovered.
 | `OtrspBox` | a `MemoryStream`, checking the bytes | no real SO2R box. Command forms are from N1MM's `N1MMPort.cs`. |
 | `ClusterClient` | a node fake over a real socket, sending the telnet negotiation, the login prompt and spot lines | no live cluster node. Which nodes send bare CR, and which send option negotiation, is guessed from N1MM's code. |
 | `StationNetwork` | the message format, round-tripped | no second station, and no N1MM on the same network. |
+| `StationLink` | the bytes of every message, and two links talking over loopback | no real N1MM has ever been on the other end. The version check is the thing to try first: N1MM turns away a station whose version is not its own, and this program has to be told what to claim. |
 | `CwDaemonSender` | the UDP messages, and a fake daemon that answers the `<ESC>h` reply request | no `cwdaemon`, no radio keyed. |
-| `MmttyEngine` and the Wine bridge | MMTTY 1.70 under Wine 10, started and stopped through `XMMT.ocx` | no sound card, so nothing has been decoded or transmitted and no `rx` or `tx` line has come from a real signal. No FSK through EXTFSK, no PTT on a serial port, and 2Tone has never been run. `docs/digital-bridge.md` |
+| `MmttyEngine` and the Wine bridge | MMTTY 1.70 under Wine 10 on a machine with a sound card: started, keyed, a message transmitted and decoded back off the air, and stopped | no radio. No FSK through EXTFSK, no PTT on a serial port, and 2Tone has never been run. Keying was wrong until 2026-09-01: `SetMmttyPTT` stops a transmission and does not start one, which is why nothing ever went out before then. `docs/digital-bridge.md` |
 | `WinkeyerSender` | the status-byte reader, on its own | no test of the serial side, and no WinKeyer. The host-mode open sequence is from the WinKeyer datasheet; the status bits are from N1MM's `Winkey.cs`. |
 
 The Cabrillo output has not been put in front of a contest sponsor's robot.
@@ -44,22 +45,358 @@ type-ahead transmit pane were checked. Nothing has been decoded from a real
 signal and nothing has gone on the air: this machine has no sound card, and
 MMTTY does not start on it.
 
-**What paces the type-ahead pump.** The pump keeps two characters in the
-engine and counts character times off the clock at the baud rate in the digital
-settings to work out when the engine has room for the next one. Two characters
-is what stops the engine running dry between characters, which would make it
-transmit idle and add that idle to the time the message takes; the price is that
-the last two characters cannot be taken back. The estimate is corrected when the
-engine reports that it has stopped transmitting, which means its buffer is
-empty.
+**The entry window's function keys on a digital mode.** They are the digital
+macros, not the CW file: N1MM loads its send buttons from the RTTYBTN set on a
+digital mode, and the CW messages carry no `{TX}`, so a key pressed there fed
+the engine without keying the transmitter and nothing went out. The first ten
+digital macros are F1 to F10; F11 and F12 stay Spot and Wipe, as in every other
+mode, and the macros past the tenth are on the digital window's own buttons.
+Right-clicking a key still opens the CW or phone messages, so the digital macros
+are edited from the digital window.
 
-The engine can be asked instead of estimated: `XMMT.ocx` has a `TxBufLen`
-property, and a probe against the control shows it answers, though what it
-answers while a message is going out could not be measured here because the
-engine will not start. Reporting it from the bridge would make the red text
-exact and would take out the one error the estimate can still make: a baud rate
-that does not match what the engine transmits at leaves a small gap between
-characters near the end of a long message.
+**How the transmit pane works.** The pane is one editable box holding the whole
+message: what the engine has transmitted, coloured red, then what is still to
+go. The red stops where the air is, not where the feeder is: the last characters
+handed over are still in the engine, and colouring those too turned every
+character red as it was typed once the transmission had caught up. An edit that
+reaches into the red is undone, and so is one into the characters the engine
+holds but has not sent yet: the engine cannot give a character back.
+
+The two are one text, so they wrap together and neither takes width from the
+other. A character moving from one half to the other only moves the colour
+boundary, so the caret and what is being typed stay where they are.
+
+Nothing goes on the air until the transmitter is keyed, which is the TX button,
+Ctrl+Enter or Alt+T. From then on the pane goes out and so does whatever is
+typed into it, and a function key pressed meanwhile goes on the end of what is
+already waiting. Enter is a new line in the message. The message ends when the
+transmitter has been told to drop and the engine says it has: what went out is
+cleared off the pane, what was typed ahead is kept, and nothing goes out again
+until TX is pressed. A new message clears the pane the same way, for the case
+where two of them run close enough together that the transmitter never drops.
+
+`{RX}` runs when the message it stands in has been handed over, wherever it
+stands in the macro. Every other action macro runs before the text unless it
+stands after `{END}`, but the transmitter cannot drop before the text has gone
+out. Running it in place, in front of the text, was what made the first click on
+a CQ button beep for half a second, unkey, then send the rest: the stop was
+issued while the engine held the two characters of the feeder's lead, so MMTTY
+sent them and dropped, and nothing was left to stop the transmitter at the end.
+The bridge log showed `> ptt 1` one millisecond after `> key 1` and before the
+first character. N1MM does the same as this now: it takes `{RX}` out of the text
+wherever it is, sends the text, then calls `StopTX`.
+
+The transmitter drops on `{RX}`, on the RX button or on Alt+T, and all three do
+what N1MM does at the end of a message, which is not what it does anywhere else.
+N1MM hands MMTTY the whole message in one `SendString` and calls
+`SetMmttyPTT(1)` with the message still in the engine's buffer. MMTTY then ends
+the transmission itself, at the last character. It never asks MMTTY how much is
+left; `TxBufLen` does not appear in N1MM at all.
+
+What sits between the text and the stop there is nothing at two of N1MM's three
+MMTTY send paths (`DigitalInterface.cs:19177` and `:19451`) and `sSleep(400)` at
+the third, the macro buttons (`:21553`). The sleep is not the mechanism — a full
+buffer is — and as a wait it is both too long on a long message and wrong on a
+short one: `TU` is 330 ms of air at 45.45 baud, so 400 ms of sleeping puts the
+stop after the buffer has emptied, which is where it does nothing. This program
+asks instead. `TypeAhead.WhenHoldingAsync` sends the stop as soon as the count
+says the engine holds something, and gives up after N1MM's 400 ms.
+
+So `{RX}` stops feeding: `TypeAhead.TakePending` hands over everything that is
+left in one piece, and the stop goes out 400 ms behind it. Nothing after that
+can be rewritten, which is what `{RX}` means. The feeder still paces the middle
+of a message, which is what makes type-ahead work; only the ending is N1MM's.
+
+`{RX}` runs as soon as the message has been given to the keyer, not after the
+message has gone out. That is the whole point: waiting for the feeder to hand
+the last character over leaves the engine empty, and a stop that reaches an
+empty engine does nothing. `EntryWindow.SendThenAsync` takes `{RX}` out of what
+stands after `{END}` and runs it there; the rest of `{END}` still waits for the
+message to go out.
+
+What the ending costs on the air, from a bridge log of one CQ: the last
+character was fed at 9616 ms and went out at about 10606 ms, and the transmitter
+did not drop until 11549 ms. That is 950 ms of dead carrier per transmission
+paid on the old timer. The key-down fallback brings it to about 230 ms. N1MM's
+ending costs nothing, because MMTTY drops the transmitter on the last character
+itself.
+
+The reason is that `SetMmttyPTT(1)` does nothing at an engine that has been fed
+one character at a time and is therefore nearly empty. Probe question 9 sent it
+with `TxBufLen` at 0 and MMTTY transmitted for another eight seconds; a bridge
+log of three CQs shows the same on the air, with the transmitter staying up
+until this program forced it down. Whether it is the empty buffer or the way the
+text arrives that MMTTY objects to is not settled: probe question 11 sent N1MM's
+exact sequence and it did not drop either, but that machine has no sound card,
+so MMTTY never really transmits and its buffer reads 0 whatever it is given.
+
+If the engine still does not drop the transmitter, the key goes down here: the
+control's `PTT` property back to false, which took 251 ms in the probe. What it
+waits for is the engine, not a clock: `StopPatience` is how long the engine may
+make no progress, not how long the whole wait may take. Measured against the
+whole wait it cut a CQ off with 21 symbols still in the engine, because a
+flushed message is seconds of transmission and the engine is entitled to all of
+it.
+
+Two things had to be right before the flush worked at all, and both were wrong
+first. The flush takes the same turn the feeder takes, so a character the feeder
+had already taken and was still handing over cannot end up behind the rest of
+the message — it did once, and the CQ went out with its first letter at the end.
+And what the engine holds is counted across feeders rather than reset when one
+starts: a feeder that started after a flush forgot a whole message the engine
+was still holding, called the transmission over and put the key down in the
+middle of it.
+`TypeAhead.WhenEmptyAsync` waits for `Aired` first, which is two things at
+once: the engine's own count at 0, and the clock saying the characters the
+engine held have had time to go out. `StopPatience` caps the wait for an engine
+that never reports itself empty.
+
+One character time is held after that and no more, because the count reaching 0
+is late news rather than early. In a bridge log the count read 0 at 10670 ms and
+the last character came back decoded at 11026 ms, and the decoder runs about
+420 ms behind the air, so that character went out at about 10606 ms — before the
+count read 0. What the character time covers is the reading, which happens every
+`TypeAhead.PollInterval`, not the transmission. Turnaround time is worth more
+than padding in a contest.
+
+MMTTY has no stop character to send instead. Its macro language ends a
+transmission with `\` at the end of a macro and stops the carrier with `~`, but
+those are read by the macro interpreter, and the only way into the engine from
+here is `PostMmttyMessage(4, ...)`, one typed character. Probe question 10 typed
+both at the end of a message: both were swallowed and the transmitter stayed up.
+
+Before this the stop was `AbortXmit` a fixed `StopPatience` after the polite
+stop, which is a race. It was lost once in the log: on one of three CQs the
+abort fired 4 ms before the count reached 0 and the carriage return at the end
+of the message never went out.
+
+Escape is the one that stops now: what has not gone to the engine is dropped and
+the engine drops what it holds.
+
+**What paces the feeder.** The clock, at one Baudot symbol every symbol time for
+the baud rate in the digital settings, with the engine kept `Ahead` characters
+ahead: one on the air and one in hand for when that one finishes. RTTY runs at a
+fixed speed, so the clock is right.
+
+A character is not a symbol. Most are one, but a digit sent while the engine is
+in the letters shift costs a shift symbol and the digit, and the letter after it
+costs a shift back: `OM5M` is six symbols, not four. Pacing one character per
+symbol time ran ahead of the air by about one part in nine — 26 characters of
+one CQ went out in 29 symbols — which showed up as the red text in the transmit
+pane running ahead of the transmission, and as the engine's count building up
+until the brake caught it. `TypeAhead.Symbols` prices a character against the
+shift the engine is in, and both the feeder and the model of what is on the air
+use it. They keep separate shift states, because the feeder is `Ahead`
+characters in front of what is being transmitted.
+
+**What a character costs cannot be calculated, so it is bounded.** The shift a
+character needs depends on settings this program cannot see: unshift-on-space is
+`TXUOS` in MMTTY's `UserPara.ini`, written per profile, and it is also a button
+on MMTTY's own display that the operator can press during a contest. The FIG
+button is another. `Symbols` assumes unshift-on-space is on, because MMTTY's
+help says that is the usual setting and because assuming it charges a symbol too
+many, which leaves the pane behind the air rather than in front of it.
+
+The engine's count is what settles it, and it needs no setting to be read. The
+count is in symbols and falls as they are transmitted, so what it drops between
+two answers is what went on the air between them. That is the air's own rate,
+measured rather than reckoned: `WentOutByCount` spends those symbols on the
+characters at the front of what has not gone out and marks them as gone.
+
+Two things make it self-correcting. The rate comes from the engine, so a clock
+that runs a little fast or slow cannot drift. And a count of 0 means the engine
+holds nothing, so everything it was given has gone out, whatever the symbols
+added up to along the way — a wrong guess about the shift is squared off at the
+end of every message rather than accumulating over a contest.
+
+The one thing to be careful of is that the count trails what the engine was last
+given by 100 to 150 ms, which the probe measured. A 0 newer than `CountLag` is
+the answer not having caught up, not an empty engine, and is passed over. An
+engine that does not count at all falls back to the clock.
+
+Not the decoder. MMTTY hears its own transmission with sound loopback on and
+reports it back, which would be the air itself, but that is the receive path: it
+is off at many stations, and with a receiver actually listening another
+station's `CQ` matches ours by chance. The transmit buffer count is the
+transmitter's own report and has neither problem.
+
+`MmttyEngine` has to declare `EngineBuffer` for any of the count to be used.
+Without it the sender falls back to the plain path: whole `SendString` calls
+instead of `PostMmttyMessage(4, ...)` one character at a time, and no count at
+all. It ran that way for a while, and a bridge log of a CQ shows what it costs.
+The feeder put 26 characters out at 165 ms each; MMTTY put them on the air at
+183 ms each, because the digits in the callsign cost a shift to figures and the
+letter after each one a shift back. By the end of the message the engine was
+1.5 s behind, and `{RX}` was answered by the hard stop rather than by MMTTY
+running dry. The brake is what holds that gap down.
+
+The clock advances by exactly one symbol time per symbol handed over,
+never from the time the poll happened. A poll is up to 50 ms late, and starting
+the next character from there made every character a little late, the lateness
+added up, and the engine ran dry and transmitted the idle tone between the
+characters of a long word.
+
+The engine's own count cannot be the pace, which the probe log shows twice over.
+`TxBufLen` read 0 for the first 150 ms after twenty-one characters were pushed
+and were already going out, and it read 0 for three seconds while the engine sat
+holding `ABCD` on Word out. The count is only to be believed while it is large
+and going down, and the feeder keeps the engine nearly empty on purpose — that is
+what leaves the message editable — so it lives in the range where the count
+reads 0. A feeder that took that 0 as room fed on it every poll and handed the
+engine about three characters for every one on the air, which is what put the
+red text ahead of the transmission.
+
+So the count is a brake. `Slack` is how many symbols the engine may be behind
+before the feeder waits for it: MMTTY answers in symbols, which is more than the
+characters it was given, so an exchange full of digits is slower than the clock
+thinks and this is what stops the clock running away on it. A count that is not
+going down means the engine is holding what it has, and the brake lets go after
+`HoldingPatience` so the space that releases a held word can get there.
+
+What the engine probe established, with MMTTY 1.70 under Wine on a machine with
+a sound card:
+
+The answer being in symbols is why `Slack` is in symbols and the feeder keeps no
+character count of its own to compare against it.
+
+The report is worth reading for the receive side as well. With MMTTY's sound
+loopback off, its help says the receive window is fed from the transmit window,
+which would be a per-character report of what has gone out with no polling at
+all. With loopback on, which is how this station runs it, the decoded text
+arrives about 420 ms after the character goes out and matches what was sent.
+
+**The protocol log.** Every line in and out of the bridge is written to
+`Diagnostics/digital-<when>.log` under the settings folder, one file per run of
+the engine, with a millisecond stamp on each. The buffer question is left out
+and its answer written only when the count changes, so what is left is the
+keying, the characters and the transmit reports. It is what says who dropped the
+transmitter and when, which nothing else in the program can answer: the engine
+probe cannot, because it runs the engine rather than the window.
+
+```sh
+./build.sh run --project tools/Nonemm.EngineProbe -- \
+    --engine ~/mmtty/MMTTY.EXE --prefix ~/.wine-nonemm --out engine-probe.log
+```
+
+It transmits on the sound card for about twenty seconds and keys no serial port
+unless `--ptt` names one. `EngineProbe` states what each answer means.
+
+**The transmitter dropping, and what ends a message.** `DigitalEngineSender`
+holds three states — `Down`, `Keyed`, `Ending` — and its class comment carries
+the transition table. The state is there rather than a flag because ending a
+message takes as long as the engine takes to transmit what it holds, which is
+seconds, and the operator can key again inside that time. With a flag, the
+ending of one message went on running and put the key down in the middle of the
+message after it. Each ending now belongs to one transmission and checks after
+every step that it is still that transmission.
+
+Coming back out of `Ending` costs something too. By then the stop is inside the
+engine, waiting for its buffer to empty, and MMTTY fed while that stands stayed
+keyed and transmitted nothing — the characters went in and never came out, and
+the pane coloured them because `TxBufLen` answered 0, which is what an engine
+that has transmitted everything also answers. So a macro pressed while the last
+one is ending waits for what the engine holds to go out, clears the stop with
+`AbortXmit`, and keys again. The wait is nothing at all in the case that shows
+it: the last message has finished playing, which is why the operator pressed the
+next key.
+
+A message ends when `{RX}` or an abort has told the engine to stop and the
+engine then reports the transmitter down. A report nobody asked for is
+ignored. MMTTY reports the
+transmitter down at other times — the buffer hands it one character at a time
+and keeps it nearly empty — and taking that as the end cleared the pane and shut
+the gate two or three characters into a message, leaving the rest of it to go
+out on the next function key.
+
+That report is not the only thing that starts the pane again, because it does
+not always come. Two macros pressed one after the other keep the transmitter up
+from the first to the last character, so the engine never reports a drop between
+them, and the pane kept both messages. Everything the engine has been given is
+locked — it cannot be taken back — so a run of macros left a pane with nothing
+in it that could be edited. `DigitalEngineSender.StartAsync` now calls
+`TypeAhead.Started` whenever it keys a new transmission, which drops the last
+message off the pane and keeps whatever was typed ahead.
+
+`TypeAhead.Ended` is called while the state lock is held, for the same reason.
+The drop arrives from the engine on a thread of its own, and a macro pressed on
+the last character of a message got as far as handing its own text over in the
+gap between reading the state and clearing the pane: the new text was wiped off
+the pane while the engine transmitted it. The order was found by reading the
+code, not on the air, and the fix has not been tested against a real engine.
+
+Which of the control's two paths carries the transmit state has not been
+settled. The bridge takes it from `OnPttEvent`, and N1MM's `DigitalInterface`
+binds an empty handler there and reads MMTTY's window message 32772 out of
+`OnTranslateMessage` instead. Both are in the same class, so one of them is a
+leftover, and `docs/digital-bridge.md` argues 32772 cannot reach
+`OnTranslateMessage` at all while the control has an event for it. `OnPttEvent`
+does report drops here, so nothing is missing; it is worth measuring only if the
+reports turn out to be coarser than the message.
+
+Keying is the keyer's, not the window's: `{TX}` and the TX button both go
+through `DigitalEngineSender.Transmit`, so the order to key is always the first
+thing the engine is told. MMTTY keys itself off a character given to it while
+the transmitter is down and drops again when it has sent it, so a key that
+arrived behind the text put a keyed-up gap in the middle of the message. A drop
+in the middle of a message that is still going out also keys the transmitter
+again, so what is left of the message does not go out into a transmitter that
+is down.
+
+The idle tone between what the operator types is MMTTY's own diddle, the Diddle
+setting on its TX tab: LTR, the standard one, is what `mmtty/UserPara.ini`
+carries here. Nothing is fed to produce it, so it needs the transmitter to stay
+keyed. Whether MMTTY holds it there with an empty buffer has not been measured:
+the probe run that says it does was on a machine with no sound card, where
+MMTTY does not really transmit.
+
+**The link between the logging computers.** N1MM has two networks and this
+program now has both. Port 12060 carries XML to other programs — a spot tool, a
+score poster — which is `StationNetwork`. Computer to computer, N1MM uses port
+12070 and a different protocol: `StationLink`.
+
+That protocol is a UDP broadcast to find the other computers and a TCP
+connection to each of them, opened both ways. A message reads
+
+```
+DATA__07%SHACK-PC%QSO%2026-09-03 12:34:56%DL1ABC%…~__DATA
+```
+
+which is the sending station's number, its computer name, the message type and
+the fields of that type. `%` and `~` cannot appear in a field, so N1MM writes
+`!` in their place. What is read and sent: contacts, edits, deletes, resyncs,
+`IAM`, the echo pair, chat, the pass frequency, transmit on and off, and where a
+station is. The rest of N1MM's forty-odd types — the score and sked windows, the
+log check, the spot lists, the serial-number pool — are passed over, which
+N1MM's own reader also does for a type it does not know.
+
+**The version has to match.** N1MM compares the version in the beacon with its
+own and, when they differ, puts up "Software versions must match. Update N1MM+."
+and drops the station. So the version this program broadcasts is a setting —
+Config ▸ Edit Networked-Computer Names — and it has to be the version of the
+N1MM copies beside it. Help ▸ About in N1MM says which. A station that
+broadcasts something else is kept in the list and the network status window
+names it, because that is the one fault where everything looks connected and
+nothing arrives.
+
+Where a station is goes out once a second when it has changed, rather than from
+each of the dozen places it can change from. In a multi-single entry that is
+what the other operators watch.
+
+**The ten-minute rule.** `BandChangeRules` counts band changes and the stay on a
+band, and the entry window shows the countdown. What was missing was the rule
+itself: only a user-defined contest carried one, so every built-in contest
+allowed anything. `BandChangeRules.ForCategory` now gives a multi-operator entry
+with one or two transmitters a ten-minute stay, which is N1MM's fallback for
+every contest that does not name a number of its own, and a contest that caps
+the changes per hour overrides `Contest.BandChangesFor`. N1MM's per-contest table
+is not repeated: it is a few hundred cases in a decompiled hash switch, and the
+contests this station enters are the ones worth reading out of it one at a time.
+
+What the link does not do yet: N1MM's resync, where a station that has been
+away asks another for the contacts it missed, and the log check that compares
+two stations' logs row by row. Both are message types this program can already
+frame; neither is written. There is also no guard against two stations taking
+the same station number.
 
 **Voice keying.** `MessageSender` was written to cover a voice keyer playing a
 recording, and nothing implements it. Each operator's recordings folder is
@@ -68,13 +405,29 @@ of an SO2R station cannot call CQ by voice. Alternating CQ therefore works on CW
 only, though nothing in it is CW-specific: a voice keyer that reports when the
 recording has finished would drive it as it stands.
 
-**The QTC window transmits on CW only.** The header, the lines, the QRV, the TU
-and the again messages go out through the keyer, with N1MM's messages and
-N1MM's defaults. Nothing goes out on SSB or RTTY: N1MM plays four recordings on
-SSB — QRV, Agn, Cfm and TU — and sends RTTY from its digital window, and this
-program has neither a voice keyer nor a digital window. N1MM's Send All, which
-keys a whole series at once, belongs to that RTTY window and is not here
-either.
+**The QTC window sends on CW and RTTY, not on SSB.** On CW the header, the
+lines, the QRV, the TU and the again messages go out through the keyer, with
+N1MM's messages and N1MM's defaults.
+
+On RTTY the station reading traffic out has Send All, which is N1MM's: the whole
+series as one message, the heading, then every line with the operator's spacing
+between them, then the ending, and `{TX}` and `{RX}` around the lot so it keys
+the transmitter and drops it again. A line is the three fields joined with
+hyphens, which is how N1MM writes them and it offers no setting for it. Snd n
+sends one line again for a station that missed it. The heading, the ending and
+the spacing are settings, N1MM's `WAESendAllHeadingText`,
+`WAESendAllEndingText` and `WAESQTCSpacing`, with its defaults; `{ENTERLF}` in
+them is new to the expander and stands for a carriage return and a line feed.
+
+A whole series is the best part of a minute of transmission at 45 baud, which is
+why it is one message rather than a button per line: the type-ahead buffer holds
+it and paces the engine, and the operator presses one button. This has been
+tested as text — what the message comes out as — and not on the air.
+
+What is not here: N1MM's four SSB recordings — QRV, Agn, Cfm and TU — which need
+a voice keyer, and its RTTY messages for the station taking traffic down, which
+are `WAERXReadyText`, `WAEAllAgnText`, `WAEAGNText` and `WAESaveQTCText`. Those
+four are typed into the digital window's transmit pane here.
 
 **Cut numbers are not sent.** N1MM can key a serial number as letters — `N` for
 9, `T` for 0 — and offers several styles. Nothing here does, in a QTC line or in
@@ -124,9 +477,11 @@ ones the station's log needs — and both of N1MM's names open a log with it. A
 sprint log would be scored by the wrong rules, and a log started here writes
 `BARTGRTTYS` into the contest name, which N1MM would read as the sprint.
 
-**Digital modes.** A contact can be logged as RTTY or another digital mode, and
-the contest rules score it, but there is no digital window: no decoding, no
-transmitting, no interface to fldigi, MMTTY or similar.
+**Digital interfaces other than MMTTY.** A contact can be logged as RTTY or
+another digital mode and the contest rules score it, and the digital window
+decodes and transmits through MMTTY over the Wine bridge. What is not here:
+fldigi, MMVARI, 2Tone and the hardware TNCs, all of which N1MM drives.
+`DigitalEngine` is the place to add them.
 
 **Contest coverage.** Twenty-five families are built in, plus whatever `.udc` files
 are in the user-defined folder. N1MM ships well over a hundred. Opening a log

src/Nonemm.App/AppSession.cs

@@ -25,6 +25,12 @@ public sealed class AppSession : IDisposable
     private int activeRadio;
     private ClusterClient? cluster;
     private StationNetwork? network;
+    private StationLink? link;
+
+    /// What was last said to the other stations about where this one is, so it
+    /// is said again only when it changes.
+    private (Frequency Where, string Mode, bool Running, int Radio) announced;
+    private DispatcherTimer? announcing;
     private MessageSender? keyer;
     private AlternatingCq? alternating;
     private MmttyEngine? digital;
@@ -124,6 +130,10 @@ public sealed class AppSession : IDisposable
 
     public StationNetwork? Network => network;
 
+    /// The link to the other logging computers, or null when the operator has
+    /// not turned it on. The network status window reads it.
+    public StationLink? Link => link;
+
     public MessageSender? Keyer => keyer;
 
     /// The digital modem, started by the digital window rather than at startup:
@@ -221,6 +231,12 @@ public sealed class AppSession : IDisposable
         Logging.Edited += (_, change) => _ = network?.SendEditAsync(
             change.Qso, Settings.Station.Callsign, change.OldCall, change.OldTimestampUtc);
         Logging.Deleted += (_, qso) => _ = network?.SendDeleteAsync(qso, Settings.Station.Callsign);
+        // and the same three to the other logging computers, which is a
+        // different protocol on a different port
+        Logging.Logged += (_, qso) => _ = link?.SendLoggedAsync(qso);
+        Logging.Edited += (_, change) =>
+            _ = link?.SendEditedAsync(change.Qso, change.OldCall, change.OldTimestampUtc);
+        Logging.Deleted += (_, qso) => _ = link?.SendDeletedAsync(qso);
         Check = new CheckWindowSources(positions[0], Calls, Bandmap);
         Available = new AvailableStations(positions[0], Bandmap);
         Save(Settings with { ContestNumber = contestNumber });
@@ -234,7 +250,9 @@ public sealed class AppSession : IDisposable
     /// multi-operator entry, following what the settings now say.
     public void ApplyNetworkSettings()
     {
+        announcing?.Stop();
         network?.Dispose();
+        link?.Dispose();
         network = null;
         if (!Settings.NetworkEnabled)
         {
@@ -253,6 +271,87 @@ public sealed class AppSession : IDisposable
         Changed?.Invoke(this, EventArgs.Empty);
     }
 
+    /// Starts, restarts or stops the link to the other logging computers. It is
+    /// separate from `ApplyNetworkSettings` because the two are separate
+    /// networks: one carries XML to other programs, the other carries contacts
+    /// to the other computers of this entry.
+    public void ApplyStationLinkSettings()
+    {
+        link?.Dispose();
+        link = null;
+        if (!Settings.StationLinkEnabled)
+        {
+            Changed?.Invoke(this, EventArgs.Empty);
+            return;
+        }
+        link = new StationLink(
+            Settings.NetworkStationName.Length > 0 ? Settings.NetworkStationName : Environment.MachineName,
+            Settings.StationLinkVersion,
+            Settings.StationNumber,
+            Settings.StationLinkPort)
+        {
+            Operator = Settings.Station.Callsign,
+        };
+        // the socket thread must not touch the log: every other change to it is
+        // made where the windows read it
+        link.UpdateArrived += (_, update) =>
+            Dispatcher.UIThread.Post(() => TakeFromNetwork(update));
+        foreach (string peer in Settings.StationLinkPeers)
+        {
+            if (Peer(peer) is var (name, address, port))
+            {
+                link.AddStation(name, address, port);
+            }
+        }
+        link.Start();
+        // where this station is has a dozen places it can change from — the
+        // radio moving, a band button, a QSY typed into the callsign box, run
+        // turning on — so it is read once a second and sent when it has
+        // changed, rather than announced from each of them
+        announcing?.Stop();
+        announced = default;
+        announcing = new DispatcherTimer { Interval = TimeSpan.FromSeconds(1) };
+        announcing.Tick += (_, _) => AnnounceWhereIAm();
+        announcing.Start();
+        Changed?.Invoke(this, EventArgs.Empty);
+    }
+
+    /// Tells the other stations where this one is, if it has moved. In a
+    /// multi-single entry this is what the other operators watch: two stations
+    /// on one band is a contact nobody can make.
+    private void AnnounceWhereIAm()
+    {
+        if (link is null || Position is not { } position)
+        {
+            return;
+        }
+        (Frequency, string, bool, int) now =
+            (position.Frequency, position.Mode.Name, position.IsRunning, position.RadioNumber);
+        if (now == announced)
+        {
+            return;
+        }
+        announced = now;
+        _ = link.SendBandAsync(position.Frequency, position.Mode, position.IsRunning, position.RadioNumber);
+    }
+
+    /// A station named by hand, written `RUN-PC@192.168.1.5` with `:port` on
+    /// the end when that station is not on the usual one. Null for anything
+    /// else, because a line the operator has half typed is not an address.
+    private (string Name, string Address, int Port)? Peer(string text)
+    {
+        string[] parts = text.Split('@');
+        if (parts.Length != 2 || parts[0].Trim().Length == 0)
+        {
+            return null;
+        }
+        string[] host = parts[1].Split(':');
+        return (
+            parts[0].Trim(),
+            host[0].Trim(),
+            host.Length > 1 && int.TryParse(host[1], out int given) ? given : Settings.StationLinkPort);
+    }
+
     /// What another station did to its log, applied to ours. Nothing goes back
     /// out to the network, and the contact is scored here from the rules
     /// instead of trusting what the sender put in the message.
@@ -329,7 +428,8 @@ public sealed class AppSession : IDisposable
         WineBridgeChannel channel = new(
             Settings.DigitalBridgePath,
             Settings.DigitalWinePrefix.Trim().Length > 0 ? Settings.DigitalWinePrefix : null,
-            Settings.DigitalWineCommand.Trim().Length > 0 ? Settings.DigitalWineCommand : "wine");
+            Settings.DigitalWineCommand.Trim().Length > 0 ? Settings.DigitalWineCommand : "wine",
+            Paths.Diagnostics);
         MmttyEngine started = new(channel, options);
         await started.StartAsync();
         digital = started;
@@ -594,7 +694,9 @@ public sealed class AppSession : IDisposable
         spotFlush.Dispose();
         DisposeRadios();
         cluster?.Dispose();
+        announcing?.Stop();
         network?.Dispose();
+        link?.Dispose();
         alternating?.Dispose();
         keyer?.Dispose();
         box?.Dispose();

src/Nonemm.App/Configuration/Settings.cs

@@ -1,6 +1,7 @@
 using System.Reflection;
 using System.Text.Json;
 using Nonemm.Core;
+using Nonemm.Network;
 using Nonemm.Session;
 
 namespace Nonemm.App.Configuration;
@@ -118,6 +119,29 @@ public sealed record Settings
 
     public IReadOnlyList<string> NetworkPeers { get; init; } = [];
 
+    /// The link to the other logging computers of a multi-operator entry, on
+    /// N1MM's port 12070. It is not the same thing as `NetworkEnabled`, which
+    /// is the XML broadcast on 12060 that other programs read; a station can
+    /// want either, or both.
+    public bool StationLinkEnabled { get; init; }
+
+    public int StationLinkPort { get; init; } = StationBeacon.DefaultPort;
+
+    /// Which station of the entry this computer is. It goes into every message
+    /// and lets a contact say which position made it.
+    public int StationNumber { get; init; } = 1;
+
+    /// The version this program claims to be on the network. N1MM refuses a
+    /// station whose version is not its own, so to work beside N1MM this has to
+    /// be the version those copies are running. The default is the version of
+    /// the N1MM this program was written against.
+    public string StationLinkVersion { get; init; } = "1.0.11364";
+
+    /// Stations named by hand, for a network where a broadcast does not reach
+    /// every computer. Each is a name and an address — `RUN-PC@192.168.1.5`,
+    /// with `:port` on the end when that station is not on the usual port.
+    public IReadOnlyList<string> StationLinkPeers { get; init; } = [];
+
     /// The call history file for this contest, or empty for none. They are
     /// published per contest, so this is not a fixed name.
     public string CallHistoryFile { get; init; } = "";
@@ -169,6 +193,16 @@ public sealed record Settings
 
     public string QtcCwTu { get; init; } = "";
 
+    /// The RTTY messages the QTC window sends, with N1MM's defaults. The
+    /// spacing goes between the lines of a series rather than between the
+    /// fields of one, and all three are macro templates: `{ENTER}` for a
+    /// carriage return, `{QTC}` for the header of the series.
+    public string QtcRttySpacing { get; init; } = QtcMessages.DefaultRttySpacing;
+
+    public string QtcRttySendAllHeading { get; init; } = QtcMessages.DefaultSendAllHeading;
+
+    public string QtcRttySendAllEnding { get; init; } = QtcMessages.DefaultSendAllEnding;
+
     /// Sub-band boundaries the operator has changed. Empty means the defaults
     /// in `BandPlan.Default`; an entry replaces one band's boundaries.
     public IReadOnlyList<StoredSubBand> SubBands { get; init; } = [];

src/Nonemm.App/Controls/TransmitPane.axaml

@@ -0,0 +1,40 @@
+<Styles xmlns="https://github.com/avaloniaui"
+        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+        xmlns:controls="using:Nonemm.App.Controls">
+  <Style Selector="controls|TransmitPane">
+    <Setter Property="Background" Value="{DynamicResource FieldBackground}" />
+    <Setter Property="Foreground" Value="{DynamicResource FieldForeground}" />
+    <Setter Property="CaretBrush" Value="{DynamicResource FieldForeground}" />
+    <Setter Property="SelectionBrush" Value="#3399FF" />
+    <Setter Property="SelectionForegroundBrush" Value="#FFFFFF" />
+    <Setter Property="Padding" Value="3,2" />
+    <Setter Property="Template">
+      <!-- a TextBox subclass gets no theme of its own, so the pane carries
+           its own template. It is the Fluent one cut down to what the pane
+           uses: no watermark, no clear button, no border of its own -->
+      <ControlTemplate>
+        <ScrollViewer Name="PART_ScrollViewer"
+                      Background="{TemplateBinding Background}"
+                      Padding="{TemplateBinding Padding}"
+                      HorizontalScrollBarVisibility="Disabled"
+                      VerticalScrollBarVisibility="Auto">
+          <controls:TransmitPresenter Name="PART_TextPresenter"
+                                      Text="{TemplateBinding Text}"
+                                      CaretIndex="{TemplateBinding CaretIndex}"
+                                      SelectionStart="{TemplateBinding SelectionStart}"
+                                      SelectionEnd="{TemplateBinding SelectionEnd}"
+                                      SelectionBrush="{TemplateBinding SelectionBrush}"
+                                      SelectionForegroundBrush="{TemplateBinding SelectionForegroundBrush}"
+                                      CaretBrush="{TemplateBinding CaretBrush}"
+                                      TextAlignment="{TemplateBinding TextAlignment}"
+                                      TextWrapping="{TemplateBinding TextWrapping}"
+                                      LineHeight="{TemplateBinding LineHeight}"
+                                      LetterSpacing="{TemplateBinding LetterSpacing}"
+                                      SentLength="{TemplateBinding SentLength}"
+                                      SentBrush="{TemplateBinding SentBrush}"
+                                      VerticalAlignment="Top" />
+        </ScrollViewer>
+      </ControlTemplate>
+    </Setter>
+  </Style>
+</Styles>

src/Nonemm.App/Controls/TransmitPane.cs

@@ -0,0 +1,37 @@
+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Media;
+
+namespace Nonemm.App.Controls;
+
+/// The digital transmit pane: one editable box in which the first `SentLength`
+/// characters — what has already gone to the engine — are drawn in `SentBrush`.
+///
+/// Avalonia's TextBox draws all of its text in one brush, so this was a label
+/// beside a box before. That took width from the box as the label grew and the
+/// label did not wrap. A TextBox builds its text through a TextPresenter, and
+/// the TextLayout under it does take a brush per run, so the pane is a TextBox
+/// with `TransmitPresenter` in place of the plain presenter. The template in
+/// `DigitalWindow.axaml` is what puts it there.
+public class TransmitPane : TextBox
+{
+    public static readonly StyledProperty<int> SentLengthProperty =
+        AvaloniaProperty.Register<TransmitPane, int>(nameof(SentLength));
+
+    public static readonly StyledProperty<IBrush?> SentBrushProperty =
+        AvaloniaProperty.Register<TransmitPane, IBrush?>(nameof(SentBrush));
+
+    /// How many characters at the front of the text have gone out.
+    public int SentLength
+    {
+        get => GetValue(SentLengthProperty);
+        set => SetValue(SentLengthProperty, value);
+    }
+
+    /// What those characters are drawn in.
+    public IBrush? SentBrush
+    {
+        get => GetValue(SentBrushProperty);
+        set => SetValue(SentBrushProperty, value);
+    }
+}

src/Nonemm.App/Controls/TransmitPresenter.cs

@@ -0,0 +1,133 @@
+using Avalonia;
+using Avalonia.Controls.Presenters;
+using Avalonia.Media;
+using Avalonia.Media.TextFormatting;
+using Avalonia.Utilities;
+
+namespace Nonemm.App.Controls;
+
+/// The presenter behind `TransmitPane`: the same text layout Avalonia builds
+/// for a TextBox, with the first `SentLength` characters given `SentBrush`.
+///
+/// TextPresenter builds its layout in `CreateTextLayout` and already passes
+/// per-run overrides for the selection, so this adds one more run to that list.
+/// The selection has to keep its own colour on top, which is why the coloured
+/// run is cut around it rather than laid over it: overlapping runs are not
+/// defined.
+public class TransmitPresenter : TextPresenter
+{
+    public static readonly StyledProperty<int> SentLengthProperty =
+        AvaloniaProperty.Register<TransmitPresenter, int>(nameof(SentLength));
+
+    public static readonly StyledProperty<IBrush?> SentBrushProperty =
+        AvaloniaProperty.Register<TransmitPresenter, IBrush?>(nameof(SentBrush));
+
+    /// The width the layout is built to. `TextPresenter` keeps its own copy and
+    /// does not hand it out, so it is read off the measure pass here.
+    private Size constraint;
+
+    public int SentLength
+    {
+        get => GetValue(SentLengthProperty);
+        set => SetValue(SentLengthProperty, value);
+    }
+
+    public IBrush? SentBrush
+    {
+        get => GetValue(SentBrushProperty);
+        set => SetValue(SentBrushProperty, value);
+    }
+
+    protected override Size MeasureOverride(Size availableSize)
+    {
+        constraint = availableSize;
+        return base.MeasureOverride(availableSize);
+    }
+
+    protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
+    {
+        base.OnPropertyChanged(change);
+        if (change.Property == SentLengthProperty || change.Property == SentBrushProperty)
+        {
+            InvalidateTextLayout();
+        }
+    }
+
+    protected override TextLayout CreateTextLayout()
+    {
+        string text = Text ?? "";
+        int sent = Math.Clamp(SentLength, 0, text.Length);
+        // nothing to colour, or a case the base class handles on its own: the
+        // password character replaces the text, and a preedit run is the input
+        // method's, not ours
+        if (sent == 0 || SentBrush is null || PasswordChar != '\0'
+            || !string.IsNullOrEmpty(PreeditText))
+        {
+            return base.CreateTextLayout();
+        }
+        Typeface typeface = new(FontFamily, FontStyle, FontWeight, FontStretch);
+        // a zero constraint is a measure with no bound, which is infinity to
+        // the layout
+        double width = constraint.Width > 0 ? constraint.Width : double.PositiveInfinity;
+        double height = constraint.Height > 0 ? constraint.Height : double.PositiveInfinity;
+        return new TextLayout(
+            text,
+            typeface,
+            FontSize,
+            Foreground,
+            TextAlignment,
+            TextWrapping,
+            null,
+            null,
+            FlowDirection,
+            width,
+            height,
+            LineHeight,
+            LetterSpacing,
+            0,
+            FontFeatures,
+            Runs(typeface, sent));
+    }
+
+    /// The coloured runs, in order and not overlapping. The selection, when
+    /// there is one with a colour of its own, cuts the coloured run in two.
+    private List<ValueSpan<TextRunProperties>> Runs(Typeface typeface, int sent)
+    {
+        int from = Math.Min(SelectionStart, SelectionEnd);
+        int to = Math.Max(SelectionStart, SelectionEnd);
+        bool selected = ShowSelectionHighlight && to > from && SelectionForegroundBrush is not null;
+        List<ValueSpan<TextRunProperties>> runs = [];
+        Add(runs, 0, selected ? Math.Min(sent, from) : sent, SentBrush, typeface);
+        if (selected)
+        {
+            Add(runs, from, to, SelectionForegroundBrush, typeface);
+            Add(runs, to, sent, SentBrush, typeface);
+        }
+        return runs;
+    }
+
+    private void Add(
+        List<ValueSpan<TextRunProperties>> runs,
+        int start,
+        int end,
+        IBrush? brush,
+        Typeface typeface)
+    {
+        if (end <= start)
+        {
+            return;
+        }
+        runs.Add(new ValueSpan<TextRunProperties>(
+            start,
+            end - start,
+            new GenericTextRunProperties(
+                typeface,
+                FontSize,
+                null,
+                brush,
+                null,
+                BaselineAlignment.Baseline,
+                null,
+                FontFeatures)));
+    }
+}

src/Nonemm.App/Dialogs/NetworkDialog.axaml

@@ -14,7 +14,23 @@
     </Grid>
     <TextBlock Text="Other stations, one address per line" FontSize="11" Opacity="0.7" Margin="0,6,0,1" />
     <TextBox Name="PeersBox" AcceptsReturn="True" Height="90" PlaceholderText="192.168.1.11" />
-    <CheckBox Name="EnabledBox" Content="Share contacts with the other stations" Margin="0,6,0,0" />
+    <CheckBox Name="EnabledBox" Content="Broadcast contacts to other programs (port 12060)" Margin="0,6,0,0" />
+    <TextBlock Text="The other logging computers" FontWeight="Bold" Margin="0,12,0,0" />
+    <TextBlock TextWrapping="Wrap" FontSize="11" Opacity="0.75"
+               Text="This is N1MM's own link between the computers of one entry, on port 12070: contacts, edits, deletes and chat, over a connection to each station. Stations are found by broadcast, so nothing has to be listed." />
+    <Grid ColumnDefinitions="90,8,110,8,*" RowDefinitions="Auto,Auto">
+      <TextBlock Text="Station number" FontSize="11" Opacity="0.7" Margin="0,6,0,1" />
+      <TextBox Name="StationNumberBox" Grid.Row="1" />
+      <TextBlock Grid.Column="2" Text="Port" FontSize="11" Opacity="0.7" Margin="0,6,0,1" />
+      <TextBox Name="LinkPortBox" Grid.Row="1" Grid.Column="2" />
+      <TextBlock Grid.Column="4" Text="Version to claim" FontSize="11" Opacity="0.7" Margin="0,6,0,1" />
+      <TextBox Name="VersionBox" Grid.Row="1" Grid.Column="4" />
+    </Grid>
+    <TextBlock TextWrapping="Wrap" FontSize="11" Opacity="0.75"
+               Text="N1MM turns away a station whose version is not its own, so this has to be the version the N1MM copies beside it are running. Help ▸ About in N1MM says which." />
+    <TextBlock Text="Stations to reach by address, one per line, as NAME@address" FontSize="11" Opacity="0.7" Margin="0,6,0,1" />
+    <TextBox Name="LinkPeersBox" AcceptsReturn="True" Height="60" PlaceholderText="RUN-PC@192.168.1.11" />
+    <CheckBox Name="LinkEnabledBox" Content="Share contacts with the other logging computers (port 12070)" Margin="0,6,0,0" />
     <StackPanel Orientation="Horizontal" HorizontalAlignment="Right" Spacing="6" Margin="0,8,0,0">
       <Button Content="Cancel" Click="OnCancel" />
       <Button Content="Save" Click="OnSave" IsDefault="True" />

src/Nonemm.App/Dialogs/NetworkDialog.axaml.cs

@@ -18,6 +18,11 @@ public sealed partial class NetworkDialog : Window
         PortBox.Text = settings.NetworkPort.ToString();
         PeersBox.Text = string.Join("\n", settings.NetworkPeers);
         EnabledBox.IsChecked = settings.NetworkEnabled;
+        StationNumberBox.Text = settings.StationNumber.ToString();
+        LinkPortBox.Text = settings.StationLinkPort.ToString();
+        VersionBox.Text = settings.StationLinkVersion;
+        LinkPeersBox.Text = string.Join("\n", settings.StationLinkPeers);
+        LinkEnabledBox.IsChecked = settings.StationLinkEnabled;
     }
 
 
@@ -31,7 +36,23 @@ public sealed partial class NetworkDialog : Window
             .Where(l => l.Length > 0)
             .ToList(),
         NetworkEnabled = EnabledBox.IsChecked == true,
+        StationNumber = int.TryParse(StationNumberBox.Text, out int number)
+            ? Math.Clamp(number, 1, 99)
+            : settings.StationNumber,
+        StationLinkPort = int.TryParse(LinkPortBox.Text, out int linkPort)
+            ? linkPort
+            : settings.StationLinkPort,
+        StationLinkVersion = (VersionBox.Text ?? "").Trim(),
+        StationLinkPeers = Lines(LinkPeersBox.Text),
+        StationLinkEnabled = LinkEnabledBox.IsChecked == true,
     });
 
+    private static List<string> Lines(string? text) =>
+        (text ?? "")
+            .Split('\n', StringSplitOptions.RemoveEmptyEntries)
+            .Select(l => l.Trim())
+            .Where(l => l.Length > 0)
+            .ToList();
+
     private void OnCancel(object? sender, RoutedEventArgs e) => Close(null);
 }

src/Nonemm.App/Dialogs/QtcSetupDialog.axaml

@@ -33,8 +33,19 @@
     </Grid>
     <TextBlock Text="The spacing goes between the fields of a QTC line, written N1MM's way with S for a space. The three again messages go out on shift and Enter in that box while taking traffic down. TU goes out when the window closes after reading a series out; empty sends nothing."
                FontSize="11" Opacity="0.7" TextWrapping="Wrap" Margin="0,-2,0,0" />
+    <TextBlock Text="RTTY messages" FontWeight="Bold" Margin="0,10,0,0" />
+    <Grid ColumnDefinitions="Auto,*" RowDefinitions="Auto,Auto,Auto">
+      <TextBlock Text="Line spacing" VerticalAlignment="Center" Margin="0,2,8,2" />
+      <TextBox Grid.Column="1" Name="RttySpacingBox" Margin="0,2" />
+      <TextBlock Grid.Row="1" Text="Send All heading" VerticalAlignment="Center" Margin="0,2,8,2" />
+      <TextBox Grid.Row="1" Grid.Column="1" Name="SendAllHeadingBox" Margin="0,2" />
+      <TextBlock Grid.Row="2" Text="Send All ending" VerticalAlignment="Center" Margin="0,2,8,2" />
+      <TextBox Grid.Row="2" Grid.Column="1" Name="SendAllEndingBox" Margin="0,2" />
+    </Grid>
+    <TextBlock Text="These go out when the All button reads a whole series out on RTTY. The spacing goes between the lines, not between the fields of one — use {ENTER} or {ENTERLF} for a carriage return. {QTC} stands for the header of the series, and the other function key macros work as well."
+               FontSize="11" Opacity="0.7" TextWrapping="Wrap" Margin="0,-2,0,0" />
     <TextBlock Name="MissingText" FontSize="11" Opacity="0.7" TextWrapping="Wrap" Margin="0,6,0,0"
-               Text="N1MM's SSB recordings and RTTY message templates are left out: sending those needs a voice keyer and a digital window, and this program has neither." />
+               Text="N1MM's SSB recordings are left out: playing those needs a voice keyer, and this program has none. So are its RTTY messages for the station taking traffic down — RX Ready, All Agn, Agn and Save — which are typed into the digital window's transmit pane here." />
     <StackPanel Orientation="Horizontal" HorizontalAlignment="Right" Spacing="6" Margin="0,8,0,0">
       <Button Content="Cancel" Click="OnCancel" />
       <Button Content="Save" Click="OnSave" IsDefault="True" />

src/Nonemm.App/Dialogs/QtcSetupDialog.axaml.cs

@@ -24,6 +24,9 @@ public sealed partial class QtcSetupDialog : Window
         CallAgainBox.Text = settings.QtcCwCallAgain;
         NumberAgainBox.Text = settings.QtcCwNumberAgain;
         TuBox.Text = settings.QtcCwTu;
+        RttySpacingBox.Text = settings.QtcRttySpacing;
+        SendAllHeadingBox.Text = settings.QtcRttySendAllHeading;
+        SendAllEndingBox.Text = settings.QtcRttySendAllEnding;
     }
 
     private void OnSave(object? sender, RoutedEventArgs e) => Close(settings with
@@ -40,6 +43,9 @@ public sealed partial class QtcSetupDialog : Window
         QtcCwCallAgain = CallAgainBox.Text ?? "",
         QtcCwNumberAgain = NumberAgainBox.Text ?? "",
         QtcCwTu = TuBox.Text ?? "",
+        QtcRttySpacing = RttySpacingBox.Text ?? "",
+        QtcRttySendAllHeading = SendAllHeadingBox.Text ?? "",
+        QtcRttySendAllEnding = SendAllEndingBox.Text ?? "",
     });
 
     private void OnCancel(object? sender, RoutedEventArgs e) => Close(null);

src/Nonemm.App/Windows/DigitalWindow.Macros.cs

@@ -127,17 +127,26 @@ public sealed partial class DigitalWindow
         }
     }
 
-    /// What is typed in the transmit pane goes into the type-ahead buffer,
-    /// which feeds the engine. A carriage return is what the engine takes as a
-    /// new line, so Enter puts one in rather than the newline the box would.
+    /// What is typed in the transmit pane goes into the type-ahead buffer.
+    /// Nothing goes on the air until the transmitter is keyed, which is the TX
+    /// button, Ctrl+Enter or Alt+T; from then on what is typed goes out as it
+    /// is typed. Enter is a new line in the message, which the engine takes as
+    /// a carriage return.
+    ///
+    /// Escape stops now: what has not gone to the engine is dropped and the
+    /// engine drops what it holds.
     private void OnTransmitKeyDown(object? sender, KeyEventArgs e)
     {
+        if (e.Key == Key.Enter && e.KeyModifiers == KeyModifiers.Control)
+        {
+            e.Handled = true;
+            StartTransmit();
+            return;
+        }
         if (e.Key == Key.Enter)
         {
             e.Handled = true;
-            int at = Math.Clamp(TransmitBox.CaretIndex, 0, TransmitBox.Text?.Length ?? 0);
-            TransmitBox.Text = (TransmitBox.Text ?? "").Insert(at, "\r");
-            TransmitBox.CaretIndex = at + 1;
+            Type("\r");
             return;
         }
         if (e.Key == Key.Escape)
@@ -148,37 +157,61 @@ public sealed partial class DigitalWindow
         }
     }
 
-    /// The operator rewrote what has not gone out yet. Only the box holds it;
-    /// what is already in the engine is in the label beside it and cannot be
-    /// reached from here.
+    /// Puts text in at the caret, never before the text that has gone out.
+    private void Type(string text)
+    {
+        string was = TransmitBox.Text ?? "";
+        int at = Math.Clamp(TransmitBox.CaretIndex, locked, was.Length);
+        TransmitBox.Text = was.Insert(at, text);
+        TransmitBox.CaretIndex = at + text.Length;
+    }
+
+    /// The operator rewrote the pane. What has already gone to the engine
+    /// cannot be taken back, so an edit that reaches into it is undone; the
+    /// rest goes to the buffer, which sends it if the transmitter is up and
+    /// holds it if it is not.
     private void OnTransmitTextChanged(object? sender, TextChangedEventArgs e)
     {
         if (showingBuffer || session.DigitalKeyer is not { } keyer)
         {
             return;
         }
-        keyer.TypeAhead.Rewrite(TransmitBox.Text ?? "", CursorInBox());
-    }
-
-    /// The cursor holds the pump back: nothing behind it goes out, so the
-    /// engine idles rather than transmitting text the operator is still
-    /// typing. With the box out of focus there is no cursor to hold anything.
-    private void OnTransmitFocus(object? sender, RoutedEventArgs e) => ShowCursor();
-
-    private void ShowCursor()
+        string now = TransmitBox.Text ?? "";
+        if (FirstDifference(pane, now) < locked)
         {
-        if (session.DigitalKeyer is { } keyer)
+            showingBuffer = true;
+            try
             {
-            keyer.TypeAhead.Cursor = TransmitBox.IsFocused ? CursorInBox() : TypeAhead.NoCursor;
+                TransmitBox.Text = pane;
+                TransmitBox.CaretIndex = locked;
             }
+            finally
+            {
+                showingBuffer = false;
+            }
+            return;
+        }
+        pane = now;
+        keyer.Buffer.Edit(now);
     }
 
-    private int CursorInBox() =>
-        Math.Clamp(TransmitBox.CaretIndex, 0, TransmitBox.Text?.Length ?? 0);
+    /// Where two versions of the pane first differ, which is the length of both
+    /// when one is the other with text added or taken off the end.
+    private static int FirstDifference(string was, string now)
+    {
+        int most = Math.Min(was.Length, now.Length);
+        int at = 0;
+        while (at < most && was[at] == now[at])
+        {
+            at++;
+        }
+        return at;
+    }
 
-    /// Draws the buffer: what has gone out in the label, what is still to go in
-    /// the box. The pump takes characters off the front, so the cursor moves
-    /// back with them and the operator can go on typing while it does.
+    /// Draws the buffer: what has gone out and what is still to go, as one
+    /// text, with the length of the first half telling the pane how much of it
+    /// to colour. A character moving from one half to the other leaves the text
+    /// the same, so the caret and what the operator is typing do not move.
     private void ShowBuffer()
     {
         if (session.DigitalKeyer is not { } keyer)
@@ -188,20 +221,29 @@ public sealed partial class DigitalWindow
         showingBuffer = true;
         try
         {
-            SentText.Text = keyer.TypeAhead.Sent;
-            string pending = keyer.TypeAhead.Pending;
-            string was = TransmitBox.Text ?? "";
-            if (was == pending)
+            string sent = keyer.Buffer.Sent;
+            string now = sent + keyer.Buffer.Pending;
+            locked = sent.Length;
+            // only what the engine has transmitted is coloured. What it is
+            // still holding cannot be taken back either, but the operator has
+            // not heard it go yet, and marking it as gone turned every
+            // character red as it was typed once the transmission caught up
+            TransmitBox.SentLength = keyer.Buffer.OnAir;
+            if ((TransmitBox.Text ?? "") == now)
             {
+                pane = now;
                 return;
             }
-            int taken = was.Length > pending.Length
-                && was.EndsWith(pending, StringComparison.Ordinal)
-                    ? was.Length - pending.Length
+            // the end of a message drops what has gone out off the front of
+            // the pane, so the caret moves back with the text it is in
+            string was = TransmitBox.Text ?? "";
+            int dropped = was.Length > now.Length && was.EndsWith(now, StringComparison.Ordinal)
+                ? was.Length - now.Length
                 : 0;
-            int caret = TransmitBox.CaretIndex;
-            TransmitBox.Text = pending;
-            TransmitBox.CaretIndex = Math.Clamp(caret - taken, 0, pending.Length);
+            int caret = TransmitBox.CaretIndex - dropped;
+            TransmitBox.Text = now;
+            TransmitBox.CaretIndex = Math.Clamp(caret, locked, now.Length);
+            pane = now;
         }
         finally
         {
@@ -211,7 +253,8 @@ public sealed partial class DigitalWindow
 
     /// N1MM's keys for the digital window: Alt+T turns the transmitter on and
     /// puts the cursor where what is typed goes out, Ctrl+K does the same, and
-    /// Alt+G takes the next call off the grab list.
+    /// Alt+G takes the next call off the grab list. Escape stops now, wherever
+    /// the focus is.
     protected override void OnKeyDown(KeyEventArgs e)
     {
         if (e.KeyModifiers == KeyModifiers.Alt && e.Key == Key.T)
@@ -235,14 +278,15 @@ public sealed partial class DigitalWindow
         if (e.Key == Key.Escape)
         {
             e.Handled = true;
+            _ = session.DigitalKeyer?.AbortAsync();
             _ = Running()?.AbortAsync();
             return;
         }
         base.OnKeyDown(e);
     }
 
-    /// Alt+T: on to transmit with the cursor in the transmit pane, off back to
-    /// receive.
+    /// Alt+T: on to transmit with the cursor in the transmit pane, off to drop
+    /// the transmitter at the end of what is waiting.
     private void ToggleTransmit()
     {
         if (Running() is not { } running)
@@ -251,31 +295,56 @@ public sealed partial class DigitalWindow
         }
         if (running.IsTransmitting)
         {
-            _ = session.DigitalKeyer?.AbortAsync();
-            _ = running.ReturnToReceiveAsync();
+            ReturnToReceive();
             return;
         }
+        StartTransmit();
+    }
+
+    private void OnTransmit(object? sender, RoutedEventArgs e) => StartTransmit();
+
+    /// Keys the transmitter and opens the gate, so what is in the pane goes out
+    /// and so does whatever is typed into it after this. The keyer does the
+    /// keying, so the engine is never fed before it is keyed.
+    private void StartTransmit()
+    {
+        if (Running() is not { } running)
+        {
+            return;
+        }
+        if (session.DigitalKeyer is { } keyer)
+        {
+            keyer.Transmit();
+        }
+        else
+        {
             _ = running.SetPttAsync(true);
+        }
         TransmitBox.Focus();
     }
 
-    private void OnTransmit(object? sender, RoutedEventArgs e)
-    {
-        _ = Running()?.SetPttAsync(true);
-        TransmitBox.Focus();
-    }
+    /// The RX button is the `{RX}` macro by hand: the transmitter drops at the
+    /// end of what is waiting rather than in the middle of it. Escape is what
+    /// stops now.
+    private void OnReceive(object? sender, RoutedEventArgs e) => ReturnToReceive();
 
-    /// The RX button stops where it is: what has not gone to the engine is
-    /// dropped, and the engine drops what it holds.
-    private void OnReceive(object? sender, RoutedEventArgs e)
+    private void ReturnToReceive()
     {
-        _ = session.DigitalKeyer?.AbortAsync();
-        _ = Running()?.AbortAsync();
+        if (Running() is not { } running)
+        {
+            return;
+        }
+        if (session.DigitalKeyer is { } keyer)
+        {
+            keyer.ReturnToReceiveWhenSent();
+            return;
+        }
+        _ = running.ReturnToReceiveAsync();
     }
 
     private void OnClearTransmit(object? sender, RoutedEventArgs e)
     {
-        session.DigitalKeyer?.TypeAhead.Clear();
+        session.DigitalKeyer?.Buffer.Clear();
         ShowBuffer();
     }
 

src/Nonemm.App/Windows/DigitalWindow.axaml

@@ -1,6 +1,7 @@
 <local:RefreshableWindow xmlns="https://github.com/avaloniaui"
         xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
         xmlns:local="using:Nonemm.App.Windows"
+        xmlns:controls="using:Nonemm.App.Controls"
         x:Class="Nonemm.App.Windows.DigitalWindow"
         Title="Digital Interface" Width="900" Height="640">
   <Window.Styles>
@@ -33,6 +34,7 @@
       <Setter Property="VerticalAlignment" Value="Center" />
       <Setter Property="Margin" Value="4,2" />
     </Style>
+    <StyleInclude Source="avares://Nonemm.App/Controls/TransmitPane.axaml" />
   </Window.Styles>
 
   <DockPanel>
@@ -166,17 +168,14 @@
 
       <Grid Grid.Row="2" ColumnDefinitions="*,Auto,150">
         <Border BorderThickness="1" BorderBrush="#40808080">
-          <!-- what has gone out, then what is still to go: the first is a
-               label so it cannot be edited, the second the box the operator
-               types in -->
-          <Grid ColumnDefinitions="Auto,*">
-            <TextBlock Name="SentText" FontFamily="monospace" Margin="3,3,0,0"
-                       VerticalAlignment="Top" />
-            <TextBox Grid.Column="1" Name="TransmitBox" AcceptsReturn="True" TextWrapping="Wrap"
-                     FontFamily="monospace" BorderThickness="0"
-                     KeyDown="OnTransmitKeyDown" TextChanged="OnTransmitTextChanged"
-                     GotFocus="OnTransmitFocus" LostFocus="OnTransmitFocus" />
-          </Grid>
+          <!-- one box for the whole message: what has gone out is coloured and
+               cannot be edited, what is still to go is typed into the same
+               text, so the two wrap together and neither takes width from the
+               other -->
+          <controls:TransmitPane Name="TransmitBox" AcceptsReturn="True" TextWrapping="Wrap"
+                                 FontFamily="monospace"
+                                 KeyDown="OnTransmitKeyDown"
+                                 TextChanged="OnTransmitTextChanged" />
         </Border>
         <Grid Grid.Column="2" RowDefinitions="*,Auto">
           <Border BorderThickness="1" BorderBrush="#40808080">

src/Nonemm.App/Windows/DigitalWindow.axaml.cs

@@ -59,6 +59,15 @@ public sealed partial class DigitalWindow : RefreshableWindow
     /// redraw is not read back as an edit by the operator.
     private bool showingBuffer;
 
+    /// The transmit pane as it was last drawn, so an edit can be told from a
+    /// redraw and the changed character found.
+    private string pane = "";
+
+    /// How much of the pane is in the engine's hands and cannot be edited. It
+    /// is more than what is coloured, which is only what has gone out over the
+    /// air.
+    private int locked;
+
     /// The buffer this window is drawing, or null while no engine is running.
     private TypeAhead? buffer;
 
@@ -82,9 +91,6 @@ public sealed partial class DigitalWindow : RefreshableWindow
         entry.Activated += WhenEntryActivated;
         BuildMacros();
         ApplySettings();
-        // the cursor holds the pump back, so it has to follow the caret as it
-        // moves, not only as the text changes
-        TransmitBox.GetObservable(TextBox.CaretIndexProperty).Subscribe(new Watcher(ShowCursor));
         Attach(session.Digital);
         Refresh();
         Closed += (_, _) =>
@@ -104,8 +110,8 @@ public sealed partial class DigitalWindow : RefreshableWindow
     {
         ReceiveScroller.Background = Themes.Brush(Themes.Current.FieldBackground);
         GrabScroller.Background = Themes.Brush(Themes.Current.FieldBackground);
-        SentText.Foreground = Transmitted;
-        SentText.FontSize = Settings.DigitalFontSize;
+        TransmitBox.Background = Themes.Brush(Themes.Current.FieldBackground);
+        TransmitBox.SentBrush = Transmitted;
         TransmitBox.FontSize = Settings.DigitalFontSize;
         if (buffer is not null)
         {
@@ -147,7 +153,7 @@ public sealed partial class DigitalWindow : RefreshableWindow
         }
         Detach();
         engine = started;
-        buffer = session.DigitalKeyer?.TypeAhead;
+        buffer = session.DigitalKeyer?.Buffer;
         if (buffer is not null)
         {
             buffer.Baud = Settings.DigitalBaud;
@@ -188,21 +194,16 @@ public sealed partial class DigitalWindow : RefreshableWindow
     private void WhenBufferChanged(object? sender, EventArgs e) =>
         Dispatcher.UIThread.Post(ShowBuffer);
 
-    /// The pane starts empty for the next message: the transmitter dropping
-    /// with nothing left to send is the end of this one. An engine that keys
-    /// itself drops between two characters as well, and that text is left
-    /// where it is.
+    /// The transmitter dropping ends the message. The buffer clears what has
+    /// gone out and keeps what the operator typed ahead, so the pane is left
+    /// with the next message in it rather than empty.
     private void WhenTransmitChanged(object? sender, bool transmitting) =>
         Dispatcher.UIThread.Post(() =>
         {
             TransmitDot.Background = transmitting
                 ? Themes.Brush(Themes.Current.TransmitLight)
                 : Brushes.Transparent;
-            if (!transmitting && buffer is { IsSending: false })
-            {
-                buffer.Clear();
             ShowBuffer();
-            }
         });
 
     private void WhenConnectionChanged(object? sender, bool connected) =>
@@ -568,18 +569,3 @@ public sealed partial class DigitalWindow : RefreshableWindow
             _ => StackOrder.Disabled,
         };
 }
-
-/// Watches one property. Avalonia hands out observables and this program has no
-/// other use for Rx, so a handler that takes no value is enough.
-internal sealed class Watcher(Action changed) : IObserver<int>
-{
-    public void OnCompleted()
-    {
-    }
-
-    public void OnError(Exception error)
-    {
-    }
-
-    public void OnNext(int value) => changed();
-}

src/Nonemm.App/Windows/EntryWindow.Macros.cs

@@ -35,14 +35,34 @@ public sealed partial class EntryWindow
 
     /// The twelve keys for this radio, which are a different twelve while
     /// running and while searching, as N1MM's file holds them.
+    ///
+    /// On a digital mode they come from the digital macros instead, which is
+    /// where N1MM reads them from as well: its entry window loads the RTTYBTN
+    /// set there rather than the CW file. Without this the keys held CW text
+    /// with no `{TX}` in it, so pressing one fed the engine without keying the
+    /// transmitter and nothing went on the air.
     private IReadOnlyList<FunctionKey> Keys() =>
-        Messages
+        Logging?.Mode.Category == ModeCategory.Digital
+            ? DigitalKeys()
+            : Messages
                 .For(
                     Logging?.Mode.Category ?? Core.ModeCategory.Cw,
                     session.Settings.CwMessageFile,
                     session.Settings.PhoneMessageFile)
                 .Keys(Logging?.IsRunning ?? false);
 
+    /// The first ten digital macros, then Spot and Wipe. F11 and F12 are what
+    /// they are in every other mode, so the labels say what the buttons do;
+    /// the macros past the tenth are on the digital window's own buttons.
+    private IReadOnlyList<FunctionKey> DigitalKeys()
+    {
+        IReadOnlyList<FunctionKey> macros = Messages.Digital(session.Settings.DigitalMessageFile).Buttons;
+        List<FunctionKey> keys = [.. macros.Take(MessageFile.KeyCount - 2)];
+        keys.Add(new FunctionKey("Spot", ""));
+        keys.Add(new FunctionKey("Wipe", ""));
+        return keys;
+    }
+
     /// Right-clicking a function key button opens the messages for the mode the
     /// radio is in, which is where import and export live too.
     private Task EditMessages() => EditMessages(Logging?.Mode.Category ?? ModeCategory.Cw);
@@ -140,7 +160,12 @@ public sealed partial class EntryWindow
         }
         if (plan.Text.Length == 0)
         {
-            // a button that only acts: {WIPE}, {LOG}, {RUN}
+            // a button that only acts: {WIPE}, {LOG}, {RUN}, or an {RX} on its
+            // own, which has nothing to wait for
+            foreach (MessageAction action in plan.After)
+            {
+                Run(action);
+            }
             return true;
         }
         if ((through ?? Sender) is not { } keyer)
@@ -170,7 +195,22 @@ public sealed partial class EntryWindow
             Status(e.Message);
             return;
         }
-        _ = RunWhenSentAsync(keyer, after);
+        // `{RX}` goes to the keyer as soon as the message is in it, which is
+        // N1MM's ending: the text in one piece, the stop 400 ms behind it, and
+        // the engine unkeys itself at the last character. Waiting for the
+        // message to go out first leaves the engine empty, and a stop that
+        // arrives there does nothing. Everything else after `{END}` still
+        // waits.
+        foreach (MessageAction action in after)
+        {
+            if (action.Command == MessageCommand.ReturnToReceive)
+            {
+                Run(action);
+            }
+        }
+        _ = RunWhenSentAsync(
+            keyer,
+            [.. after.Where(action => action.Command != MessageCommand.ReturnToReceive)]);
     }
 
     /// Waits for the keyer, turns the transmit light off, and runs whatever
@@ -292,14 +332,30 @@ public sealed partial class EntryWindow
                 Logging.Stack.Clear();
                 ShowCallStack();
                 break;
-            // the digital engine keys itself while it has text to send, so
-            // {TX} and {RX} only matter when a macro wants the transmitter
-            // held open around what it sends
+            // {TX} keys the transmitter and {RX} drops it. {RX} waits until
+            // everything waiting has gone out, so it ends the transmission
+            // rather than cutting it off
             case MessageCommand.StartTransmit:
+                // through the keyer, which keys the engine before it feeds it
+                // the message
+                if (session.DigitalKeyer is { } keying)
+                {
+                    keying.Transmit();
+                }
+                else
+                {
                     _ = session.Digital?.SetPttAsync(true);
+                }
                 break;
             case MessageCommand.ReturnToReceive:
+                if (session.DigitalKeyer is { } digital)
+                {
+                    digital.ReturnToReceiveWhenSent();
+                }
+                else
+                {
                     _ = session.Digital?.ReturnToReceiveAsync();
+                }
                 break;
         }
     }

src/Nonemm.App/Windows/EntryWindow.Menu.cs

@@ -431,6 +431,9 @@ public sealed partial class EntryWindow
 
     private void OnShowScore(object? sender, RoutedEventArgs e) => Show(() => new ScoreWindow(session));
 
+    private void OnShowNetworkStatus(object? sender, RoutedEventArgs e) =>
+        Show(() => new NetworkStatusWindow(session));
+
     /// N1MM's grey line window: where the daylight is now, and where it will be.
     private void OnShowGrayline(object? sender, RoutedEventArgs e) =>
         Show(() => new GraylineWindow(session));
@@ -518,10 +521,21 @@ public sealed partial class EntryWindow
         {
             session.Save(updated);
             session.ApplyNetworkSettings();
-            Status(updated.NetworkEnabled ? "networked with the other stations" : "networking off");
+            session.ApplyStationLinkSettings();
+            Status(Networking(updated));
         }
     }
 
+    /// What the two networks are now doing, for the status line.
+    private static string Networking(Settings settings) =>
+        (settings.NetworkEnabled, settings.StationLinkEnabled) switch
+        {
+            (true, true) => "broadcasting contacts and linked to the other computers",
+            (true, false) => "broadcasting contacts to other programs",
+            (false, true) => "linked to the other logging computers",
+            _ => "networking off",
+        };
+
     private async void OnKeyerSettings(object? sender, RoutedEventArgs e)
     {
         KeyerDialog dialog = new(session.Settings);

src/Nonemm.App/Windows/EntryWindow.axaml

@@ -119,6 +119,7 @@
         <MenuItem Header="Call Stack" Click="OnShowCallStack" />
         <MenuItem Header="Check" Click="OnShowCheck" />
         <MenuItem Header="Log" Click="OnShowLog" InputGesture="Ctrl+L" />
+        <MenuItem Header="Network Status" Click="OnShowNetworkStatus" />
         <MenuItem Header="Grey Line" Click="OnShowGrayline" />
         <MenuItem Header="Score Summary" Click="OnShowScore" />
         <MenuItem Header="Telnet" Click="OnShowTelnet" />

src/Nonemm.App/Windows/NetworkStatusWindow.axaml

@@ -0,0 +1,23 @@
+<local:RefreshableWindow xmlns="https://github.com/avaloniaui"
+        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+        xmlns:local="using:Nonemm.App.Windows"
+        x:Class="Nonemm.App.Windows.NetworkStatusWindow"
+        Title="Network status" Width="820" Height="320">
+  <DockPanel Margin="8">
+    <StackPanel DockPanel.Dock="Bottom" Spacing="6" Margin="0,8,0,0">
+      <StackPanel Orientation="Horizontal" Spacing="6">
+        <TextBox Name="TalkBox" Width="420" PlaceholderText="a line to the other operators" />
+        <Button Content="Send" Click="OnTalk" IsDefault="True" />
+        <Button Content="Echo" Click="OnEcho" />
+        <Button Name="LinkButton" Content="Setup…" Click="OnSetup" />
+      </StackPanel>
+      <TextBlock Name="StateText" FontSize="11" Opacity="0.75" TextWrapping="Wrap" />
+    </StackPanel>
+    <ScrollViewer>
+      <StackPanel Spacing="6">
+        <Grid Name="Table" />
+        <TextBlock Name="TalkText" FontFamily="monospace" FontSize="11" TextWrapping="Wrap" />
+      </StackPanel>
+    </ScrollViewer>
+  </DockPanel>
+</local:RefreshableWindow>

src/Nonemm.App/Windows/NetworkStatusWindow.axaml.cs

@@ -0,0 +1,236 @@
+using Avalonia.Controls;
+using Avalonia.Interactivity;
+using Avalonia.Media;
+using Avalonia.Threading;
+using Nonemm.App.Dialogs;
+using Nonemm.App.Configuration;
+using Nonemm.App.Theming;
+using Nonemm.Network;
+
+namespace Nonemm.App.Windows;
+
+/// The other computers of this entry: who they are, where they are on the
+/// bands, and whether anything is arriving from them. N1MM's network status
+/// window, with its columns.
+///
+/// A row per station, this computer included, which is how N1MM shows it: the
+/// operator reads its own station number and the version it is claiming off the
+/// same window as everybody else's.
+///
+/// The window redraws on every message, which on a busy network is several a
+/// second. That is what a status window is for, and the table is a dozen rows.
+public sealed partial class NetworkStatusWindow : RefreshableWindow
+{
+    /// The columns, in N1MM's order as far as this program has the answers.
+    private static readonly string[] Columns =
+        ["Computer", "Nr", "Address", "Operator", "Band", "Mode", "Run", "TX", "Pass", "Last", "Heard", "Sent", "Read", "Echo"];
+
+    /// How much chat is kept on screen.
+    private const int TalkLines = 8;
+
+    private readonly AppSession session;
+    private readonly List<string> talk = [];
+    private readonly DispatcherTimer clock;
+    private StationLink? link;
+
+    public NetworkStatusWindow(AppSession session)
+    {
+        this.session = session;
+        InitializeComponent();
+        Attach();
+        // the Heard column is a countdown, so it has to redraw with nothing
+        // arriving
+        clock = new DispatcherTimer { Interval = TimeSpan.FromSeconds(1) };
+        clock.Tick += (_, _) => Refresh();
+        clock.Start();
+        Closed += (_, _) =>
+        {
+            clock.Stop();
+            Detach();
+        };
+        Refresh();
+    }
+
+    public override void Refresh()
+    {
+        Attach();
+        Table.Children.Clear();
+        Table.ColumnDefinitions.Clear();
+        Table.RowDefinitions.Clear();
+        if (link is null)
+        {
+            StateText.Text =
+                "the link to the other logging computers is off — Setup turns it on";
+            TalkText.Text = "";
+            return;
+        }
+        foreach (string _ in Columns)
+        {
+            Table.ColumnDefinitions.Add(new ColumnDefinition(GridLength.Auto));
+        }
+        AddRow(0, Columns, header: true);
+        int row = 1;
+        DateTime now = DateTime.UtcNow;
+        foreach (NetworkedStation station in link.Stations.OrderBy(s => s.StationNumber))
+        {
+            AddRow(row++, Cells(station, now), refused: station.Refused.Length > 0 && !station.IsMine);
+        }
+        StateText.Text = State();
+        TalkText.Text = string.Join("\n", talk);
+    }
+
+    private static string[] Cells(NetworkedStation station, DateTime now) =>
+    [
+        station.ComputerName + (station.IsMine ? " (this one)" : ""),
+        station.StationNumber > 0 ? station.StationNumber.ToString() : "",
+        station.Address,
+        station.Operator,
+        station.Band?.Name ?? "",
+        station.Mode?.Name ?? "",
+        station.IsRunning ? "run" : "",
+        station.IsTransmitting ? "TX" : "",
+        station.PassFrequency.Hertz > 0 ? $"{station.PassFrequency.Kilohertz:0.0} {station.PassCall}" : "",
+        station.LastMessage,
+        station.IsMine ? "" : Ago(now - station.LastHeardUtc),
+        station.Sent.ToString(),
+        station.Read.ToString(),
+        station.EchoTime is { } echo ? $"{echo.TotalMilliseconds:0} ms" : "",
+    ];
+
+    /// How long ago, in the shortest form that says it. A station heard from
+    /// less than a second ago reads as now rather than as 0 s.
+    private static string Ago(TimeSpan since) => since switch
+    {
+        { TotalSeconds: < 2 } => "now",
+        { TotalMinutes: < 1 } => $"{since.Seconds} s",
+        { TotalHours: < 1 } => $"{since.Minutes} min",
+        _ => "over an hour",
+    };
+
+    /// What the link is doing, under the table. A station that broadcast the
+    /// wrong version is named here: it is the one fault that leaves everything
+    /// looking connected and nothing arriving.
+    private string State()
+    {
+        if (link is null)
+        {
+            return "";
+        }
+        List<NetworkedStation> others = link.Stations.Where(s => !s.IsMine).ToList();
+        int connected = others.Count(s => s.IsConnected);
+        string what = $"station {link.StationNumber} as {link.ComputerName}, "
+            + $"claiming version {session.Settings.StationLinkVersion} — "
+            + $"{connected} of {others.Count} other stations connected";
+        List<string> refused = others
+            .Where(s => s.Refused.Length > 0)
+            .Select(s => $"{s.ComputerName}: {s.Refused}")
+            .ToList();
+        return refused.Count > 0 ? $"{what}\n{string.Join("\n", refused)}" : what;
+    }
+
+    private void Attach()
+    {
+        if (ReferenceEquals(link, session.Link))
+        {
+            return;
+        }
+        Detach();
+        link = session.Link;
+        if (link is null)
+        {
+            return;
+        }
+        link.StationsChanged += WhenStationsChanged;
+        link.TalkArrived += WhenTalkArrived;
+        link.Failed += WhenFailed;
+    }
+
+    private void Detach()
+    {
+        if (link is null)
+        {
+            return;
+        }
+        link.StationsChanged -= WhenStationsChanged;
+        link.TalkArrived -= WhenTalkArrived;
+        link.Failed -= WhenFailed;
+        link = null;
+    }
+
+    /// The link raises its events on a socket thread, so everything that draws
+    /// is posted.
+    private void WhenStationsChanged(object? sender, EventArgs e) =>
+        Dispatcher.UIThread.Post(Refresh);
+
+    private void WhenTalkArrived(object? sender, string text) =>
+        Dispatcher.UIThread.Post(() => Say(text));
+
+    private void WhenFailed(object? sender, string why) =>
+        Dispatcher.UIThread.Post(() => Say(why));
+
+    private void Say(string text)
+    {
+        talk.Add(text);
+        if (talk.Count > TalkLines)
+        {
+            talk.RemoveRange(0, talk.Count - TalkLines);
+        }
+        Refresh();
+    }
+
+    private async void OnTalk(object? sender, RoutedEventArgs e)
+    {
+        if (link is null || (TalkBox.Text ?? "").Trim() is not { Length: > 0 } text)
+        {
+            return;
+        }
+        TalkBox.Text = "";
+        Say($"[{link.ComputerName}] {text}");
+        if (await link.SendTalkAsync(text) == 0)
+        {
+            Say("nobody is connected");
+        }
+    }
+
+    private async void OnEcho(object? sender, RoutedEventArgs e)
+    {
+        if (link is not null)
+        {
+            await link.SendEchoRequestAsync();
+        }
+    }
+
+    private async void OnSetup(object? sender, RoutedEventArgs e)
+    {
+        NetworkDialog dialog = new(session.Settings);
+        if (await dialog.ShowDialog<Settings?>(this) is { } updated)
+        {
+            session.Save(updated);
+            session.ApplyNetworkSettings();
+            session.ApplyStationLinkSettings();
+            Refresh();
+        }
+    }
+
+    private void AddRow(int row, IReadOnlyList<string> cells, bool header = false, bool refused = false)
+    {
+        Table.RowDefinitions.Add(new RowDefinition(GridLength.Auto));
+        for (int column = 0; column < cells.Count; column++)
+        {
+            TextBlock text = new()
+            {
+                Text = cells[column],
+                FontWeight = header ? FontWeight.Bold : FontWeight.Normal,
+                Margin = new Avalonia.Thickness(4, 2, 8, 2),
+                FontSize = 12,
+            };
+            if (refused)
+            {
+                text.Foreground = Themes.Brush(Themes.Current.BadBackground);
+            }
+            Grid.SetRow(text, row);
+            Grid.SetColumn(text, column);
+            Table.Children.Add(text);
+        }
+    }
+}

src/Nonemm.App/Windows/QtcWindow.axaml

@@ -21,6 +21,7 @@
       <Button Name="ReadyButton" Content="RX Ready" Click="OnReady" />
       <Button Name="HeaderAgainButton" Content="Hdr Agn" Click="OnHeaderAgain" />
       <Button Name="HeaderCfmButton" Content="Cfm" Click="OnHeaderConfirm" />
+      <Button Name="SendAllButton" Content="Send All" Click="OnSendAll" />
     </StackPanel>
     <Grid Name="Lines" Margin="0,6,0,0" />
     <TextBlock Name="KeysText" FontSize="11" Opacity="0.7" Margin="0,6,0,0" />

src/Nonemm.App/Windows/QtcWindow.axaml.cs

@@ -19,8 +19,10 @@ namespace Nonemm.App.Windows;
 /// the log and cannot be edited — what is being reported is what was worked.
 ///
 /// On CW it puts the traffic on the air through the entry window's keyer, the
-/// way N1MM's QTC window sends through its own entry window. On SSB and RTTY
-/// nothing is sent: one needs a voice keyer and the other a digital window.
+/// way N1MM's QTC window sends through its own entry window. On RTTY it sends
+/// through the digital engine the same way, but a line at a time rather than a
+/// field at a time, and Send All reads the whole series out in one message. On
+/// SSB nothing is sent: that needs a voice keyer, and there is none here.
 public sealed partial class QtcWindow : Window
 {
     private static IBrush Saved => Themes.Brush(Themes.Current.GoodBackground);
@@ -83,7 +85,9 @@ public sealed partial class QtcWindow : Window
         if (!IsCw)
         {
             return isSending
-                ? $"{traffic.Remaining(station)} of the ten QTCs for {station.Text} are still free"
+                ? IsRtty
+                    ? "Send All reads the whole series out — Snd n sends one QTC again"
+                    : $"{traffic.Remaining(station)} of the ten QTCs for {station.Text} are still free"
                 : "Type the header, then a line per QTC: time, callsign, serial number.";
         }
         return isSending
@@ -91,10 +95,17 @@ public sealed partial class QtcWindow : Window
             : "Shift 1 = ask time  Shift 2 = ask call  Shift 3 = ask serial";
     }
 
-    /// Traffic goes out on CW alone. The contest says which mode it is, not
-    /// what the radio happens to be on, which is how N1MM decides too.
+    /// Which mode the traffic goes out on. The contest says, not what the radio
+    /// happens to be on, which is how N1MM decides too.
     private bool IsCw => position.Contest.Modes is [ModeCategory.Cw];
 
+    private bool IsRtty => position.Contest.Modes.Contains(ModeCategory.Digital);
+
+    /// The header of the series as it goes out in a message, which is N1MM's
+    /// `QTC 3/10`. The box shows the station callsign after it on RTTY, and
+    /// that is for the operator rather than for the air.
+    private string SeriesHeader => ready.Count > 0 ? $"QTC {ready[0].SeriesText}" : "";
+
     private void SetDirection(bool sending)
     {
         if (sending == isSending)
@@ -118,6 +129,9 @@ public sealed partial class QtcWindow : Window
         HeaderAgainButton.Content = isSending && IsCw ? "Snd Hdr" : "Hdr Agn";
         HeaderAgainButton.IsVisible = !isSending || IsCw;
         HeaderCfmButton.IsVisible = !isSending;
+        // N1MM's Send All, which reads a whole series out in one message. It
+        // needs the engine to hold the text, so it is RTTY only
+        SendAllButton.IsVisible = isSending && IsRtty;
         ClearButton.IsVisible = !isSending;
         CloseButton.Content = IsCw ? "Exit" : "Close";
         HeaderBox.IsReadOnly = isSending;
@@ -144,7 +158,7 @@ public sealed partial class QtcWindow : Window
             TextBox number = Box(at, 2);
             Button again = new()
             {
-                Content = isSending && IsCw ? $"Snd{at + 1}" : $"Agn{at + 1}",
+                Content = isSending && (IsCw || IsRtty) ? $"Snd{at + 1}" : $"Agn{at + 1}",
                 Margin = new Avalonia.Thickness(2, 1, 2, 1),
             };
             Button confirm = new() { Content = $"Cfm{at + 1}", Margin = new Avalonia.Thickness(2, 1, 2, 1) };
@@ -195,13 +209,11 @@ public sealed partial class QtcWindow : Window
             return;
         }
         ready.AddRange(traffic.ToSend(station, session.Settings.QtcLinesPerSeries));
-        // the contest is what makes it RTTY, not whatever the radio is on now
-        bool rtty = position.Contest.Modes.Contains(ModeCategory.Digital);
         HeaderBox.Text = ready.Count == 0
             ? ""
-            : rtty
-                ? $"QTC {ready[0].SeriesText} - {station.Text}"
-                : $"QTC {ready[0].SeriesText}";
+            : IsRtty
+                ? $"{SeriesHeader} - {station.Text}"
+                : SeriesHeader;
         for (int at = 0; at < rows.Count; at++)
         {
             bool has = at < ready.Count;
@@ -353,7 +365,7 @@ public sealed partial class QtcWindow : Window
     {
         if (isSending)
         {
-            if (IsCw)
+            if (IsCw || IsRtty)
             {
                 SendLine(at);
                 return;
@@ -375,7 +387,9 @@ public sealed partial class QtcWindow : Window
     private void SendLine(int at)
     {
         lastSent = at;
-        _ = send(QtcMessages.Line(
+        _ = send(IsRtty
+            ? QtcMessages.SendOne(session.Settings.QtcRttySpacing, RttyLine(at))
+            : QtcMessages.Line(
                 rows[at].Time.Text ?? "",
                 rows[at].Call.Text ?? "",
                 rows[at].Number.Text ?? "",
@@ -391,6 +405,43 @@ public sealed partial class QtcWindow : Window
         CloseButton.Focus();
     }
 
+    private string RttyLine(int at) => QtcMessages.RttyLine(
+        rows[at].Time.Text ?? "",
+        rows[at].Call.Text ?? "",
+        rows[at].Number.Text ?? "");
+
+    /// N1MM's Send All: the whole series in one message. RTTY runs at 45 baud,
+    /// so a series is the best part of a minute of transmission, and sending it
+    /// a line at a time means the operator presses a button between each one
+    /// while the transmitter is up. The heading, the spacing between the lines
+    /// and the ending are the operator's, and the message keys the transmitter
+    /// and drops it again itself.
+    private void OnSendAll(object? sender, RoutedEventArgs e)
+    {
+        List<string> lines = [];
+        for (int at = 0; at < rows.Count; at++)
+        {
+            if (!rows[at].IsEmpty)
+            {
+                lines.Add(RttyLine(at));
+            }
+        }
+        string message = QtcMessages.SendAll(
+            session.Settings.QtcRttySendAllHeading,
+            session.Settings.QtcRttySendAllEnding,
+            session.Settings.QtcRttySpacing,
+            SeriesHeader,
+            lines);
+        if (message.Length == 0)
+        {
+            StatusText.Text = "nothing to send";
+            return;
+        }
+        lastSent = lines.Count - 1;
+        _ = send(message);
+        CloseButton.Focus();
+    }
+
     /// 1, 2, 3 and 4 as N1MM numbers them: the time, the call, the serial
     /// number and the header.
     private static int FieldOf(Key key) =>

src/Nonemm.Contests/BandChangeRules.cs

@@ -22,6 +22,48 @@ public sealed record BandChangeRules(
     /// A contest that does not limit band changes at all.
     public static readonly BandChangeRules None = new(0);
 
+    /// How long a multi-operator entry with one transmitter has to stay on a
+    /// band. It is the ten-minute rule, and it is N1MM's fallback for every
+    /// contest that does not name a number of its own.
+    public static readonly TimeSpan MultiOneStay = TimeSpan.FromMinutes(10);
+
+    /// What a contest allows an entry in this category, before the contest has
+    /// its say. These are N1MM's defaults, from
+    /// `ContestInstance.BandChangeTimerDuration` and `BandChangeCountMax`:
+    ///
+    /// | Category | Changes counted | Stay |
+    /// |---|---|---|
+    /// | single operator | no | none |
+    /// | multi-operator, one transmitter | no | ten minutes |
+    /// | multi-operator, two transmitters | no | ten minutes |
+    /// | multi-operator, more | no | none |
+    ///
+    /// The count is off by default because N1MM's own fallback for it is zero:
+    /// the contests that cap band changes per hour name the cap themselves, and
+    /// a contest class that does so overrides `Contest.BandChangesFor`. The
+    /// stay is the other way round — ten minutes is the fallback, and a contest
+    /// with no rule of its own gets it.
+    ///
+    /// A station with a transmitter per band changes band by moving to another
+    /// radio, so nothing is counted for it.
+    public static BandChangeRules ForCategory(ContestEntry entry)
+    {
+        if (!entry.OperatorCategory.StartsWith("MULTI", StringComparison.OrdinalIgnoreCase))
+        {
+            return None;
+        }
+        // N1MM reads the number of transmitters off the operator category on
+        // Cabrillo 2.0 and off the transmitter category after it. Both are
+        // checked, so an entry written either way is read
+        bool one = Is(entry, "MULTI-ONE", "ONE");
+        bool two = Is(entry, "MULTI-TWO", "TWO");
+        return one || two ? None with { MinimumStay = MultiOneStay } : None;
+    }
+
+    private static bool Is(ContestEntry entry, string operatorCategory, string transmitterCategory) =>
+        entry.OperatorCategory.Equals(operatorCategory, StringComparison.OrdinalIgnoreCase)
+        || entry.TransmitterCategory.Equals(transmitterCategory, StringComparison.OrdinalIgnoreCase);
+
     public bool IsCounted => Max > 0;
 
     /// True for a contest that says how long a station has to stay on a band.

src/Nonemm.Contests/Contest.cs

@@ -77,7 +77,12 @@ public interface Contest
 
     /// How many band changes the entry may make, and over what stretch of time
     /// they are counted. Most contests do not limit them.
-    BandChangeRules BandChangesFor(ContestEntry entry) => BandChangeRules.None;
+    /// What this contest allows in the way of band changes. The default is
+    /// what the entry's category alone says, which is the ten-minute rule for a
+    /// multi-operator entry with one or two transmitters and nothing for a
+    /// single operator. A contest that caps the changes per hour, or asks for a
+    /// different stay, says so here.
+    BandChangeRules BandChangesFor(ContestEntry entry) => BandChangeRules.ForCategory(entry);
 
     /// How long a gap between contacts has to be before it counts as time off.
     /// N1MM asks each contest and takes 30 minutes when it says nothing.

src/Nonemm.Digital/DigitalEngine.cs

@@ -28,4 +28,18 @@ public interface DigitalEngine : IDisposable
 
     /// Drops whatever has not gone out yet, which is what Escape does.
     Task AbortAsync(CancellationToken cancellation = default);
+
+    /// N1MM's `{TX}`: the transmitter comes up now, before there is anything
+    /// to send.
+    Task KeyAsync(CancellationToken cancellation = default);
+
+    /// N1MM's `{RX}`: the transmitter drops once the modem has transmitted what
+    /// it still holds.
+    Task ReturnToReceiveAsync(CancellationToken cancellation = default);
+
+    /// The key back down, without waiting for anything. MMTTY needs this: its
+    /// `SetMmttyPTT(1)` leaves the transmitter up whatever N1MM's source says,
+    /// so `{RX}` waits for the modem to say its buffer is empty and then puts
+    /// the key down here.
+    Task ReleaseKeyAsync(CancellationToken cancellation = default);
 }

src/Nonemm.Digital/DigitalEngineSender.cs

@@ -6,26 +6,87 @@ namespace Nonemm.Digital;
 /// through the same expander, the same `{END}` handling and the same ESM as a
 /// CW message does.
 ///
-/// A message is not handed to the engine whole. It goes into the type-ahead
-/// buffer, which keeps the engine a couple of characters ahead of the operator,
-/// so the rest can still be rewritten. `Finished` is raised when the buffer has
-/// run dry and the engine has transmitted what it was given.
+/// A message goes into the type-ahead buffer rather than to the engine whole,
+/// so the part that has not gone out can still be rewritten. `Finished` is
+/// raised when the buffer has run dry, with the last characters of the message
+/// in the engine and about a third of a second of it still to go out.
+///
+/// The buffer is paced by the clock at `baud`, and the engine's own count of
+/// what it has left is a brake on it.
+///
+/// Three states, which is what the transmitter can be doing:
+///
+/// | State | Key | Gate | Engine |
+/// |---|---|---|---|
+/// | `Down` | down | shut | holds nothing |
+/// | `Keyed` | up | open, the feeder paces text over | holds `Ahead` characters |
+/// | `Ending` | up | shut, everything left was flushed in one piece | holds the rest of the message |
+///
+/// | From | Event | To |
+/// |---|---|---|
+/// | `Down` | `Transmit`, or a message to send | `Keyed` |
+/// | `Keyed` | more text | `Keyed` |
+/// | `Keyed` | `{RX}` | `Ending` |
+/// | `Keyed` | the engine drops with the message unfinished | `Keyed`, keyed again |
+/// | `Ending` | the engine drops | `Down` |
+/// | `Ending` | the engine never drops | `Down`, the key put down here |
+/// | `Ending` | `Transmit`, or a message to send | `Keyed`, once the engine is empty and its stop is cleared |
+/// | any | `AbortAsync` | `Down` |
+///
+/// The last two rows are why there is a state at all rather than a flag.
+/// Ending a message takes as long as the engine takes to transmit what it
+/// holds, and the operator can key again inside that time. An ending that went
+/// on running put the key down in the middle of the message after it.
+///
+/// Coming back out of `Ending` is not free either. The stop is inside the
+/// engine by then, waiting for its buffer to empty, and MMTTY fed while that
+/// stands stayed keyed and transmitted nothing: the characters went in and
+/// never came out. So a new message waits for what the engine holds to go out,
+/// clears the stop with an abort, and keys again.
 public sealed class DigitalEngineSender : MessageSender
 {
+    /// How long the engine may make no progress at all, after it has been told
+    /// to stop, before the key goes down anyway. It is not a limit on the whole
+    /// wait: a flushed message is seconds of transmission and the engine is
+    /// entitled to all of it. Measured against the whole wait instead, it cut
+    /// a CQ off with 21 symbols still in the engine.
+    public static readonly TimeSpan StopPatience = TimeSpan.FromSeconds(1.5);
+
+    /// The longest the stop waits for the engine to say it has the message.
+    /// The stop does nothing at all if it arrives at an engine with an empty
+    /// buffer, so it goes out as soon as the count says there is something to
+    /// stop, and after this long whether the count says so or not. It is
+    /// N1MM's number, which N1MM sleeps outright.
+    public static readonly TimeSpan StopDelay = TimeSpan.FromMilliseconds(400);
+
     private readonly DigitalEngine engine;
 
+    private readonly Lock gate = new();
+
+    /// Where the transmitter is, as far as this program knows.
+    private Keying state = Keying.Down;
+
+    /// Which transmission this is. It goes up whenever one starts or is
+    /// abandoned, and the ending of a message checks it after every step: an
+    /// ending belongs to one transmission and must not act on the next one.
+    /// Without it the ending of one message put the key down in the middle of
+    /// the message after it.
+    private int transmission;
+
     public DigitalEngineSender(DigitalEngine engine, double baud = TypeAhead.DefaultBaud)
     {
         this.engine = engine;
-        TypeAhead = new TypeAhead(
+        Buffer = engine is EngineBuffer counter
+            ? new TypeAhead(counter, baud)
+            : new TypeAhead(
                 (character, cancellation) => engine.SendAsync(character.ToString(), cancellation),
                 baud);
-        TypeAhead.Drained += WhenDrained;
+        Buffer.Given += WhenGiven;
         engine.TransmitChanged += WhenTransmitChanged;
     }
 
     /// What is waiting to go out, which the digital window shows and edits.
-    public TypeAhead TypeAhead { get; }
+    public TypeAhead Buffer { get; }
 
     public bool IsReady => engine.IsConnected;
 
@@ -35,17 +96,158 @@ public sealed class DigitalEngineSender : MessageSender
 
     public event EventHandler? Finished;
 
-    public Task SendAsync(string text, CancellationToken cancellation = default)
+    /// A message to send. Text is the operator asking for the transmitter, so
+    /// it keys as well: text arriving while the last message was ending
+    /// abandons that ending, and text arriving with the transmitter down brings
+    /// it up rather than letting MMTTY key itself off the first character.
+    public async Task SendAsync(string text, CancellationToken cancellation = default)
     {
-        TypeAhead.Append(text);
-        return Task.CompletedTask;
+        await StartAsync(cancellation).ConfigureAwait(false);
+        Buffer.Append(text);
     }
 
-    /// Escape and the RX button: what has not gone to the engine is dropped,
-    /// and the engine drops what it still holds.
+    private async Task TransmitAsync()
+    {
+        await StartAsync().ConfigureAwait(false);
+        Buffer.Transmit();
+    }
+
+    /// The TX button, Ctrl+Enter and `{TX}`: the transmitter comes up and what
+    /// is in the pane goes out, and so does whatever is typed into it from now
+    /// on.
+    ///
+    /// Keying belongs here rather than in the window so that it is always the
+    /// first thing the engine is told. MMTTY keys itself off a character it is
+    /// given while the transmitter is down, sends it and drops again, so a key
+    /// that arrives behind the text puts a keyed-up gap in the middle of a
+    /// message.
+    public void Transmit() => _ = TransmitAsync();
+
+    /// N1MM's `{RX}`: everything still waiting goes to the engine in one piece
+    /// and the engine is then asked to stop. A macro that ends with it goes out
+    /// in full, and so does anything the operator has typed ahead of it.
+    ///
+    /// It does not wait for the feeder to hand the message over first. Waiting
+    /// is what made the stop useless: by the time the last character had gone
+    /// over, the engine was empty again, and MMTTY ignores a stop that reaches
+    /// it empty.
+    public void ReturnToReceiveWhenSent() => _ = StopAsync();
+
+    /// A transmission begins, or the one that was ending goes on. True when the
+    /// engine has to be keyed, which is every time but one already keyed and
+    /// still going.
+    private async Task StartAsync(CancellationToken cancellation = default)
+    {
+        bool ending;
+        lock (gate)
+        {
+            ending = state == Keying.Ending;
+        }
+        if (ending)
+        {
+            // the last message is ending and its stop is inside the engine,
+            // waiting for the buffer to empty. Feeding an engine with that
+            // standing left MMTTY keyed with nothing going out and the
+            // characters swallowed, so what it still holds is let out and the
+            // stop is cleared with an abort before it is keyed again
+            await WaitUntilAiredAsync(StopPatience).ConfigureAwait(false);
+            await engine.AbortAsync(cancellation).ConfigureAwait(false);
+        }
+        bool key;
+        lock (gate)
+        {
+            key = state != Keying.Keyed;
+            if (key)
+            {
+                transmission++;
+            }
+            state = Keying.Keyed;
+        }
+        if (key)
+        {
+            // the pane starts again on the message that has gone out. The
+            // engine reporting the drop does this too, but it never reports one
+            // when a message follows the last close enough to keep the
+            // transmitter up, and the pane then held the whole run with none of
+            // it editable
+            Buffer.Started();
+            await engine.KeyAsync(cancellation).ConfigureAwait(false);
+        }
+    }
+
+    /// Waits until the engine says it holds something, or `patience` runs out.
+    /// The stop does nothing at an engine with an empty buffer, so it goes out
+    /// as soon as there is something to stop. N1MM sleeps 400 ms here instead,
+    /// which is the same wait without the question: it never reads the count.
+    /// A sleep is also wrong on a short message — `TU` is 330 ms of air at
+    /// 45.45 baud, so 400 ms of it puts the stop back where it does nothing.
+    private async Task WaitUntilHoldingAsync(TimeSpan patience)
+    {
+        DateTime giveUp = DateTime.UtcNow + patience;
+        while (Buffer.EngineHolds <= 0 && DateTime.UtcNow < giveUp)
+        {
+            await Task.Delay(TypeAhead.PollInterval).ConfigureAwait(false);
+        }
+    }
+
+    /// Waits until the engine has transmitted everything it was given, or until
+    /// it has made no progress for `patience`.
+    ///
+    /// `patience` is not a limit on the whole wait: a flushed message is
+    /// seconds of transmission and the engine is entitled to all of it. What it
+    /// catches is an engine that has stopped moving. Measured against the whole
+    /// wait, it cut a CQ off with 21 symbols still in the engine.
+    private async Task WaitUntilAiredAsync(TimeSpan patience)
+    {
+        if (Buffer.Outstanding <= 0 && Buffer.EngineHolds <= 0 && !Buffer.IsSending)
+        {
+            return;
+        }
+        TaskCompletionSource aired = new(TaskCreationOptions.RunContinuationsAsynchronously);
+        void WhenAired(object? sender, EventArgs e) => aired.TrySetResult();
+        Buffer.Aired += WhenAired;
+        try
+        {
+            int holds = Buffer.EngineHolds;
+            int outstanding = Buffer.Outstanding;
+            DateTime giveUp = DateTime.UtcNow + patience;
+            while (!aired.Task.IsCompleted && DateTime.UtcNow < giveUp)
+            {
+                await Task.Delay(TypeAhead.PollInterval).ConfigureAwait(false);
+                if (Buffer.EngineHolds < holds || Buffer.Outstanding < outstanding)
+                {
+                    holds = Buffer.EngineHolds;
+                    outstanding = Buffer.Outstanding;
+                    giveUp = DateTime.UtcNow + patience;
+                }
+            }
+        }
+        finally
+        {
+            Buffer.Aired -= WhenAired;
+        }
+    }
+
+    /// True while `mine` is still the transmission being ended. False once
+    /// something has started another one, ended this one, or aborted.
+    private bool Ending(int mine)
+    {
+        lock (gate)
+        {
+            return state == Keying.Ending && transmission == mine;
+        }
+    }
+
+    /// Escape: what has not gone to the engine is dropped, and the engine drops
+    /// what it still holds.
     public Task AbortAsync(CancellationToken cancellation = default)
     {
-        TypeAhead.Drop();
+        lock (gate)
+        {
+            state = Keying.Down;
+            transmission++;
+        }
+        Buffer.Drop();
         return engine.AbortAsync(cancellation);
     }
 
@@ -56,29 +258,134 @@ public sealed class DigitalEngineSender : MessageSender
 
     public void Dispose()
     {
-        TypeAhead.Drained -= WhenDrained;
-        TypeAhead.Dispose();
+        Buffer.Given -= WhenGiven;
+        Buffer.Dispose();
         engine.TransmitChanged -= WhenTransmitChanged;
     }
 
-    private void WhenDrained(object? sender, EventArgs e) => Finished?.Invoke(this, EventArgs.Empty);
+    /// The message is finished when the last character has been handed to the
+    /// engine, not when the engine has transmitted it. What stands after
+    /// `{END}` runs there, `{RX}` among it, and `{RX}` has to reach the engine
+    /// while the engine still holds something to send.
+    private void WhenGiven(object? sender, EventArgs e) => Finished?.Invoke(this, EventArgs.Empty);
 
-    /// The engine dropping the transmitter empties its buffer, which the
-    /// type-ahead buffer is told so it stops waiting for characters that have
-    /// already gone. It ends the message only when there is nothing left to
-    /// send: an engine that keys itself off what it is given drops between two
-    /// characters of a message the pump is still feeding, and that is not the
-    /// end of anything.
+    /// Tells the engine to stop, waits for it to transmit what it still holds,
+    /// and puts the key down.
+    ///
+    /// This is N1MM's ending, which is not the same as its keying. N1MM hands
+    /// MMTTY the whole message with `SendString` and calls `SetMmttyPTT(1)`
+    /// with the message still in the engine, and MMTTY ends the transmission
+    /// itself at the last character. So the feeding stops here: what is left
+    /// goes over in one piece and the engine is asked to stop on a full buffer.
+    /// Nothing after the flush can be rewritten, which is what `{RX}` means.
+    ///
+    /// What sits between the two is the count, not a sleep. Two of N1MM's three
+    /// MMTTY send paths call the stop straight after the text and the third
+    /// sleeps 400 ms first, which is a fixed wait for something this program
+    /// can ask about: the stop goes out as soon as the engine says it holds
+    /// something. A fixed sleep is also wrong on a short message — `TU` is
+    /// 330 ms of air at 45.45 baud, so 400 ms of sleeping puts the stop back
+    /// where it does nothing.
+    ///
+    /// The key going down is the fallback for an engine that ignores the stop,
+    /// which is what MMTTY did every time it was asked on an empty buffer. It
+    /// waits for the engine's own count to reach 0 and one symbol time on
+    /// top, which covers the count being read every `TypeAhead.PollInterval`
+    /// rather than the transmission. Every millisecond here is turnaround time
+    /// in a contest.
+    private async Task StopAsync()
+    {
+        int mine;
+        lock (gate)
+        {
+            if (state == Keying.Down)
+            {
+                return;
+            }
+            state = Keying.Ending;
+            mine = transmission;
+        }
+        string rest = await Buffer
+            .FlushAsync((text, cancellation) => engine.SendAsync(text, cancellation))
+            .ConfigureAwait(false);
+        if (rest.Length > 0 && Ending(mine))
+        {
+            await WaitUntilHoldingAsync(StopDelay).ConfigureAwait(false);
+        }
+        if (!Ending(mine))
+        {
+            return;
+        }
+        await engine.ReturnToReceiveAsync().ConfigureAwait(false);
+        await WaitUntilAiredAsync(StopPatience).ConfigureAwait(false);
+        if (!Ending(mine))
+        {
+            return;
+        }
+        await Task.Delay(Buffer.SymbolTime).ConfigureAwait(false);
+        if (Ending(mine))
+        {
+            await engine.ReleaseKeyAsync().ConfigureAwait(false);
+        }
+    }
+
+    /// The engine dropping the transmitter ends the message, but only after
+    /// `{RX}` or an abort asked it to stop. The pane then starts again: what
+    /// has gone out is cleared, what the operator typed ahead is kept, and
+    /// nothing more is fed until the transmitter is keyed again.
+    ///
+    /// A drop nobody asked for is the engine keying itself off what it is
+    /// given, which happens between the characters of a message that is still
+    /// going out, and is not the end of anything.
     private void WhenTransmitChanged(object? sender, bool transmitting)
     {
         if (transmitting)
         {
             return;
         }
-        TypeAhead.EngineIdle();
-        if (!TypeAhead.IsSending)
+        bool unfinished = Buffer.IsTransmitting && Buffer.IsSending;
+        bool ended;
+        lock (gate)
+        {
+            ended = state == Keying.Ending;
+            if (ended)
+            {
+                state = Keying.Down;
+                transmission++;
+                // inside the lock, so a message starting at this moment cannot
+                // have its own text cleared by the end of the one before it.
+                // The drop arrives from the engine on its own thread: a macro
+                // pressed on the last character of a message got as far as
+                // handing its text to the engine before this ran, and the pane
+                // was then wiped while the text went out
+                Buffer.Ended();
+            }
+        }
+        if (!ended)
+        {
+            // the engine dropped in the middle of a message. What is left of it
+            // would go out into a transmitter that is down, so it comes back
+            // up; the engine keying itself off the next character would leave
+            // that character half sent
+            if (unfinished)
+            {
+                _ = engine.KeyAsync();
+            }
+            return;
+        }
+        if (!Buffer.IsSending)
         {
             Finished?.Invoke(this, EventArgs.Empty);
         }
     }
+
+    /// Where the transmitter is. `Ending` is one message: everything left of it
+    /// has gone to the engine and nothing more is fed, and the engine is
+    /// transmitting what it holds.
+    private enum Keying
+    {
+        Down,
+        Keyed,
+        Ending,
+    }
 }

src/Nonemm.Digital/EngineBuffer.cs

@@ -0,0 +1,23 @@
+namespace Nonemm.Digital;
+
+/// An engine that holds a transmit buffer of its own, takes characters into it
+/// one at a time, and says how many it still has. MMTTY does: its type-ahead is
+/// what its own keyboard drives, and `XMMT.ocx` exposes both the keystroke and
+/// the count.
+///
+/// A backspace is a character like any other. MMTTY's help says it erases from
+/// the end of the buffer and works only until the letter has been transmitted,
+/// which is what makes `EngineTypeAhead` possible.
+public interface EngineBuffer
+{
+    /// The answer to `AskBufferedAsync`: how many characters are left to
+    /// transmit, or -1 when the engine would not say.
+    event EventHandler<int>? Buffered;
+
+    /// One character into the engine's buffer.
+    Task TypeAsync(char character, CancellationToken cancellation = default);
+
+    /// Asks how many characters are left. There is no event for it, so it is
+    /// polled.
+    Task AskBufferedAsync(string property = "", CancellationToken cancellation = default);
+}

src/Nonemm.Digital/MmttyEngine.cs

@@ -7,7 +7,11 @@ namespace Nonemm.Digital;
 /// N1MM's: the engine is given a title, the PTT port out of Mmtty.INI and a
 /// command line, and an engine that fails to initialise is started again up to
 /// ten times, which is what N1MM's retry does.
-public sealed class MmttyEngine : DigitalEngine
+///
+/// It holds a buffer of its own and says how much of it is left, so the
+/// type-ahead feeds it one character at a time the way N1MM does and paces on
+/// the count.
+public sealed class MmttyEngine : DigitalEngine, EngineBuffer
 {
     private const int StartAttempts = 10;
 
@@ -54,6 +58,10 @@ public sealed class MmttyEngine : DigitalEngine
     /// Anything the bridge says about itself, for the status line.
     public event EventHandler<string>? Reported;
 
+    /// The answer to `AskBufferedAsync`: how many characters the engine still
+    /// has to transmit.
+    public event EventHandler<int>? Buffered;
+
     public async Task StartAsync(CancellationToken cancellation = default)
     {
         settings = MmttySettings.Read(options.SettingsPath);
@@ -75,15 +83,34 @@ public sealed class MmttyEngine : DigitalEngine
     public Task TypeAsync(char character, CancellationToken cancellation = default) =>
         PostAsync(MmttyMessage.TypeCharacter, character, cancellation);
 
-    /// N1MM's abort, which is what its RX button and Escape do: drop PTT and
-    /// let the engine stop where it is.
-    public Task AbortAsync(CancellationToken cancellation = default) =>
-        SetPttAsync(false, cancellation);
+    /// Asks how many characters the engine still holds. The control has no
+    /// event for it, so it is polled; the answer arrives on `Buffered`. A
+    /// property name other than `TxBufLen` is only for finding out what the
+    /// control answers to.
+    public Task AskBufferedAsync(string property = "", CancellationToken cancellation = default) =>
+        bridge.SendAsync(BridgeLine.Write("buffer", property), cancellation);
 
-    /// N1MM's `{TX}` and `{RX}`. MMTTY sends what is in its buffer before it
-    /// drops PTT, so a macro that ends with `{RX}` still goes out in full.
+    /// Escape and the RX button: stop now and leave what has not gone out
+    /// unsent. N1MM's `AbortXmit`.
+    public Task AbortAsync(CancellationToken cancellation = default) =>
+        bridge.SendAsync(BridgeLine.Write("ptt", "0"), cancellation);
+
+    /// The control's `PTT` property back to false, which is the key the other
+    /// way rather than a stop. N1MM never does this: it keys with the property
+    /// and leaves `SetMmttyPTT(1)` to drop the transmitter.
+    public Task ReleaseKeyAsync(CancellationToken cancellation = default) =>
+        bridge.SendAsync(BridgeLine.Write("key", "0"), cancellation);
+
+    /// N1MM's `{TX}` and `{RX}`. Keying is the control's `PTT` property;
+    /// unkeying is `SetMmttyPTT(1)`, which waits for the buffer to empty first,
+    /// so a macro that ends with `{RX}` still goes out in full.
     public Task SetPttAsync(bool on, CancellationToken cancellation = default) =>
-        bridge.SendAsync(BridgeLine.Write("ptt", on ? "1" : "0"), cancellation);
+        bridge.SendAsync(
+            on ? BridgeLine.Write("key", "1") : BridgeLine.Write("ptt", "1"),
+            cancellation);
+
+    public Task KeyAsync(CancellationToken cancellation = default) =>
+        SetPttAsync(true, cancellation);
 
     public Task ReturnToReceiveAsync(CancellationToken cancellation = default) =>
         SetPttAsync(false, cancellation);
@@ -154,6 +181,9 @@ public sealed class MmttyEngine : DigitalEngine
             case "rx":
                 Received?.Invoke(this, ((char)Number(fields, 0)).ToString());
                 break;
+            case "buffer":
+                Buffered?.Invoke(this, Number(fields, 0));
+                break;
             case "tx":
                 IsTransmitting = Number(fields, 0) == 1;
                 TransmitChanged?.Invoke(this, IsTransmitting);

src/Nonemm.Digital/TypeAhead.cs

@@ -5,69 +5,154 @@ namespace Nonemm.Digital;
 /// The text waiting to go out, held here rather than handed to the engine in
 /// one piece.
 ///
-/// A digital engine takes a whole message and transmits it at the baud rate,
-/// which is slow: a callsign and a report take several seconds. Once the engine
-/// has the message nothing can be changed, so the operator who sees a wrong
-/// call go out has to stop the transmission and start again. This keeps the
-/// message here instead and feeds it to the engine a few characters at a time,
-/// so everything that has not gone out yet can still be rewritten, added to or
-/// deleted.
+/// A digital engine transmits at the baud rate, which is slow: a callsign and a
+/// report take several seconds. Once the engine has a character nothing can
+/// take it back — MMTTY treats a backspace as another character to transmit
+/// rather than as an edit, which the engine probe showed on the air. So the
+/// message is held here and the engine is kept just short of running dry, and
+/// everything behind that can still be rewritten, added to or deleted.
 ///
-/// `Sent` is what has gone to the engine and cannot be taken back. `Pending` is
-/// what is still to go. `Cursor` is how much of the pending text may go out,
-/// which is where the operator is typing: the pump stops when it reaches the
-/// cursor, because the operator has not finished the word yet.
+/// Two things happen to a character and they are not the same: it is **given**
+/// to the engine, and later it **goes out** on the air. Everything here is one
+/// or the other.
 ///
-/// The pump keeps `Lead` characters in the engine rather than one. An engine
-/// that runs out of characters partway through a message does not wait: it
-/// transmits idle until the next one arrives, so every gap the pump leaves is
-/// added to the time the message takes. Keeping a second character queued
-/// behind the one being transmitted means the engine never runs dry, and the
-/// cost is that the last `Lead` characters cannot be taken back rather than the
-/// last one.
+/// | Given to the engine | Gone out on the air |
+/// |---|---|
+/// | `Sent`, and it cannot be taken back | `OnAir`, how much of `Sent` has been transmitted |
+/// | `Outstanding`, given but not out yet | |
+/// | `Given` is raised when there is nothing left to give | `Aired` is raised when the engine has transmitted it too |
 ///
-/// `Lead` characters at the front are timed off the clock at the baud rate,
-/// which is an estimate of how far the engine has got. It is corrected by
-/// `EngineIdle`: an engine that has stopped transmitting has an empty buffer,
-/// whatever the estimate says.
+/// `Pending` is the rest: what has not been given to the engine and can still
+/// be rewritten. `EngineHolds` is the engine's own answer to the same question
+/// as `Outstanding`, in Baudot symbols rather than characters.
+///
+/// Nothing goes out until `Transmit`, which is the TX button, Ctrl+Enter or a
+/// function key. From then on everything in `Pending` goes out, and so does
+/// anything added to it: the operator can go on typing and can press a function
+/// key, and both follow what is already going. `Ended` shuts the gate again
+/// without dropping what is waiting, so text typed ahead survives the end of a
+/// message.
+///
+/// The pace is the clock at the baud rate: one character every
+/// `SymbolTime`, with the engine kept `Ahead` characters ahead so it never
+/// runs dry and transmits the idle tone in the middle of a word.
+/// RTTY runs at a fixed speed, so the clock is right, and the
+/// engine's own count of what it has left cannot replace it. The engine probe
+/// shows why: `TxBufLen` read 0 for the first 150 ms after twenty-one
+/// characters were pushed, and read 0 again for three seconds while the engine
+/// was still holding `ABCD` on Word out. The count is only to be believed while
+/// it is large and going down, and the feeder keeps the engine nearly empty,
+/// so a feeder that paced on the count fed on a reading of 0 and ran ahead of
+/// the air.
+///
+/// The count is a brake instead. `Slack` is how many Baudot symbols the engine
+/// may be behind — MMTTY answers in symbols, which is more than the characters
+/// it was given, because a digit costs a shift to figures and the letter after
+/// it a shift back. An exchange full of digits is slower than the clock thinks,
+/// and this is what stops the clock running away on it.
+///
+/// A count that is not going down means the engine is holding what it has:
+/// MMTTY on Word out keeps a word until the space after it arrives, and that
+/// space is the character the brake would hold back. So the brake lets go after
+/// `HoldingPatience`.
 public sealed class TypeAhead : IDisposable
 {
-    /// A RTTY character is a start bit, five data bits and a stop bit and a
-    /// half.
-    public const double BitsPerCharacter = 7.5;
+    /// A Baudot symbol is a start bit, five data bits and a stop bit and a
+    /// half. A character is one symbol, or two when it needs a shift first.
+    public const double BitsPerSymbol = 7.5;
 
     /// MMTTY's own default, and the speed nearly every RTTY contest runs at.
     public const double DefaultBaud = 45.45;
 
-    /// How many characters may sit in the engine. Two is the smallest number
-    /// that keeps the engine transmitting without a gap: one on the air and one
-    /// behind it. At 45.45 baud that puts the last third of a second of the
-    /// message beyond reach.
-    public const int DefaultLead = 2;
+    /// How many characters the engine is kept ahead by. Two is the smallest
+    /// number that keeps it transmitting without a gap: one on the air and one
+    /// in hand for when that one finishes. An engine left with an empty buffer
+    /// transmits the idle tone instead, which is audible between the characters
+    /// of a long word. At 45.45 baud this puts the last third of a second of
+    /// the message beyond reach.
+    public const int DefaultAhead = 2;
+
+    /// How many Baudot symbols the engine may be behind the clock before the
+    /// feeder waits for it. Six is about a second at 45.45 baud, and well above
+    /// what `Ahead` characters of ordinary text come to, so the brake only
+    /// bites on a run of digits, which costs more symbols than the clock
+    /// thinks.
+    public const int DefaultSlack = 6;
 
     /// How much of the text that has gone out is kept. It is there to be read
     /// back, not to be a log.
     public const int KeptSent = 2000;
 
-    /// `Cursor` set to this lets everything pending go out, which is where it
-    /// stands while the operator is not typing into the pane.
-    public const int NoCursor = int.MaxValue;
+    /// How far behind the engine's count runs. The probe pushed twenty-one
+    /// characters and read 0 for three answers, 100 to 150 ms, before the count
+    /// caught up with them. A 0 newer than this says nothing.
+    public static readonly TimeSpan CountLag = TimeSpan.FromMilliseconds(250);
 
-    /// The longest the pump sleeps between looks at the buffer. It is what
-    /// stands between the operator moving the cursor on and the next character
-    /// going out, so it is short against a character time.
-    private static readonly TimeSpan LongestTick = TimeSpan.FromMilliseconds(10);
+    /// How often the engine is asked how much it has left, and how closely the
+    /// feeder follows its own clock. It is well under a character time at any
+    /// speed RTTY is worked at.
+    public static readonly TimeSpan PollInterval = TimeSpan.FromMilliseconds(50);
 
     private readonly Func<char, CancellationToken, Task> send;
+    private readonly EngineBuffer? counter;
     private readonly Lock gate = new();
+
+    /// Held while a character or a flush is on its way to the engine, so the
+    /// two cannot cross. Without it a flush overtook the character the feeder
+    /// had already taken and was still sending, and that character went out
+    /// behind the rest of the message.
+    private readonly SemaphoreSlim handing = new(1, 1);
     private readonly StringBuilder pending = new();
     private readonly StringBuilder sent = new();
-
     private CancellationTokenSource? stopping;
-    private Task pump = Task.CompletedTask;
-    private int cursor = NoCursor;
-    private int inEngine;
-    private DateTime nextOut = DateTime.MinValue;
+
+    /// True once `Transmit` has been called and until the message ends: the
+    /// gate between what the operator has typed and the engine.
+    private bool open;
+
+    /// True while no feeder is running, so anything waiting for the end of the
+    /// message runs at once rather than waiting for a feeder that never starts.
+    private bool idle = true;
+
+    /// True once there has been nothing left to hand to the engine, so the end
+    /// of a message is announced once rather than at every poll after it.
+    private bool given = true;
+
+    /// The engine's last answer: how many symbols it still had to transmit, or
+    /// -1 before it has answered at all.
+    private int counted = -1;
+
+    /// How many characters at the front of `sent` the engine has transmitted.
+    private int aired;
+
+    /// The count the engine last answered, and how many symbols it has been
+    /// seen to drop that have not been charged to a character yet. Together
+    /// they are the air's own rate: a count that falls by four means four
+    /// symbols left the engine.
+    private int lastCount = -1;
+    private int budget;
+
+    /// When the engine was last given a character. Its count reads 0 for the
+    /// first 150 ms after a push, so a 0 within `CountLag` of one is the answer
+    /// not having caught up rather than an empty engine.
+    private DateTime gaveAt = DateTime.MinValue;
+
+    /// How many characters the engine holds that have not gone out yet, and
+    /// when the one it is transmitting now is finished. The engine transmits at
+    /// the baud rate, so what it holds goes out one character time apart. The
+    /// feeder keeps these, and a flush adds to them in one go.
+    ///
+    /// They outlive the feeder, which starts and stops with the text: a feeder
+    /// that started after a flush and reset them forgot a whole message the
+    /// engine was still holding, and the key went down in the middle of it.
+    private int outstanding;
+    private DateTime nextOut = DateTime.UtcNow;
+
+    /// Whether the engine is in the figures shift, on the air and at the front
+    /// of the feeder. The two run apart: the feeder is `Ahead` characters in
+    /// front of what is being transmitted.
+    private bool airShift;
+    private bool feedShift;
 
     public TypeAhead(Func<char, CancellationToken, Task> send, double baud = DefaultBaud)
     {
@@ -75,15 +160,36 @@ public sealed class TypeAhead : IDisposable
         Baud = baud;
     }
 
-    /// The speed the engine transmits at, which is what the estimate of its
-    /// buffer is paced by.
+    /// An engine that holds a buffer of its own and can say how much of it is
+    /// left, which is what paces the feeder.
+    public TypeAhead(EngineBuffer engine, double baud = DefaultBaud)
+        : this((character, cancellation) => engine.TypeAsync(character, cancellation), baud)
+    {
+        counter = engine;
+        engine.Buffered += WhenBuffered;
+    }
+
+    /// The speed the engine transmits at, which is what paces the feeder.
     public double Baud { get; set; }
 
-    /// How many characters may sit in the engine at once.
-    public int Lead { get; set; } = DefaultLead;
+    /// How many characters the engine is kept ahead by.
+    public int Ahead { get; set; } = DefaultAhead;
 
-    public TimeSpan CharacterTime =>
-        TimeSpan.FromSeconds(BitsPerCharacter / (Baud > 0 ? Baud : DefaultBaud));
+    /// How many symbols the engine may be behind before the feeder waits.
+    public int Slack { get; set; } = DefaultSlack;
+
+    /// True once the engine has said how much it holds.
+    public bool Counts { get; private set; }
+
+    /// How long one Baudot symbol takes on the air. A character takes one of
+    /// these, or two when the engine has to shift to figures or back first.
+    public TimeSpan SymbolTime =>
+        TimeSpan.FromSeconds(BitsPerSymbol / (Baud > 0 ? Baud : DefaultBaud));
+
+    /// How long the brake may hold before it lets go. Three character times is
+    /// longer than any gap between transmitted characters and short enough that
+    /// a word the engine is holding goes out at once.
+    public TimeSpan HoldingPatience => SymbolTime * 3;
 
     /// What has gone to the engine.
     public string Sent
@@ -97,6 +203,30 @@ public sealed class TypeAhead : IDisposable
         }
     }
 
+    /// How many characters of `Sent` have gone out over the air.
+    ///
+    /// The engine is kept `Ahead` characters ahead, so the last characters
+    /// handed to it are still in its buffer. They cannot be taken back, but
+    /// they have not been heard yet, so the pane draws them as text still to
+    /// go. Without this, a character typed after the engine has caught up went
+    /// red as it was typed: the feeder hands it over at once, and the buffer
+    /// had no other measure of the air.
+    ///
+    /// It is counted on the same clock as the feeder: the engine transmits at
+    /// the baud rate, so a character handed to an engine that is already
+    /// transmitting goes out one character time after the one before it, and a
+    /// character handed to an empty engine one character time from now.
+    public int OnAir
+    {
+        get
+        {
+            lock (gate)
+            {
+                return Math.Min(aired, sent.Length);
+            }
+        }
+    }
+
     /// What is still to go.
     public string Pending
     {
@@ -109,53 +239,50 @@ public sealed class TypeAhead : IDisposable
         }
     }
 
-    /// How many of the pending characters may go out. The window sets it to
-    /// where the operator's cursor is; `NoCursor` while nobody is typing.
-    public int Cursor
+    /// True while what is waiting is being fed to the engine.
+    public bool IsTransmitting
     {
         get
         {
             lock (gate)
             {
-                return cursor;
-            }
-        }
-        set
-        {
-            lock (gate)
-            {
-                cursor = Math.Max(0, value);
+                return open;
             }
         }
     }
 
-    /// True while there is text to send or the engine is estimated to be still
-    /// transmitting what it was given.
     public bool IsSending
     {
         get
         {
             lock (gate)
             {
-                Advance(DateTime.UtcNow);
-                return pending.Length > 0 || inEngine > 0;
+                return pending.Length > 0;
             }
         }
     }
 
     /// The text moved: a character went out, or a message was added. Raised on
-    /// the pump's thread, so a handler that touches the screen has to post.
+    /// the feeder's thread, so a handler that touches the screen has to post.
     public event EventHandler? Changed;
 
-    /// Everything that was waiting has gone out, and the engine is estimated to
-    /// have transmitted it. This is what tells the entry window that a message
-    /// is finished, so what stands after `{END}` runs and `{RX}` drops the
-    /// transmitter at the right moment.
-    public event EventHandler? Drained;
+    /// Everything that was waiting has been handed to the engine, which still
+    /// holds the last `Ahead` characters of it. This is what tells the entry
+    /// window that a message is finished, so what stands after `{END}` runs and
+    /// `{RX}` reaches the engine while it has something left to send.
+    public event EventHandler? Given;
+
+    /// Everything that was waiting has gone out and the engine has transmitted
+    /// it, which is `Ahead` characters later than `Given`.
+    public event EventHandler? Aired;
 
     /// A message to send. It goes on the end of what is already waiting, so two
     /// function keys pressed together send one after the other rather than one
-    /// over the other.
+    /// over the other, and a function key pressed while the operator is typing
+    /// follows what has been typed.
+    ///
+    /// A function key is the operator asking for the message, so it also opens
+    /// the gate: there is no second key to press.
     public void Append(string text)
     {
         if (text.Length == 0)
@@ -165,51 +292,167 @@ public sealed class TypeAhead : IDisposable
         lock (gate)
         {
             pending.Append(text);
-            if (cursor != NoCursor)
-            {
-                // text added behind the operator's cursor is still text to
-                // send, so the cursor moves out with it
-                cursor += text.Length;
-            }
+            open = true;
+            given = false;
         }
         Changed?.Invoke(this, EventArgs.Empty);
         Start();
     }
 
-    /// The operator rewrote what has not gone out yet.
-    public void Rewrite(string text, int wanted)
+    /// The operator rewrote the pane. `text` is the whole pane, what has gone
+    /// out and what is still to go, and `changedAt` is where in it the first
+    /// changed character is.
+    ///
+    /// The text that has gone out cannot be changed, so as much of `text` as
+    /// matches it is dropped and the rest becomes the pending text. Reading the
+    /// whole pane rather than the pending half is what makes this safe against
+    /// the feeder: a character the feeder took between the window reading the
+    /// pane and this call is still at the front of `text`, and is dropped with
+    /// the rest of what has gone out.
+    ///
+    /// An edit does not open or close the gate. Typing before the transmitter
+    /// is keyed stays off the air; typing while it is keyed goes out behind
+    /// what is already going.
+    public void Edit(string text)
     {
         lock (gate)
         {
+            int gone = Math.Min(sent.Length, text.Length);
             pending.Clear();
-            pending.Append(text);
-            cursor = Math.Clamp(wanted, 0, text.Length);
+            pending.Append(text, gone, text.Length - gone);
+            given = given && pending.Length == 0;
         }
         Start();
     }
 
-    /// The engine has stopped transmitting, so whatever it was given has gone
-    /// out. This corrects the estimate: an engine that is faster than the
-    /// estimate would otherwise be left waiting for a character it could have
-    /// had.
-    public void EngineIdle()
+    /// The TX button, Ctrl+Enter and Alt+T: what is in the pane goes out, and
+    /// so does whatever is added to it, until the transmitter drops.
+    public void Transmit()
     {
         lock (gate)
         {
-            inEngine = 0;
+            open = true;
+            given = given && pending.Length == 0;
+        }
+        Start();
+    }
+
+    /// The message is over: the transmitter has dropped, so the pane starts
+    /// again with what has gone out cleared and whatever was typed ahead kept.
+    public void Ended()
+    {
+        lock (gate)
+        {
+            open = false;
+            sent.Clear();
+            aired = 0;
+            outstanding = 0;
+        }
+        Changed?.Invoke(this, EventArgs.Empty);
+    }
+
+    /// A new transmission begins. What the last one sent is dropped from the
+    /// pane, and what the operator typed ahead is kept. The gate is left as it
+    /// is: the caller opens it with `Append` or `Transmit`.
+    ///
+    /// `Ended` does the same at the end of a message, but only when the engine
+    /// reports that the transmitter dropped. Two messages sent one after the
+    /// other keep the transmitter up, so that report never comes, and without
+    /// this the pane kept every message of the run and none of it could be
+    /// edited: text that has gone to the engine cannot be taken back.
+    public void Started()
+    {
+        bool had;
+        lock (gate)
+        {
+            had = sent.Length > 0;
+            sent.Clear();
+            aired = 0;
+        }
+        if (had)
+        {
+            Changed?.Invoke(this, EventArgs.Empty);
         }
     }
 
-    /// Drops what has not gone out. Escape and the RX button do this: what is
-    /// already in the engine cannot be stopped from here, and the engine's own
-    /// abort takes care of that.
+    /// The engine's own answer: how many symbols it still holds, or -1 before
+    /// it has answered at all. It is in symbols, which is more than the
+    /// characters it was given, because a digit costs a shift to figures and
+    /// the letter after it a shift back.
+    public int EngineHolds => Volatile.Read(ref counted);
+
+    /// Everything still waiting goes to the engine in one piece through `push`,
+    /// and nothing more can be rewritten. Returns what was sent.
+    ///
+    /// The feeder cannot be handing a character over at the same time: this
+    /// takes the same turn the feeder takes, so a character already on its way
+    /// arrives first and the rest follows it in order.
+    public async Task<string> FlushAsync(
+        Func<string, CancellationToken, Task> push,
+        CancellationToken cancellation = default)
+    {
+        await handing.WaitAsync(cancellation).ConfigureAwait(false);
+        try
+        {
+            string rest = TakePending();
+            if (rest.Length > 0)
+            {
+                await push(rest, cancellation).ConfigureAwait(false);
+            }
+            return rest;
+        }
+        finally
+        {
+            handing.Release();
+        }
+    }
+
+    /// Everything still waiting, taken in one piece and marked as gone to the
+    /// engine. The caller sends it, and nothing more can be rewritten.
+    ///
+    /// `{RX}` uses this. N1MM hands MMTTY the whole message with `SendString`
+    /// and asks it to stop 400 ms later, with the message still in the engine's
+    /// buffer, and MMTTY ends the transmission itself at exactly the right
+    /// moment. `SetMmttyPTT(1)` sent to an engine that has been fed one
+    /// character at a time, and is therefore nearly empty, does nothing at all.
+    private string TakePending()
+    {
+        string rest;
+        lock (gate)
+        {
+            rest = pending.ToString();
+            pending.Clear();
+            sent.Append(rest);
+            if (sent.Length > KeptSent)
+            {
+                int dropped = sent.Length - KeptSent;
+                sent.Remove(0, dropped);
+                aired = Math.Max(0, aired - dropped);
+            }
+            given = true;
+            // inside the same lock as the pending text, so the feeder cannot
+            // see an empty buffer and call the message over before the engine
+            // is counted as holding what it was just given
+            Gave(rest.Length, DateTime.UtcNow);
+        }
+        if (rest.Length > 0)
+        {
+            Changed?.Invoke(this, EventArgs.Empty);
+        }
+        return rest;
+    }
+
+    /// Drops what has not gone out. Escape does this: what is already in the
+    /// engine cannot be stopped from here, and the engine's own abort takes
+    /// care of that.
     public void Drop()
     {
         lock (gate)
         {
             pending.Clear();
-            inEngine = 0;
-            cursor = NoCursor;
+            open = false;
+            // the engine's own abort goes with this, so it holds nothing either
+            outstanding = 0;
         }
         Changed?.Invoke(this, EventArgs.Empty);
     }
@@ -222,111 +465,369 @@ public sealed class TypeAhead : IDisposable
         {
             pending.Clear();
             sent.Clear();
-            inEngine = 0;
-            cursor = NoCursor;
+            aired = 0;
+            open = false;
         }
         Changed?.Invoke(this, EventArgs.Empty);
     }
 
     public void Dispose()
     {
+        if (counter is not null)
+        {
+            counter.Buffered -= WhenBuffered;
+        }
         stopping?.Cancel();
         stopping?.Dispose();
         stopping = null;
+        handing.Dispose();
     }
 
     private void Start()
     {
         lock (gate)
         {
-            if (!pump.IsCompleted)
+            if (!idle)
             {
                 return;
             }
+            idle = false;
             stopping?.Dispose();
             stopping = new CancellationTokenSource();
-            pump = Task.Run(() => RunAsync(stopping.Token));
+            _ = Task.Run(() => RunAsync(stopping.Token));
         }
     }
 
-    /// Hands the engine a character whenever it has room for one, until there
-    /// is nothing left to send. A pump held at the cursor keeps running: the
-    /// operator is typing, and the next character is theirs to release.
+    /// Hands the engine a character every character time, keeping it `Ahead`
+    /// characters ahead of the air, unless its own count says it is more than
+    /// `Slack` symbols behind and still moving.
     private async Task RunAsync(CancellationToken cancellation)
     {
+        // when the next character is owed. It advances by exactly one character
+        // time per character handed over, never from the time the poll happened:
+        // a poll is up to `PollInterval` late, and starting the next character
+        // from there made every one late by a little and the engine run dry
+        DateTime due = DateTime.MinValue;
+        DateTime moved = DateTime.UtcNow;
+        int last = int.MaxValue;
         try
         {
             while (!cancellation.IsCancellationRequested)
             {
-                if (Take() is { } next)
+                if (counter is not null)
                 {
+                    await counter.AskBufferedAsync("", cancellation).ConfigureAwait(false);
+                }
+                DateTime now = DateTime.UtcNow;
+                int left = Volatile.Read(ref counted);
+                if (left < last)
+                {
+                    moved = now;
+                }
+                last = left;
+                bool wentOut = Counts ? WentOutByCount(left, now) : WentOutByClock(now);
+                if (wentOut)
+                {
+                    Changed?.Invoke(this, EventArgs.Empty);
+                }
+                bool behind = left > Slack && now - moved < HoldingPatience;
+                // the feeder owes at most `Ahead` characters at any moment,
+                // so a start, a brake letting go or an empty pane does not turn
+                // into a burst that puts the whole message beyond reach
+                DateTime earliest = now - (SymbolTime * (Ahead - 1));
+                if (due < earliest)
+                {
+                    due = earliest;
+                }
+                await handing.WaitAsync(cancellation).ConfigureAwait(false);
+                try
+                {
+                    while (!behind && now >= due && Take(now) is { } next)
+                    {
+                        due += SymbolTime * Symbols(next, ref feedShift);
                         await send(next, cancellation).ConfigureAwait(false);
                         Changed?.Invoke(this, EventArgs.Empty);
-                    continue;
                     }
-                if (!IsSending)
+                }
+                finally
+                {
+                    handing.Release();
+                }
+                if (NothingLeftToGive())
+                {
+                    Given?.Invoke(this, EventArgs.Empty);
+                }
+                // the engine is still holding what it has not transmitted, so
+                // the message is not over and the last characters of it have
+                // not been marked as gone out yet
+                if (left <= 0 && Outstanding == 0 && now >= due && AllAired())
                 {
-                    Drained?.Invoke(this, EventArgs.Empty);
                     return;
                 }
-                await Task.Delay(Tick, cancellation).ConfigureAwait(false);
+                await Task.Delay(PollInterval, cancellation).ConfigureAwait(false);
             }
         }
         catch (OperationCanceledException)
         {
         }
-    }
-
-    private TimeSpan Tick
-    {
-        get
-        {
-            TimeSpan quarter = CharacterTime / 4;
-            return quarter < LongestTick ? quarter : LongestTick;
-        }
-    }
-
-    /// The next character to send, or null when there is none to send now: the
-    /// engine is full, nothing is waiting, or what is waiting is behind the
-    /// cursor.
-    private char? Take()
+        finally
         {
             lock (gate)
             {
-            DateTime now = DateTime.UtcNow;
-            Advance(now);
-            if (pending.Length == 0 || cursor == 0 || inEngine >= Lead)
+                idle = true;
+            }
+        }
+    }
+
+    /// Everything waiting has gone out and the engine has transmitted it, or
+    /// the gate has closed on what is left. `Aired` says so. False when text
+    /// arrived while the feeder was deciding, which is what keeps a message
+    /// added at the last moment from being stranded.
+    private bool AllAired()
+    {
+        lock (gate)
+        {
+            if (open && pending.Length > 0)
+            {
+                return false;
+            }
+            idle = true;
+        }
+        Aired?.Invoke(this, EventArgs.Empty);
+        return true;
+    }
+
+    /// True the first time there is nothing left to hand to the engine, and
+    /// false again once more text arrives.
+    private bool NothingLeftToGive()
+    {
+        lock (gate)
+        {
+            if (given || (open && pending.Length > 0))
+            {
+                return false;
+            }
+            given = true;
+            return true;
+        }
+    }
+
+    /// One character the engine held has gone out, on the clock. False when it
+    /// is still transmitting it or holds nothing.
+    ///
+    /// The character that goes out is the first one that has not, which is
+    /// `sent[aired]`, and what it costs says when the one after it is due.
+    /// What the engine has transmitted, from the engine itself. Its count is in
+    /// symbols and falls as they go out, so what it drops between two answers
+    /// is what went on the air between them. That is the air's own rate, and
+    /// nothing here has to know what a character costs to follow it: the
+    /// symbols are spent on the characters at the front of what has not gone
+    /// out, at whatever `Symbols` says they cost.
+    ///
+    /// A count of 0 is the end of it: the engine holds nothing, so everything
+    /// it was given has gone out, whatever the symbols added up to. That is
+    /// what makes a wrong guess about the shift correct itself every message
+    /// rather than accumulating.
+    ///
+    /// The count reads 0 for the first 150 ms after a push, so a 0 that new is
+    /// passed over: it is the answer trailing what the engine was given, not an
+    /// engine that has transmitted it.
+    private bool WentOutByCount(int symbols, DateTime now)
+    {
+        lock (gate)
+        {
+            if (symbols < 0 || (symbols == 0 && now - gaveAt < CountLag))
+            {
+                return false;
+            }
+            if (lastCount > symbols)
+            {
+                budget += lastCount - symbols;
+            }
+            lastCount = symbols;
+            bool moved = false;
+            while (aired < sent.Length && outstanding > 0 && budget >= NextSymbols())
+            {
+                budget -= NextSymbols();
+                OneOut();
+                moved = true;
+            }
+            if (symbols == 0)
+            {
+                budget = 0;
+                while (aired < sent.Length && outstanding > 0)
+                {
+                    OneOut();
+                    moved = true;
+                }
+            }
+            return moved;
+        }
+    }
+
+    /// One character off the front of what has not gone out. The caller holds
+    /// `gate`.
+    private void OneOut()
+    {
+        bool figures = airShift;
+        Symbols(sent[aired], ref figures);
+        airShift = figures;
+        aired++;
+        outstanding--;
+    }
+
+    /// What the engine has transmitted, on the clock, for an engine that does
+    /// not count.
+    private bool WentOutByClock(DateTime now)
+    {
+        bool moved = false;
+        while (WentOut(now))
+        {
+            moved = true;
+        }
+        return moved;
+    }
+
+    private bool WentOut(DateTime now)
+    {
+        lock (gate)
+        {
+            if (outstanding <= 0 || now < nextOut || aired >= sent.Length)
+            {
+                return false;
+            }
+            bool figures = airShift;
+            Symbols(sent[aired], ref figures);
+            airShift = figures;
+            outstanding--;
+            aired++;
+            nextOut += SymbolTime * NextSymbols();
+            return true;
+        }
+    }
+
+    /// Marks everything up to `mark` as gone out, whatever the clock had
+    /// reached. True when that moved. The caller holds `gate`.
+    private bool AirTo(int mark, DateTime now)
+    {
+        if (mark <= aired)
+        {
+            return false;
+        }
+        while (aired < mark && aired < sent.Length)
+        {
+            bool figures = airShift;
+            Symbols(sent[aired], ref figures);
+            airShift = figures;
+            aired++;
+            outstanding = Math.Max(0, outstanding - 1);
+        }
+        nextOut = now + SymbolTime * NextSymbols();
+        return true;
+    }
+
+    /// How many symbols the character now at the front of what has not gone out
+    /// takes. The caller holds `gate`.
+    private int NextSymbols()
+    {
+        bool figures = airShift;
+        return aired < sent.Length ? Symbols(sent[aired], ref figures) : 1;
+    }
+
+    /// `count` more characters are in the engine as of `now`. The caller holds
+    /// `gate`: what the engine holds and what is still to go are one fact and
+    /// are written together.
+    private void Gave(int count, DateTime now)
+    {
+        if (count <= 0)
+        {
+            return;
+        }
+        if (outstanding == 0)
+        {
+            nextOut = now + (SymbolTime * NextSymbols());
+        }
+        outstanding += count;
+        // the count reads 0 for the first 150 ms after a push, so a 0 from here
+        // on is the answer not having caught up rather than an empty engine.
+        // What it has already been seen to drop still stands: those symbols
+        // went out whatever is given after them.
+        gaveAt = now;
+    }
+
+    /// How many characters the engine has been given that have not gone out
+    /// yet, counted on the clock rather than on the engine's own answer.
+    public int Outstanding
+    {
+        get
+        {
+            lock (gate)
+            {
+                return outstanding;
+            }
+        }
+    }
+
+    private void WhenBuffered(object? sender, int left)
+    {
+        Counts = left >= 0;
+        Volatile.Write(ref counted, left);
+    }
+
+    /// How many symbols `character` takes, and the shift it leaves the engine
+    /// in. A letter sent while the engine is in figures costs a shift symbol
+    /// and the character, and the same the other way.
+    ///
+    /// This is why the clock alone ran ahead of the air. It paced one character
+    /// every symbol time, and a callsign with a digit in it takes more than
+    /// that: 26 characters of one CQ went out in 29 symbols, one part in nine
+    /// slower than the clock thought.
+    ///
+    /// A space is taken to put the engine back in letters, which is
+    /// unshift-on-space. It is a setting — `TXUOS` in MMTTY's `UserPara.ini`,
+    /// per profile, and a button on its own display that the operator can press
+    /// mid-contest — so this cannot be read once and believed. It is assumed on
+    /// because MMTTY's help says that is the usual setting, and because being
+    /// wrong that way charges a symbol too many and leaves the pane behind the
+    /// air rather than in front of it. `AllowedOutstanding` is what corrects
+    /// the rest.
+    private static int Symbols(char character, ref bool figures)
+    {
+        if (character is ' ' or '\r' or '\n')
+        {
+            figures = false;
+            return 1;
+        }
+        bool wants = !char.IsAsciiLetter(character);
+        if (wants == figures)
+        {
+            return 1;
+        }
+        figures = wants;
+        return 2;
+    }
+
+    /// The next character to send, or null when there is none to send now:
+    /// nothing is waiting, or the gate is shut.
+    private char? Take(DateTime now)
+    {
+        lock (gate)
+        {
+            if (!open || pending.Length == 0)
             {
                 return null;
             }
             char next = pending[0];
             pending.Remove(0, 1);
-            if (cursor != NoCursor)
-            {
-                cursor--;
-            }
-            if (inEngine == 0)
-            {
-                nextOut = now + CharacterTime;
-            }
-            inEngine++;
             sent.Append(next);
+            Gave(1, now);
             if (sent.Length > KeptSent)
             {
-                sent.Remove(0, sent.Length - KeptSent);
+                int dropped = sent.Length - KeptSent;
+                sent.Remove(0, dropped);
+                aired = Math.Max(0, aired - dropped);
             }
             return next;
         }
     }
-
-    /// Takes off the estimate the characters the engine has had time to
-    /// transmit since the last look. Called with the lock held.
-    private void Advance(DateTime now)
-    {
-        while (inEngine > 0 && now >= nextOut)
-        {
-            inEngine--;
-            nextOut += CharacterTime;
-        }
-    }
 }

src/Nonemm.Digital/WineBridgeChannel.cs

@@ -12,17 +12,28 @@ public sealed class WineBridgeChannel : BridgeChannel
     private readonly string wine;
     private readonly string bridgePath;
     private readonly string? prefix;
+    private readonly string? logFolder;
     private readonly Queue<string> lastErrors = new();
     private readonly SemaphoreSlim writing = new(1, 1);
+    private readonly Lock writingLog = new();
     private Process? bridge;
+    private StreamWriter? log;
+    private DateTime started;
+    private int lastCount = int.MinValue;
 
     /// `prefix` is the WINEPREFIX to run in. Left null, Wine uses its default,
-    /// which is what a station with one prefix wants.
-    public WineBridgeChannel(string bridgePath, string? prefix = null, string wine = "wine")
+    /// which is what a station with one prefix wants. `logFolder`, when it is
+    /// given, is where the protocol log for this run is written.
+    public WineBridgeChannel(
+        string bridgePath,
+        string? prefix = null,
+        string wine = "wine",
+        string? logFolder = null)
     {
         this.bridgePath = bridgePath;
         this.prefix = prefix;
         this.wine = wine;
+        this.logFolder = logFolder;
     }
 
     public event EventHandler<string>? LineReceived;
@@ -31,6 +42,7 @@ public sealed class WineBridgeChannel : BridgeChannel
 
     public Task StartAsync(CancellationToken cancellation = default)
     {
+        OpenLog();
         bridge = Start(bridgePath);
         _ = ReadOutputAsync(bridge);
         _ = ReadErrorsAsync(bridge);
@@ -46,6 +58,7 @@ public sealed class WineBridgeChannel : BridgeChannel
             await running.StandardInput.WriteLineAsync(line.AsMemory(), cancellation)
                 .ConfigureAwait(false);
             await running.StandardInput.FlushAsync(cancellation).ConfigureAwait(false);
+            Log(">", line);
         }
         finally
         {
@@ -84,6 +97,71 @@ public sealed class WineBridgeChannel : BridgeChannel
         }
         bridge?.Dispose();
         writing.Dispose();
+        lock (writingLog)
+        {
+            log?.Dispose();
+            log = null;
+        }
+    }
+
+    /// One file per run of the engine, named for when it started. It is the
+    /// whole protocol with a millisecond stamp on every line, which is what
+    /// says who keyed the transmitter and when.
+    private void OpenLog()
+    {
+        if (logFolder is null)
+        {
+            return;
+        }
+        try
+        {
+            Directory.CreateDirectory(logFolder);
+            started = DateTime.Now;
+            log = new StreamWriter(
+                Path.Combine(logFolder, $"digital-{started:yyyyMMdd-HHmmss}.log"),
+                append: false)
+            {
+                AutoFlush = true,
+            };
+        }
+        catch (IOException)
+        {
+            // a log that cannot be written stops nothing
+            log = null;
+        }
+        catch (UnauthorizedAccessException)
+        {
+            log = null;
+        }
+    }
+
+    /// The buffer question goes out every 50 ms and is answered just as often,
+    /// which would bury everything else. The question is left out and the
+    /// answer is written only when the count has changed.
+    private void Log(string direction, string line)
+    {
+        if (log is null)
+        {
+            return;
+        }
+        (string verb, string[] fields) = BridgeLine.Read(line);
+        if (verb == "buffer")
+        {
+            if (direction == ">" || fields.Length == 0)
+            {
+                return;
+            }
+            if (!int.TryParse(fields[0], out int count) || count == lastCount)
+            {
+                return;
+            }
+            lastCount = count;
+        }
+        lock (writingLog)
+        {
+            log?.WriteLine(
+                $"{(DateTime.Now - started).TotalMilliseconds,9:0} ms  {direction} {line.Replace('\t', ' ')}");
+        }
     }
 
     private Process Start(string program, params string[] arguments)
@@ -113,6 +191,7 @@ public sealed class WineBridgeChannel : BridgeChannel
     {
         while (await running.StandardOutput.ReadLineAsync().ConfigureAwait(false) is { } line)
         {
+            Log("<", line);
             LineReceived?.Invoke(this, line);
         }
         await running.WaitForExitAsync().ConfigureAwait(false);

src/Nonemm.Network/NetworkedStation.cs

@@ -0,0 +1,98 @@
+using Nonemm.Core;
+
+namespace Nonemm.Network;
+
+/// One computer on the network, as far as this one can tell. The network status
+/// window shows a row per station and these are its columns.
+///
+/// Everything here is what the station last said, with the time it said it.
+/// Nothing is asked for on demand: the beacons and the messages carry it, and a
+/// station that has gone quiet keeps the last thing it said with an older
+/// `LastHeardUtc`, which is how the window shows it as missing rather than
+/// blank.
+public sealed class NetworkedStation
+{
+    public NetworkedStation(string computerName, string address, int port)
+    {
+        ComputerName = computerName;
+        Address = address;
+        Port = port;
+        // now is when it was heard of, whether that was a beacon or the
+        // operator naming it. Left at nothing, `StationLink.Forget` reads a
+        // station as having been quiet since the beginning of time and drops
+        // the connection to it the moment it is opened
+        LastHeardUtc = DateTime.UtcNow;
+    }
+
+    /// The name in every message the station sends, and what a contact from it
+    /// is stamped with. N1MM uses the computer's own network name.
+    public string ComputerName { get; }
+
+    public string Address { get; internal set; }
+
+    public int Port { get; internal set; }
+
+    /// Which station of the entry this is. N1MM numbers them so a contact can
+    /// say which position made it; the number arrives in `IAM`.
+    public int StationNumber { get; internal set; }
+
+    /// The version the station broadcast. N1MM refuses to talk to a station
+    /// whose version is not its own, so a mismatch here is why nothing works.
+    public string Version { get; internal set; } = "";
+
+    public string Operator { get; internal set; } = "";
+
+    public string ContestName { get; internal set; } = "";
+
+    /// Where the station is, from the last `BANDMAP` it sent.
+    public Frequency Frequency { get; internal set; }
+
+    public Mode? Mode { get; internal set; }
+
+    public Band? Band => Bands.ForFrequency(Frequency);
+
+    /// True while the station is running rather than searching. In a
+    /// multi-single entry this is the run station.
+    public bool IsRunning { get; internal set; }
+
+    public bool IsTransmitting { get; internal set; }
+
+    public int RadioNumber { get; internal set; } = 1;
+
+    /// A frequency this station has passed, from `PASSFREQ`, and who is on it.
+    public Frequency PassFrequency { get; internal set; }
+
+    public string PassCall { get; internal set; } = "";
+
+    /// When anything last arrived from the station, and what it was. The window
+    /// shows both: a station whose last message is minutes old is not there any
+    /// more, whatever its connection says.
+    public DateTime LastHeardUtc { get; internal set; }
+
+    public string LastMessage { get; internal set; } = "";
+
+    /// How many messages have gone each way since the program started. N1MM
+    /// shows the same two numbers, and a Send that climbs while Read stands
+    /// still is a station that is not listening.
+    public int Sent { get; internal set; }
+
+    public int Read { get; internal set; }
+
+    /// True while a connection to the station is open. It is not the same as
+    /// the station being there: the connection can stand for a while after the
+    /// other program has stopped.
+    public bool IsConnected { get; internal set; }
+
+    /// How long the last echo took to come back, or null when none has.
+    public TimeSpan? EchoTime { get; internal set; }
+
+    /// This computer, which is in the list as well. N1MM shows it too, so an
+    /// operator can read its own station number and version off the same
+    /// window.
+    public bool IsMine { get; internal init; }
+
+    /// Set when the station broadcast a version that is not ours. N1MM turns
+    /// such a station away, so this program says why rather than failing to
+    /// connect for no visible reason.
+    public string Refused { get; internal set; } = "";
+}

src/Nonemm.Network/QsoRecord.cs

@@ -0,0 +1,142 @@
+using System.Globalization;
+using Nonemm.Core;
+
+namespace Nonemm.Network;
+
+/// A contact as it travels between two logging computers: the thirty-five
+/// fields of N1MM's `QSOString`, in N1MM's order.
+///
+/// The order is what matters and it cannot be changed, because the other end
+/// reads the fields by position. It is taken from `MultiOpManager.cs:1007`.
+///
+/// | At | Field | At | Field |
+/// |---|---|---|---|
+/// | 0 | timestamp | 18 | sent serial |
+/// | 1 | callsign | 19 | points |
+/// | 2 | frequency, kHz | 20 | multiplier 1 |
+/// | 3 | transmit frequency | 21 | multiplier 2 |
+/// | 4 | mode | 22 | power |
+/// | 5 | contest name | 23 | band, MHz |
+/// | 6 | sent report | 24 | WPX prefix |
+/// | 7 | received report | 25 | exchange 1 |
+/// | 8 | country prefix | 26 | radio number |
+/// | 9 | station prefix | 27 | operator |
+/// | 10 | QTH | 28 | grid square |
+/// | 11 | name | 29 | contest number |
+/// | 12 | comment | 30 | multiplier 3 |
+/// | 13 | received serial | 31 | misc text |
+/// | 14 | section | 32 | contact type |
+/// | 15 | precedence | 33 | run 1 or run 2 |
+/// | 16 | check | 34 | continent |
+/// | 17 | zone | | |
+///
+/// The contact carries its points and its multiplier flags, and they are read
+/// rather than trusted: the log works them out again from the contest rules, so
+/// two stations cannot disagree about a score because one of them was running
+/// an older set of rules.
+public static class QsoRecord
+{
+    /// How many fields there are. A message with fewer is read as far as it
+    /// goes, because a station on another version sends a shorter one.
+    public const int FieldCount = 35;
+
+    public static List<string> Write(Qso qso) =>
+    [
+        StationRecord.Written(qso.TimestampUtc),
+        qso.Call.Text,
+        StationRecord.Written(qso.Frequency.Kilohertz),
+        StationRecord.Written(
+            (qso.QsxFrequency.Hertz == 0 ? qso.Frequency : qso.QsxFrequency).Kilohertz),
+        qso.Mode.Name,
+        qso.ContestName,
+        qso.SentReport,
+        qso.ReceivedReport,
+        qso.CountryPrefix,
+        qso.StationPrefix,
+        qso.Qth,
+        qso.Name,
+        qso.Comment,
+        Text(qso.ReceivedNumber),
+        qso.Section,
+        qso.Precedence,
+        Text(qso.Check),
+        Text(qso.Zone),
+        Text(qso.SentNumber),
+        Text(qso.Points),
+        StationRecord.Written(qso.IsMultiplier1),
+        StationRecord.Written(qso.IsMultiplier2),
+        qso.Power,
+        StationRecord.Written(qso.Band?.MegahertzLabel ?? 0),
+        qso.WpxPrefix,
+        qso.Exchange1,
+        Text(qso.RadioNumber),
+        qso.Operator,
+        qso.GridSquare,
+        Text(qso.ContestNumber),
+        StationRecord.Written(qso.IsMultiplier3),
+        qso.MiscText,
+        qso.ContactType,
+        Text(qso.RunPosition),
+        qso.Continent,
+    ];
+
+    /// The contact the fields describe. `at` is where the fields start, which
+    /// is not always 0: an edit puts the old timestamp in front of them and a
+    /// replace the old callsign as well.
+    ///
+    /// `stationName` is the computer that sent it, kept on the contact so the
+    /// log window can say where each row came from.
+    ///
+    /// The band, field 23, is not read back: this program works the band out
+    /// from the frequency, and reading it would believe a station that
+    /// disagreed with the band plan about where 14.2 MHz is.
+    public static Qso Read(StationRecord record, int at, string stationName)
+    {
+        Frequency frequency = Frequency.FromKilohertz(record.Decimal(at + 2));
+        Frequency transmit = Frequency.FromKilohertz(record.Decimal(at + 3));
+        return new Qso
+        {
+            Id = Qso.NewId(),
+            TimestampUtc = record.Time(at),
+            Call = Callsign.Parse(record.Field(at + 1)),
+            Frequency = frequency,
+            QsxFrequency = transmit == frequency ? Frequency.Zero : transmit,
+            Mode = Modes.Parse(record.Field(at + 4)) ?? Modes.Cw,
+            ContestName = record.Field(at + 5),
+            SentReport = record.Field(at + 6),
+            ReceivedReport = record.Field(at + 7),
+            CountryPrefix = record.Field(at + 8),
+            StationPrefix = record.Field(at + 9),
+            Qth = record.Field(at + 10),
+            Name = record.Field(at + 11),
+            Comment = record.Field(at + 12),
+            ReceivedNumber = record.Number(at + 13),
+            Section = record.Field(at + 14),
+            Precedence = record.Field(at + 15),
+            Check = record.Number(at + 16),
+            Zone = record.Number(at + 17),
+            SentNumber = record.Number(at + 18),
+            Points = record.Number(at + 19),
+            IsMultiplier1 = record.Flag(at + 20),
+            IsMultiplier2 = record.Flag(at + 21),
+            Power = record.Field(at + 22),
+            WpxPrefix = record.Field(at + 24),
+            Exchange1 = record.Field(at + 25),
+            RadioNumber = Math.Max(1, record.Number(at + 26)),
+            Operator = record.Field(at + 27),
+            GridSquare = record.Field(at + 28),
+            ContestNumber = record.Number(at + 29),
+            IsMultiplier3 = record.Flag(at + 30),
+            MiscText = record.Field(at + 31),
+            ContactType = record.Field(at + 32),
+            RunPosition = record.Number(at + 33),
+            Continent = record.Field(at + 34),
+            StationName = stationName,
+            NetworkedComputerNumber = record.StationNumber,
+            // it was made at another radio, on another computer
+            IsOriginal = false,
+        };
+    }
+
+    private static string Text(int number) => number.ToString(CultureInfo.InvariantCulture);
+}

src/Nonemm.Network/StationBeacon.cs

@@ -0,0 +1,71 @@
+using System.Globalization;
+
+namespace Nonemm.Network;
+
+/// The message every computer broadcasts to say it is on the network, which is
+/// how the others find it without anybody typing in an address.
+///
+/// It goes out as a UDP broadcast to port 12070 and carries six fields:
+///
+/// | Field | What it is |
+/// |---|---|
+/// | `ComputerName` | the name the station is known by, and the name in every message it sends |
+/// | `Address` | the address the others open a connection to |
+/// | `Port` | the port they open it on, 12070 unless the operator moved it |
+/// | `Version` | the program version, which has to match — see below |
+/// | `Operator` | who is at that radio |
+/// | `VpnAdapter` | the adapter the beacon went out of, or empty on an ordinary network |
+///
+/// **N1MM will not talk to a station whose version is not its own.** It
+/// compares the fourth field with its own version and, when they differ, puts
+/// up "Software versions must match. Update N1MM+." and drops the station. So
+/// the version this program broadcasts is a setting rather than its own
+/// version: to work alongside N1MM it has to say what that copy of N1MM says.
+/// A beacon with the wrong number of fields is refused the same way.
+public sealed record StationBeacon(
+    string ComputerName,
+    string Address,
+    int Port,
+    string Version,
+    string Operator,
+    string VpnAdapter = "")
+{
+    /// N1MM's port for talking to another copy of itself. It listens on both
+    /// UDP and TCP here: the beacons arrive on the first and the contacts on
+    /// the second.
+    public const int DefaultPort = 12070;
+
+    /// How many fields N1MM requires. It splits on `%` and counts, and the
+    /// trailing separator leaves an empty seventh.
+    public const int FieldCount = 7;
+
+    public string ToWire() => string.Join(
+        StationRecord.FieldSeparator,
+        [ComputerName, Address, Port.ToString(CultureInfo.InvariantCulture), Version, Operator, VpnAdapter, ""]);
+
+    /// Null for anything that is not a beacon. N1MM's own reader says so out
+    /// loud — it tells the operator that the other station is on an old
+    /// version — but a beacon is broadcast, so anything on the network can land
+    /// here and most of it is not worth a message.
+    public static StationBeacon? Read(string text)
+    {
+        // N1MM's own check: XML on this port is the port-12060 contact
+        // broadcast pointed at the wrong place
+        if (text.Contains("xml version", StringComparison.OrdinalIgnoreCase))
+        {
+            return null;
+        }
+        string[] parts = text.Split(StationRecord.FieldSeparator);
+        if (parts.Length != FieldCount)
+        {
+            return null;
+        }
+        return new StationBeacon(
+            parts[0],
+            parts[1],
+            int.TryParse(parts[2], CultureInfo.InvariantCulture, out int port) ? port : DefaultPort,
+            parts[3],
+            parts[4],
+            parts[5]);
+    }
+}

src/Nonemm.Network/StationLink.cs

@@ -0,0 +1,555 @@
+using System.Collections.Concurrent;
+using System.Net;
+using System.Net.Sockets;
+using System.Text;
+using Nonemm.Core;
+
+namespace Nonemm.Network;
+
+/// The link between the computers of one multi-operator entry, on N1MM's own
+/// port 12070.
+///
+/// Two sockets, which is how N1MM does it:
+///
+/// | Socket | What goes over it |
+/// |---|---|
+/// | UDP, broadcast | the beacon that says a computer is here, every `BeaconInterval` |
+/// | TCP, one per station | contacts, edits, deletes, chat, everything else |
+///
+/// A station is found rather than configured: the beacon carries the address
+/// and the port to open a connection to, so an operator plugs a laptop in and
+/// the others see it. A connection is opened to every station whose beacon
+/// arrives, and both ends do it, so there are two connections per pair of
+/// stations — one each way. That is N1MM's arrangement: a station writes to the
+/// connection it opened and reads from the one that was opened to it.
+///
+/// **The version must match.** N1MM compares the version in the beacon with
+/// its own and turns away anything else, so `version` is what this program
+/// claims to be and has to be the version of the N1MM copies it is running
+/// beside. A station that says something else is kept in the list with
+/// `Refused` set, so the operator can see why it is not talking.
+///
+/// This does not score anything or touch the log. It hands what arrives to
+/// whoever owns the log through `UpdateArrived`, the same as `StationNetwork`
+/// does with the XML broadcasts on port 12060. The two run side by side: 12060
+/// is for other programs, 12070 is for other logging computers.
+public sealed class StationLink : IDisposable
+{
+    /// How often the beacon goes out. N1MM broadcasts on startup, when the
+    /// network status window asks, and on a timer; ten seconds is short enough
+    /// that a station that joins is seen at once and long enough to be nothing
+    /// on a network carrying contest traffic.
+    public static readonly TimeSpan BeaconInterval = TimeSpan.FromSeconds(10);
+
+    /// How long a station may say nothing before the window shows it as gone.
+    /// Three beacons.
+    public static readonly TimeSpan Patience = TimeSpan.FromSeconds(30);
+
+    private readonly int port;
+    private readonly string computerName;
+    private readonly string version;
+    private readonly CancellationTokenSource stopping = new();
+    private readonly ConcurrentDictionary<string, NetworkedStation> stations = new(StringComparer.OrdinalIgnoreCase);
+    private readonly ConcurrentDictionary<string, TcpClient> writers = new(StringComparer.OrdinalIgnoreCase);
+    private readonly NetworkedStation mine;
+    /// When the last echo request went out, which is what the round trip is
+    /// measured against.
+    private DateTime echoSentAt;
+    private UdpClient? beacons;
+    private TcpListener? listener;
+    private readonly List<Task> loops = [];
+
+    public StationLink(string computerName, string version, int stationNumber = 1, int port = StationBeacon.DefaultPort)
+    {
+        this.computerName = computerName.ToUpperInvariant();
+        this.version = version;
+        this.port = port;
+        StationNumber = stationNumber;
+        mine = new NetworkedStation(this.computerName, "", port)
+        {
+            IsMine = true,
+        };
+        mine.StationNumber = stationNumber;
+        mine.Version = version;
+        stations[this.computerName] = mine;
+    }
+
+    /// Which station of the entry this computer is. It goes into every message
+    /// and into every contact this computer logs.
+    public int StationNumber { get; }
+
+    public string ComputerName => computerName;
+
+    /// Who is at this radio, sent in the beacon so the other stations can show
+    /// it.
+    public string Operator
+    {
+        get => mine.Operator;
+        set => mine.Operator = value;
+    }
+
+    /// Every station, this computer included, in the order they were first
+    /// heard.
+    public IReadOnlyList<NetworkedStation> Stations => stations.Values.ToList();
+
+    /// A contact another station logged, edited or deleted.
+    public event EventHandler<ContactUpdate>? UpdateArrived;
+
+    /// A line of chat from another operator.
+    public event EventHandler<string>? TalkArrived;
+
+    /// Anything about the list of stations changed: one joined, one went quiet,
+    /// one moved band. The status window redraws on this.
+    public event EventHandler? StationsChanged;
+
+    public event EventHandler<string>? Failed;
+
+    public void Start()
+    {
+        if (loops.Count > 0)
+        {
+            return;
+        }
+        try
+        {
+            beacons = new UdpClient();
+            beacons.Client.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.ReuseAddress, true);
+            beacons.Client.Bind(new IPEndPoint(IPAddress.Any, port));
+            beacons.EnableBroadcast = true;
+            listener = new TcpListener(new IPEndPoint(IPAddress.Any, port));
+            listener.Start();
+        }
+        catch (SocketException e)
+        {
+            Failed?.Invoke(this, $"could not take port {port}: {e.Message}");
+            return;
+        }
+        loops.Add(Task.Run(() => BeaconLoopAsync(stopping.Token)));
+        loops.Add(Task.Run(() => ListenForBeaconsAsync(stopping.Token)));
+        loops.Add(Task.Run(() => AcceptAsync(stopping.Token)));
+    }
+
+    /// A station named by hand rather than found by a beacon, and a connection
+    /// opened to it. N1MM offers the same thing for a network where a broadcast
+    /// does not reach every computer — its predefined stations — and it is also
+    /// what a station on the other side of a VPN needs.
+    public NetworkedStation AddStation(string name, string address, int stationPort)
+    {
+        NetworkedStation station = stations.GetOrAdd(
+            name.ToUpperInvariant(),
+            known => new NetworkedStation(known, address, stationPort));
+        station.Address = address;
+        station.Port = stationPort;
+        station.LastHeardUtc = DateTime.UtcNow;
+        StationsChanged?.Invoke(this, EventArgs.Empty);
+        if (!writers.ContainsKey(station.ComputerName))
+        {
+            _ = ConnectAsync(station, stopping.Token);
+        }
+        return station;
+    }
+
+    /// A message to every station that is connected. Returns how many it
+    /// reached, so a caller that has to know whether anybody heard can say so.
+    public async Task<int> SendAsync(StationRecord record, CancellationToken cancellation = default)
+    {
+        byte[] message = Encoding.UTF8.GetBytes(record.ToWire());
+        int reached = 0;
+        foreach ((string name, TcpClient writer) in writers)
+        {
+            try
+            {
+                await writer.GetStream().WriteAsync(message, cancellation).ConfigureAwait(false);
+                reached++;
+                if (stations.TryGetValue(name, out NetworkedStation? station))
+                {
+                    station.Sent++;
+                }
+            }
+            catch (Exception e) when (e is IOException or SocketException or ObjectDisposedException)
+            {
+                Drop(name, e.Message);
+            }
+        }
+        if (reached > 0)
+        {
+            mine.Sent += reached;
+            StationsChanged?.Invoke(this, EventArgs.Empty);
+        }
+        return reached;
+    }
+
+    /// A message to one station. Returns false when it could not be written,
+    /// which drops the connection: the next beacon opens a new one.
+    public async Task<bool> SendToAsync(
+        NetworkedStation station,
+        StationRecord record,
+        CancellationToken cancellation = default)
+    {
+        if (!writers.TryGetValue(station.ComputerName, out TcpClient? writer))
+        {
+            return false;
+        }
+        try
+        {
+            await writer
+                .GetStream()
+                .WriteAsync(Encoding.UTF8.GetBytes(record.ToWire()), cancellation)
+                .ConfigureAwait(false);
+        }
+        catch (Exception e) when (e is IOException or SocketException or ObjectDisposedException)
+        {
+            Drop(station.ComputerName, e.Message);
+            return false;
+        }
+        station.Sent++;
+        mine.Sent++;
+        StationsChanged?.Invoke(this, EventArgs.Empty);
+        return true;
+    }
+
+    public Task<int> SendLoggedAsync(Qso qso, CancellationToken cancellation = default) =>
+        SendAsync(StationMessages.Logged(qso, StationNumber, computerName), cancellation);
+
+    public Task<int> SendEditedAsync(
+        Qso qso,
+        string oldCall,
+        DateTime oldTimestampUtc,
+        CancellationToken cancellation = default) =>
+        SendAsync(
+            StationMessages.Edited(qso, oldCall, oldTimestampUtc, StationNumber, computerName),
+            cancellation);
+
+    public Task<int> SendDeletedAsync(Qso qso, CancellationToken cancellation = default) =>
+        SendAsync(StationMessages.Deleted(qso, StationNumber, computerName), cancellation);
+
+    public Task<int> SendTalkAsync(string text, CancellationToken cancellation = default) =>
+        SendAsync(StationMessages.Talk(text, StationNumber, computerName), cancellation);
+
+    /// Says where this station is. It goes out whenever the radio moves or the
+    /// operator turns run on or off, and it is what the other stations show and
+    /// what the band-change rule counts.
+    public Task<int> SendBandAsync(
+        Frequency frequency,
+        Mode mode,
+        bool running,
+        int radioNumber,
+        CancellationToken cancellation = default)
+    {
+        mine.Frequency = frequency;
+        mine.Mode = mode;
+        mine.IsRunning = running;
+        mine.RadioNumber = radioNumber;
+        StationsChanged?.Invoke(this, EventArgs.Empty);
+        return SendAsync(
+            StationMessages.OnBand(frequency, mode, running, radioNumber, StationNumber, computerName),
+            cancellation);
+    }
+
+    public Task<int> SendTransmittingAsync(
+        bool transmitting,
+        int radioNumber,
+        CancellationToken cancellation = default)
+    {
+        mine.IsTransmitting = transmitting;
+        return SendAsync(
+            StationMessages.Transmitting(transmitting, radioNumber, StationNumber, computerName),
+            cancellation);
+    }
+
+    /// Asks every station whether it is there. The answer sets `EchoTime`.
+    public Task<int> SendEchoRequestAsync(CancellationToken cancellation = default)
+    {
+        echoSentAt = DateTime.UtcNow;
+        return SendAsync(StationMessages.EchoRequest(StationNumber, computerName, DateTime.UtcNow), cancellation);
+    }
+
+    public void Dispose()
+    {
+        stopping.Cancel();
+        foreach (TcpClient writer in writers.Values)
+        {
+            writer.Dispose();
+        }
+        writers.Clear();
+        listener?.Stop();
+        beacons?.Dispose();
+        stopping.Dispose();
+    }
+
+    /// The beacon, on every interface that can broadcast. It carries this
+    /// computer's name, address and port, so a station that hears it knows
+    /// where to open a connection.
+    private async Task BeaconLoopAsync(CancellationToken cancellation)
+    {
+        while (!cancellation.IsCancellationRequested)
+        {
+            try
+            {
+                StationBeacon beacon = new(computerName, Address(), port, version, mine.Operator);
+                byte[] message = Encoding.UTF8.GetBytes(beacon.ToWire());
+                await beacons!
+                    .SendAsync(message, new IPEndPoint(IPAddress.Broadcast, port), cancellation)
+                    .ConfigureAwait(false);
+                mine.Address = beacon.Address;
+                mine.LastHeardUtc = DateTime.UtcNow;
+                Forget();
+            }
+            catch (OperationCanceledException)
+            {
+                return;
+            }
+            catch (SocketException e)
+            {
+                Failed?.Invoke(this, $"could not broadcast: {e.Message}");
+            }
+            try
+            {
+                await Task.Delay(BeaconInterval, cancellation).ConfigureAwait(false);
+            }
+            catch (OperationCanceledException)
+            {
+                return;
+            }
+        }
+    }
+
+    private async Task ListenForBeaconsAsync(CancellationToken cancellation)
+    {
+        while (!cancellation.IsCancellationRequested)
+        {
+            try
+            {
+                UdpReceiveResult received = await beacons!.ReceiveAsync(cancellation).ConfigureAwait(false);
+                if (StationBeacon.Read(Encoding.UTF8.GetString(received.Buffer)) is not { } beacon)
+                {
+                    continue;
+                }
+                if (beacon.ComputerName.Equals(computerName, StringComparison.OrdinalIgnoreCase))
+                {
+                    continue;
+                }
+                Heard(beacon, received.RemoteEndPoint.Address.ToString());
+            }
+            catch (OperationCanceledException)
+            {
+                return;
+            }
+            catch (SocketException e)
+            {
+                Failed?.Invoke(this, e.Message);
+            }
+        }
+    }
+
+    /// A station said it is here. Its own address is believed only as far as
+    /// the packet: a station behind a router announces an address nothing can
+    /// reach, so the address the packet came from is what a connection is
+    /// opened to.
+    private void Heard(StationBeacon beacon, string from)
+    {
+        NetworkedStation station = stations.GetOrAdd(
+            beacon.ComputerName,
+            name => new NetworkedStation(name, from, beacon.Port));
+        station.Address = from;
+        station.Port = beacon.Port;
+        station.Version = beacon.Version;
+        station.Operator = beacon.Operator;
+        station.LastHeardUtc = DateTime.UtcNow;
+        if (beacon.Version != version)
+        {
+            station.Refused = $"version {beacon.Version}, this station is {version}";
+            station.IsConnected = false;
+            StationsChanged?.Invoke(this, EventArgs.Empty);
+            return;
+        }
+        station.Refused = "";
+        StationsChanged?.Invoke(this, EventArgs.Empty);
+        if (!writers.ContainsKey(station.ComputerName))
+        {
+            _ = ConnectAsync(station, stopping.Token);
+        }
+    }
+
+    private async Task ConnectAsync(NetworkedStation station, CancellationToken cancellation)
+    {
+        TcpClient writer = new();
+        try
+        {
+            await writer.ConnectAsync(station.Address, station.Port, cancellation).ConfigureAwait(false);
+        }
+        catch (Exception e) when (e is SocketException or OperationCanceledException)
+        {
+            writer.Dispose();
+            station.Refused = e.Message;
+            Failed?.Invoke(this, $"could not open a connection to {station.ComputerName} at {station.Address}:{station.Port}: {e.Message}");
+            StationsChanged?.Invoke(this, EventArgs.Empty);
+            return;
+        }
+        if (!writers.TryAdd(station.ComputerName, writer))
+        {
+            writer.Dispose();
+            return;
+        }
+        station.IsConnected = true;
+        station.LastHeardUtc = DateTime.UtcNow;
+        StationsChanged?.Invoke(this, EventArgs.Empty);
+        // N1MM's first message on a new connection, which tells the other end
+        // which station of the entry this is. It goes to that station alone:
+        // the others were told when their own connection opened
+        await SendToAsync(station, StationMessages.IAm(StationNumber, computerName), cancellation)
+            .ConfigureAwait(false);
+    }
+
+    private async Task AcceptAsync(CancellationToken cancellation)
+    {
+        while (!cancellation.IsCancellationRequested)
+        {
+            try
+            {
+                TcpClient reader = await listener!.AcceptTcpClientAsync(cancellation).ConfigureAwait(false);
+                _ = Task.Run(() => ReadAsync(reader, cancellation), cancellation);
+            }
+            catch (OperationCanceledException)
+            {
+                return;
+            }
+            catch (SocketException e)
+            {
+                Failed?.Invoke(this, e.Message);
+            }
+        }
+    }
+
+    /// One connection that was opened to this station. Whatever arrives is
+    /// added to what has not been read yet, and every whole message in it is
+    /// handed on: TCP gives no promise about where a read ends.
+    private async Task ReadAsync(TcpClient reader, CancellationToken cancellation)
+    {
+        byte[] buffer = new byte[8192];
+        string held = "";
+        try
+        {
+            using (reader)
+            {
+                NetworkStream stream = reader.GetStream();
+                while (!cancellation.IsCancellationRequested)
+                {
+                    int count = await stream.ReadAsync(buffer, cancellation).ConfigureAwait(false);
+                    if (count == 0)
+                    {
+                        return;
+                    }
+                    held += Encoding.UTF8.GetString(buffer, 0, count);
+                    while (StationRecord.Read(ref held) is { } record)
+                    {
+                        Arrived(record);
+                    }
+                }
+            }
+        }
+        catch (Exception e) when (e is IOException or SocketException or OperationCanceledException)
+        {
+        }
+    }
+
+    /// What one message means. The station it came from is credited with it
+    /// whatever the type is, so the window's Read count and Last heard are
+    /// right even for the messages this program does nothing with.
+    private void Arrived(StationRecord record)
+    {
+        NetworkedStation station = stations.GetOrAdd(
+            record.ComputerName,
+            name => new NetworkedStation(name, "", port));
+        station.LastHeardUtc = DateTime.UtcNow;
+        station.LastMessage = record.Type;
+        station.Read++;
+        mine.Read++;
+        switch (record.Type)
+        {
+            case "IAM":
+                station.StationNumber = record.Number(0);
+                break;
+            case "BANDMAP":
+                station.Frequency = Frequency.FromKilohertz(record.Decimal(0));
+                station.Mode = Modes.Parse(record.Field(1));
+                station.IsRunning = record.Flag(2);
+                station.RadioNumber = Math.Max(1, record.Number(3));
+                break;
+            case "XMIT":
+                station.IsTransmitting = record.Flag(0);
+                station.RadioNumber = Math.Max(1, record.Number(1));
+                break;
+            case "PASSFREQ":
+                station.PassFrequency = Frequency.FromKilohertz(record.Decimal(0));
+                station.PassCall = record.Field(1);
+                break;
+            case "ECHOREQ":
+                _ = SendAsync(StationMessages.Echo(record, StationNumber, computerName));
+                break;
+            case "ECHO":
+                station.EchoTime = DateTime.UtcNow - echoSentAt;
+                break;
+            case "TALK":
+                TalkArrived?.Invoke(this, record.Field(0));
+                break;
+            default:
+                if (StationMessages.Read(record) is { } update)
+                {
+                    UpdateArrived?.Invoke(this, update);
+                }
+                break;
+        }
+        StationsChanged?.Invoke(this, EventArgs.Empty);
+    }
+
+    /// Closes a connection that has failed or gone quiet. The station stays in
+    /// the list: it
+    /// is still part of the entry, and the window showing it as not connected
+    /// is the point.
+    private void Drop(string name, string why)
+    {
+        if (writers.TryRemove(name, out TcpClient? writer))
+        {
+            writer.Dispose();
+        }
+        if (stations.TryGetValue(name, out NetworkedStation? station))
+        {
+            station.IsConnected = false;
+            station.Refused = why;
+        }
+        StationsChanged?.Invoke(this, EventArgs.Empty);
+    }
+
+    /// Marks as not connected any station that has said nothing for
+    /// `Patience`, and lets go of its connection so the next beacon opens a
+    /// new one.
+    private void Forget()
+    {
+        DateTime cutoff = DateTime.UtcNow - Patience;
+        foreach (NetworkedStation station in stations.Values)
+        {
+            if (!station.IsMine && station.IsConnected && station.LastHeardUtc < cutoff)
+            {
+                Drop(station.ComputerName, "said nothing for half a minute");
+            }
+        }
+    }
+
+    /// This computer's address on the network it can reach the others on. It is
+    /// only for the beacon: the other end uses the address the packet came
+    /// from, so a wrong answer here costs nothing.
+    private static string Address()
+    {
+        try
+        {
+            using Socket probe = new(AddressFamily.InterNetwork, SocketType.Dgram, ProtocolType.Udp);
+            probe.Connect("8.8.8.8", 65530);
+            return probe.LocalEndPoint is IPEndPoint local ? local.Address.ToString() : "";
+        }
+        catch (SocketException)
+        {
+            return "";
+        }
+    }
+}

src/Nonemm.Network/StationMessages.cs

@@ -0,0 +1,169 @@
+using System.Globalization;
+using Nonemm.Core;
+
+namespace Nonemm.Network;
+
+/// The messages two logging computers send each other, and what to do with one
+/// that arrives.
+///
+/// N1MM has around forty of these. The ones here are the ones a multi-operator
+/// entry cannot run without:
+///
+/// | Type | What it says | Read | Sent |
+/// |---|---|---|---|
+/// | `QSO` | a contact was logged | yes | yes |
+/// | `ReEditQSO` | a contact was edited | yes | yes |
+/// | `QSODELETE` | a contact was removed | yes | yes |
+/// | `ReSyncQSO` | a contact again, in answer to a resync | yes | yes |
+/// | `IAM` | which station number the sender has taken | yes | yes |
+/// | `ECHOREQ` | are you there | yes | yes |
+/// | `ECHO` | yes | yes | yes |
+/// | `TALK` | a line of chat between operators | yes | yes |
+/// | `PASSFREQ` | a frequency being passed to another radio | yes | yes |
+/// | `XMIT` | a station started or stopped transmitting | yes | yes |
+/// | `BANDMAP` | which band and mode a station is on | yes | yes |
+///
+/// The rest — the score and sked windows, the log check, the spot lists, the
+/// serial-number pool — are passed over. An unknown type is not an error: N1MM
+/// adds them between versions, and a station that cannot read one is no worse
+/// off than a station that has not been told.
+public static class StationMessages
+{
+    /// A logged contact. Every field is N1MM's `QSOString`, with the timestamp
+    /// the contact had before this message in front of them; for a new contact
+    /// that is the same timestamp.
+    public static StationRecord Logged(Qso qso, int stationNumber, string computerName) =>
+        Record("QSO", stationNumber, computerName, [StationRecord.Written(qso.TimestampUtc), .. QsoRecord.Write(qso)]);
+
+    /// A contact that has been edited. The old timestamp and the old callsign
+    /// say which row to replace, because that pair is what N1MM keys a contact
+    /// on rather than an identifier of its own.
+    public static StationRecord Edited(
+        Qso qso,
+        string oldCall,
+        DateTime oldTimestampUtc,
+        int stationNumber,
+        string computerName) =>
+        Record(
+            "ReEditQSO",
+            stationNumber,
+            computerName,
+            [StationRecord.Written(oldTimestampUtc), oldCall, .. QsoRecord.Write(qso)]);
+
+    /// The same contact sent again because another station asked for it. It is
+    /// a separate type so the other end can tell a resync from a contact just
+    /// made and not count it twice in the rate window.
+    public static StationRecord Resynced(Qso qso, int stationNumber, string computerName) =>
+        Record(
+            "ReSyncQSO",
+            stationNumber,
+            computerName,
+            [StationRecord.Written(qso.TimestampUtc), .. QsoRecord.Write(qso)]);
+
+    public static StationRecord Deleted(Qso qso, int stationNumber, string computerName) =>
+        Record(
+            "QSODELETE",
+            stationNumber,
+            computerName,
+            [StationRecord.Written(qso.TimestampUtc), qso.Call.Text, Text(qso.ContestNumber), qso.Id]);
+
+    /// Which station number this computer has taken. N1MM numbers the stations
+    /// of an entry so a contact can say which position made it.
+    public static StationRecord IAm(int stationNumber, string computerName) =>
+        Record("IAM", stationNumber, computerName, [Text(stationNumber)]);
+
+    /// Are you there. N1MM sends the date and the time of day, and uses the
+    /// answer to show how long the round trip took.
+    public static StationRecord EchoRequest(int stationNumber, string computerName, DateTime now) =>
+        Record(
+            "ECHOREQ",
+            stationNumber,
+            computerName,
+            [StationRecord.WrittenDate(now), now.ToString("HH:mm:ss", CultureInfo.InvariantCulture)]);
+
+    /// The answer, carrying back what the request said so the asker can work
+    /// out the round trip without keeping anything.
+    public static StationRecord Echo(StationRecord request, int stationNumber, string computerName) =>
+        Record("ECHO", stationNumber, computerName, [request.Field(0), request.Field(1)]);
+
+    public static StationRecord Talk(string text, int stationNumber, string computerName) =>
+        Record("TALK", stationNumber, computerName, [$"[{computerName}] {text}"]);
+
+    /// A frequency handed to another radio, which is N1MM's pass. The
+    /// callsign is who is on it.
+    public static StationRecord PassFrequency(
+        Frequency frequency,
+        string call,
+        int stationNumber,
+        string computerName) =>
+        Record(
+            "PASSFREQ",
+            stationNumber,
+            computerName,
+            [StationRecord.Written(frequency.Kilohertz), call]);
+
+    /// A station started or stopped transmitting. The other stations of a
+    /// multi-single entry need this: two of them keying at once is one signal
+    /// too many.
+    public static StationRecord Transmitting(
+        bool transmitting,
+        int radioNumber,
+        int stationNumber,
+        string computerName) =>
+        Record(
+            "XMIT",
+            stationNumber,
+            computerName,
+            [StationRecord.Written(transmitting), Text(radioNumber)]);
+
+    /// Where a station is: the band and the mode it is on, and whether it is
+    /// running. This is what fills the band and Running columns of the network
+    /// status window, and what the band-change rule counts.
+    public static StationRecord OnBand(
+        Frequency frequency,
+        Mode mode,
+        bool running,
+        int radioNumber,
+        int stationNumber,
+        string computerName) =>
+        Record(
+            "BANDMAP",
+            stationNumber,
+            computerName,
+            [
+                StationRecord.Written(frequency.Kilohertz),
+                mode.Name,
+                StationRecord.Written(running),
+                Text(radioNumber),
+            ]);
+
+    /// What a message that has arrived means, or null for one this program does
+    /// nothing with.
+    public static ContactUpdate? Read(StationRecord record) => record.Type switch
+    {
+        "QSO" or "RESYNCQSO" => new ContactLogged(
+            QsoRecord.Read(record, 1, record.ComputerName),
+            record.ComputerName),
+        "REEDITQSO" => new ContactReplaced(
+            QsoRecord.Read(record, 2, record.ComputerName),
+            record.Field(1),
+            record.Time(0),
+            record.ComputerName),
+        "QSODELETE" => new ContactDeleted(
+            record.Field(3),
+            record.Field(1),
+            record.Time(0),
+            record.Number(2),
+            record.ComputerName),
+        _ => null,
+    };
+
+    private static StationRecord Record(
+        string type,
+        int stationNumber,
+        string computerName,
+        IReadOnlyList<string> fields) =>
+        new(stationNumber, computerName, type, fields);
+
+    private static string Text(int number) => number.ToString(CultureInfo.InvariantCulture);
+}

src/Nonemm.Network/StationRecord.cs

@@ -0,0 +1,149 @@
+using System.Globalization;
+using System.Text;
+
+namespace Nonemm.Network;
+
+/// One message on the wire between two logging computers, and the frame around
+/// it.
+///
+/// This is not the XML on port 12060 that `ContactMessage` writes. That one is
+/// N1MM talking to other programs — a spotting tool, a score poster. This is
+/// N1MM talking to another copy of itself on port 12070, and the format is
+/// different: fields separated by `%`, the whole message ended with `~`, and
+/// the lot wrapped in `DATA__` and `__DATA`.
+///
+/// A frame reads
+///
+/// ```
+/// DATA__07%SHACK-PC%QSO%2026-09-03 12:34:56%DL1ABC%…~__DATA
+/// ```
+///
+/// which is the sending station's number in two digits, its computer name, the
+/// message type, and then as many fields as the type has. `%` and `~` cannot
+/// appear in a field, so N1MM writes `!` in their place; this does the same, so
+/// a comment with a per-cent sign in it arrives as N1MM would have sent it
+/// rather than splitting the message in half.
+///
+/// Both delimiters are needed. TCP hands over whatever has arrived, which is
+/// half a message as often as two of them, so `~` says where a message ends;
+/// `DATA__` and `__DATA` are N1MM's own and are kept because N1MM looks for
+/// them.
+public sealed record StationRecord(int StationNumber, string ComputerName, string Type, IReadOnlyList<string> Fields)
+{
+    public const string FramePrefix = "DATA__";
+
+    public const string FrameSuffix = "__DATA";
+
+    /// What separates the fields, and what ends a message.
+    public const char FieldSeparator = '%';
+
+    public const char MessageEnd = '~';
+
+    /// What N1MM puts in place of a delimiter that turns up inside a field.
+    public const char Escape = '!';
+
+    private const string TimeFormat = "yyyy-MM-dd HH:mm:ss";
+
+    private const string DateFormat = "yyyy-MM-dd";
+
+    /// The message as it goes on the wire, frame and all.
+    public string ToWire()
+    {
+        StringBuilder text = new();
+        text.Append(FramePrefix);
+        text.Append(StationNumber.ToString("00", CultureInfo.InvariantCulture));
+        text.Append(FieldSeparator);
+        text.Append(Clean(ComputerName.ToUpperInvariant()));
+        text.Append(FieldSeparator);
+        text.Append(Clean(Type));
+        text.Append(FieldSeparator);
+        foreach (string field in Fields)
+        {
+            text.Append(Clean(field));
+            text.Append(FieldSeparator);
+        }
+        text.Append(MessageEnd);
+        text.Append(FrameSuffix);
+        return text.ToString();
+    }
+
+    /// One message read out of `text`, which is whatever has arrived so far.
+    /// Null while no whole message is there yet; the caller keeps the rest and
+    /// adds what arrives next to it.
+    ///
+    /// The frame markers are taken off wherever they stand, because a reader
+    /// that has fallen behind holds several frames at once and the `__DATA`
+    /// that ends one sits in front of the `DATA__` that starts the next.
+    public static StationRecord? Read(ref string text)
+    {
+        int end = text.IndexOf(MessageEnd, StringComparison.Ordinal);
+        if (end < 0)
+        {
+            return null;
+        }
+        string message = text[..end];
+        text = text[(end + 1)..];
+        message = message
+            .Replace(FramePrefix, "", StringComparison.Ordinal)
+            .Replace(FrameSuffix, "", StringComparison.Ordinal);
+        string[] parts = message.Split(FieldSeparator);
+        // the station number, the computer name and the type, and then the
+        // trailing separator leaves one empty field on the end
+        if (parts.Length < 4)
+        {
+            return null;
+        }
+        return new StationRecord(
+            int.TryParse(parts[0], CultureInfo.InvariantCulture, out int number) ? number : 0,
+            parts[1],
+            parts[2].ToUpperInvariant(),
+            parts[3..^1]);
+    }
+
+    /// The field at `at`, or empty when the message is shorter than that. A
+    /// station running another version sends fewer fields than this one reads,
+    /// and a short message is worth more than no message.
+    public string Field(int at) => at >= 0 && at < Fields.Count ? Fields[at] : "";
+
+    public int Number(int at) =>
+        int.TryParse(Field(at), CultureInfo.InvariantCulture, out int value) ? value : 0;
+
+    /// A field written as N1MM's `uNum`, which is two decimal places with a
+    /// dot whatever the machine's own separator is.
+    public double Decimal(int at) =>
+        double.TryParse(Field(at), NumberStyles.Float, CultureInfo.InvariantCulture, out double value)
+            ? value
+            : 0;
+
+    /// N1MM writes a boolean as Visual Basic prints one, which is `True` or
+    /// `False`. A number is read as well, because its own log holds -1 and 0
+    /// for the same thing.
+    public bool Flag(int at) =>
+        Field(at).Trim() is { Length: > 0 } text
+        && (text.Equals("True", StringComparison.OrdinalIgnoreCase) || Number(at) != 0);
+
+    public DateTime Time(int at) =>
+        DateTime.TryParseExact(
+            Field(at),
+            TimeFormat,
+            CultureInfo.InvariantCulture,
+            DateTimeStyles.AssumeUniversal | DateTimeStyles.AdjustToUniversal,
+            out DateTime value)
+            ? value
+            : default;
+
+    public static string Written(DateTime time) =>
+        time.ToString(TimeFormat, CultureInfo.InvariantCulture);
+
+    public static string WrittenDate(DateTime time) =>
+        time.ToString(DateFormat, CultureInfo.InvariantCulture);
+
+    public static string Written(double number) =>
+        number.ToString("0.00", CultureInfo.InvariantCulture);
+
+    public static string Written(bool flag) => flag ? "True" : "False";
+
+    /// A field with the delimiters taken out of it, which is what N1MM sends.
+    private static string Clean(string field) =>
+        field.Replace(FieldSeparator, Escape).Replace(MessageEnd, Escape);
+}

src/Nonemm.Session/MessageExpander.cs

@@ -113,6 +113,9 @@ public static class MessageExpander
             // the digital window's carriage return, which starts a new line on
             // the other station's screen
             "ENTER" => "\r",
+            // the same with a line feed behind it, which is what N1MM's own
+            // RTTY message defaults are written with
+            "ENTERLF" => "\r\n",
             _ => null,
         };
 

src/Nonemm.Session/MessagePlan.cs

@@ -10,6 +10,12 @@ namespace Nonemm.Session;
 /// N1MM's rule, which this follows: an action macro runs before the message is
 /// sent, unless it stands after `{END}`, and then it runs when the message has
 /// finished. Text after `{END}` is dropped, because the message is over.
+///
+/// `{RX}` is the exception. It drops the transmitter, which cannot happen
+/// before the message it stands in has gone out, so it always runs with the
+/// actions that follow the message. N1MM does the same: it takes `{RX}` out of
+/// the text wherever it stands, sends the text, and stops the transmitter
+/// after it.
 public sealed record MessagePlan(
     IReadOnlyList<MessageAction> Before,
     string Text,
@@ -45,7 +51,8 @@ public sealed record MessagePlan(
                 }
                 else if (Action(name) is { } action)
                 {
-                    (ended ? after : before).Add(action);
+                    (ended || action.Command == MessageCommand.ReturnToReceive ? after : before)
+                        .Add(action);
                 }
                 continue;
             }

src/Nonemm.Session/QtcMessages.cs

@@ -1,10 +1,15 @@
+using System.Text;
+
 namespace Nonemm.Session;
 
-/// What a station sends on CW while exchanging QTC traffic, and N1MM's
-/// defaults for the parts an operator can change.
+/// What a station sends while exchanging QTC traffic, and N1MM's defaults for
+/// the parts an operator can change.
 ///
-/// CW only. N1MM sends recorded voice messages on SSB and its digital window's
-/// templates on RTTY, and this program has neither.
+/// CW and RTTY. The two are laid out differently: on CW a line is the three
+/// fields with the operator's spacing between them and each one is keyed by
+/// hand, and on RTTY the fields are joined with hyphens and the whole series
+/// goes out as one message. N1MM plays recordings on SSB, and there is no voice
+/// keyer here.
 public static class QtcMessages
 {
     /// The gap between the fields of a QTC line, written the way N1MM writes
@@ -28,6 +33,59 @@ public static class QtcMessages
     /// What the station reading traffic out asks before it starts.
     public const string AreYouReady = "QRV?";
 
+    /// What goes between the QTC lines on RTTY. N1MM keeps it as a macro rather
+    /// than as characters, so an operator can put a carriage return there,
+    /// which is what its default does.
+    public const string DefaultRttySpacing = "{ENTER}";
+
+    /// What stands in front of the series and what closes it. `{QTC}` stands
+    /// for the header, so the default reads the series out twice for a station
+    /// that missed it the first time.
+    public const string DefaultSendAllHeading = "{ENTERLF}{QTC} {QTC}";
+
+    public const string DefaultSendAllEnding = "{ENTERLF}QSL?? BK DE {MYCALL} K";
+
+    /// One line of traffic on RTTY: the three fields joined with hyphens. N1MM
+    /// writes them this way and offers no setting for it, so neither does this.
+    public static string RttyLine(string time, string call, string number) =>
+        $"{time.Trim()}-{call.Trim()}-{number.Trim()}";
+
+    /// A whole series as one message, which is N1MM's Send All: the heading,
+    /// then every line, then the ending, with `spacing` between them. It keys
+    /// the transmitter itself and drops it at the end, so the operator presses
+    /// one button and the series goes out.
+    ///
+    /// The text macros in it are left as they stand — `{ENTER}`, `{MYCALL}` and
+    /// the rest are filled in by the message expander, the same as in a
+    /// function key. `{QTC}` is filled in here, because only this window knows
+    /// which series is going out.
+    ///
+    /// A series with no lines in it sends nothing.
+    public static string SendAll(
+        string heading,
+        string ending,
+        string spacing,
+        string header,
+        IReadOnlyList<string> lines)
+    {
+        if (lines.Count == 0)
+        {
+            return "";
+        }
+        StringBuilder text = new();
+        text.Append("{TX}").Append(WithHeader(heading, header)).Append(spacing);
+        foreach (string line in lines)
+        {
+            text.Append(line).Append(' ').Append(spacing);
+        }
+        return text.Append(WithHeader(ending, header)).Append("{RX}").ToString();
+    }
+
+    /// One line of a series on its own, for a station that asks for it again.
+    /// It keys and drops the transmitter the same way Send All does.
+    public static string SendOne(string spacing, string line) =>
+        $"{{TX}}{spacing}{line}{spacing}{{RX}}";
+
     public static string Spacing(string setting) =>
         setting.Replace("S", " ", StringComparison.Ordinal);
 
@@ -43,6 +101,11 @@ public static class QtcMessages
     /// The message that ends the exchange, sent whichever way the traffic went.
     /// `{QTC}` in it stands for the header of the series, so `TU {QTC} 73`
     /// goes out as `TU QTC 3/10 73`.
-    public static string Tu(string template, string header) =>
+    public static string Tu(string template, string header) => WithHeader(template, header);
+
+    /// `{QTC}` filled in with the header of the series. It is filled in here
+    /// rather than by the message expander because only the QTC window knows
+    /// which series is going out.
+    private static string WithHeader(string template, string header) =>
         template.Replace("{QTC}", header.Trim().ToUpperInvariant(), StringComparison.OrdinalIgnoreCase);
 }

tests/Nonemm.Contests.Tests/BandChangeRulesTests.cs

@@ -14,6 +14,58 @@ public class BandChangeRulesTests
             TimestampUtc = Start.AddMinutes(minutes).AddSeconds(seconds),
         };
 
+    /// N1MM's fallback, which is the ten-minute rule: a multi-operator entry
+    /// with one transmitter has to stay ten minutes on a band, and nothing caps
+    /// how many times it may move.
+    [Theory]
+    [InlineData("MULTI-ONE", "ONE", 10)]
+    [InlineData("MULTI-OP", "ONE", 10)]
+    [InlineData("MULTI-TWO", "TWO", 10)]
+    [InlineData("MULTI-OP", "TWO", 10)]
+    [InlineData("MULTI-OP", "UNLIMITED", 0)]
+    [InlineData("SINGLE-OP", "ONE", 0)]
+    public void TheCategoryAloneSaysHowLongAStationStaysOnABand(
+        string operatorCategory,
+        string transmitters,
+        int minutes)
+    {
+        BandChangeRules rules = BandChangeRules.ForCategory(new ContestEntry
+        {
+            OperatorCategory = operatorCategory,
+            TransmitterCategory = transmitters,
+        });
+
+        Assert.Equal(TimeSpan.FromMinutes(minutes), rules.MinimumStay);
+        Assert.False(rules.IsCounted);
+    }
+
+    /// A contest that says nothing about band changes still gets the rule for
+    /// the category, which is what N1MM does.
+    [Fact]
+    public void AContestThatSaysNothingStillGetsTheTenMinuteRule()
+    {
+        Contest contest = new Rules.CqWorldWide(ModeCategory.Cw);
+
+        BandChangeRules rules = contest.BandChangesFor(new ContestEntry
+        {
+            OperatorCategory = "MULTI-ONE",
+            TransmitterCategory = "ONE",
+        });
+
+        Assert.True(rules.HasStayTimer);
+        Assert.Equal(TimeSpan.FromMinutes(10), rules.MinimumStay);
+    }
+
+    [Fact]
+    public void ASingleOperatorEntryHasNoBandChangeRule()
+    {
+        Contest contest = new Rules.CqWorldWide(ModeCategory.Cw);
+
+        Assert.Equal(
+            BandChangeRules.None,
+            contest.BandChangesFor(new ContestEntry { OperatorCategory = "SINGLE-OP" }));
+    }
+
     [Fact]
     public void EachMoveToAnotherBandIsOneChange() =>
         Assert.Equal(

tests/Nonemm.Digital.Tests/DigitalEngineSenderTests.cs

@@ -0,0 +1,404 @@
+using System.Text;
+using Nonemm.Digital;
+
+namespace Nonemm.Digital.Tests;
+
+/// The message sender over a digital engine. The engine here reports what it
+/// was given and when it was told to stop; the pace is a baud rate no radio
+/// uses so the tests do not wait for RTTY.
+public class DigitalEngineSenderTests
+{
+    /// Long enough for the stop, which waits for the engine to empty and then
+    /// one character time on top of it.
+    private static readonly TimeSpan Patience = TimeSpan.FromSeconds(15);
+
+    private const double Fast = 1500;
+
+    /// A quarter of a second per character, so a test can catch the engine
+    /// still holding what it was handed.
+    private const double Slow = 30;
+
+    [Fact]
+    public async Task TheEngineDroppingTheTransmitterOnItsOwnDoesNotEndTheMessage()
+    {
+        FakeEngine engine = new();
+        using DigitalEngineSender sender = new(engine, baud: Fast);
+
+        await sender.SendAsync("CQ TEST DE OM5M");
+        await WaitForAsync(() => engine.Sent.Length >= 2);
+        engine.Drop();
+
+        await WaitForAsync(() => engine.Sent == "CQ TEST DE OM5M");
+        Assert.Equal("CQ TEST DE OM5M", engine.Sent);
+    }
+
+    [Fact]
+    public async Task TheDropAfterReturnToReceiveEndsTheMessage()
+    {
+        FakeEngine engine = new();
+        using DigitalEngineSender sender = new(engine, baud: Fast);
+        bool finished = false;
+        sender.Finished += (_, _) => finished = true;
+
+        await sender.SendAsync("CQ TEST");
+        sender.ReturnToReceiveWhenSent();
+        await WaitForAsync(() => engine.Stopped);
+        engine.Drop();
+
+        await WaitForAsync(() => finished && sender.Buffer.Sent.Length == 0);
+        Assert.True(finished);
+        Assert.Equal("", sender.Buffer.Sent);
+        Assert.Equal("CQ TEST", engine.Sent);
+    }
+
+    [Fact]
+    public async Task TheEngineDroppingInTheMiddleOfAMessagePutsTheTransmitterBackUp()
+    {
+        FakeEngine engine = new();
+        using DigitalEngineSender sender = new(engine, baud: Fast);
+
+        sender.Transmit();
+        await sender.SendAsync("CQ TEST DE OM5M");
+        await WaitForAsync(() => engine.Sent.Length >= 2);
+        int keyed = engine.Keyed;
+        engine.Drop();
+
+        await WaitForAsync(() => engine.Keyed > keyed);
+        Assert.Equal(keyed + 1, engine.Keyed);
+    }
+
+    /// MMTTY's stop does nothing when its buffer is empty already, so the
+    /// stop has to reach the engine while it still holds the last characters.
+    [Fact]
+    public async Task ReturnToReceiveReachesTheEngineWhileItStillHoldsTheLastCharacters()
+    {
+        FakeEngine engine = new();
+        using DigitalEngineSender sender = new(engine, baud: Slow);
+        sender.Transmit();
+
+        await sender.SendAsync("AB");
+        sender.ReturnToReceiveWhenSent();
+
+        await WaitForAsync(() => engine.Stopped);
+        Assert.True(engine.Stopped);
+        Assert.True(sender.Buffer.OnAir < 2, "the engine had already transmitted everything");
+    }
+
+    /// MMTTY's stop leaves the transmitter up, so `{RX}` puts the key down
+    /// itself once the engine has transmitted what it holds.
+    [Fact]
+    public async Task ReturnToReceivePutsTheKeyDownAfterTheEngineHasEmptied()
+    {
+        FakeEngine engine = new();
+        using DigitalEngineSender sender = new(engine, baud: Slow);
+        bool finished = false;
+        sender.Finished += (_, _) => finished = true;
+        sender.Transmit();
+
+        await sender.SendAsync("AB");
+        sender.ReturnToReceiveWhenSent();
+
+        await WaitForAsync(() => engine.Released);
+        Assert.True(engine.Released);
+        Assert.Equal("AB", engine.Sent);
+        Assert.False(engine.Aborted, "the message was cut off instead of being let finish");
+        await WaitForAsync(() => finished);
+        Assert.True(finished);
+    }
+
+    /// N1MM's ending: what is left of the message goes to the engine in one
+    /// piece and the engine is asked to stop with its buffer full, because
+    /// `SetMmttyPTT(1)` does nothing at an engine that has been fed one
+    /// character at a time and is therefore nearly empty.
+    [Fact]
+    public async Task ReturnToReceiveHandsTheRestOfTheMessageOverInOnePiece()
+    {
+        FakeEngine engine = new();
+        using DigitalEngineSender sender = new(engine, baud: Slow);
+        sender.Transmit();
+
+        await sender.SendAsync("CQ TEST DE OM5M");
+        sender.ReturnToReceiveWhenSent();
+
+        await WaitForAsync(() => engine.Stopped);
+        Assert.Equal("CQ TEST DE OM5M", engine.Sent);
+        Assert.Equal("", sender.Buffer.Pending);
+        Assert.Contains(engine.Pushes, push => push.Length > 1);
+    }
+
+    /// The stop reaches the engine while the message is still in it, which at
+    /// 30 baud is most of the four seconds the message takes.
+    [Fact]
+    public async Task TheStopReachesTheEngineWhileItStillHoldsTheMessage()
+    {
+        FakeEngine engine = new();
+        using DigitalEngineSender sender = new(engine, baud: Slow);
+        sender.Transmit();
+
+        await sender.SendAsync("CQ TEST DE OM5M");
+        sender.ReturnToReceiveWhenSent();
+
+        await WaitForAsync(() => engine.Stopped);
+        Assert.True(sender.Buffer.OnAir < "CQ TEST DE OM5M".Length,
+            "the engine had transmitted the whole message before it was told to stop");
+    }
+
+    /// A message takes longer to transmit than the engine is given to make
+    /// progress, and the two are not the same thing. Measured as one, the key
+    /// went down partway through a CQ.
+    [Fact]
+    public async Task TheKeyWaitsForTheWholeMessageAndNotForTheStopPatience()
+    {
+        const string message = "CQ CQ DE OM5M OM5M K";
+        FakeEngine engine = new();
+        using DigitalEngineSender sender = new(engine, baud: Slow);
+        sender.Transmit();
+
+        await sender.SendAsync(message);
+        DateTime from = DateTime.UtcNow;
+        sender.ReturnToReceiveWhenSent();
+
+        await WaitForAsync(() => engine.Released);
+        TimeSpan waited = DateTime.UtcNow - from;
+        Assert.Equal(message, engine.Sent);
+        Assert.True(
+            waited > DigitalEngineSender.StopPatience,
+            $"the key went down after {waited.TotalMilliseconds:0} ms, before the message was out");
+    }
+
+    /// Transmitting again while the last message is still ending. The ending
+    /// takes as long as the engine takes to transmit what it holds, and the
+    /// operator can key inside that time; the old ending must not put the key
+    /// down in the middle of the new message.
+    [Fact]
+    public async Task TransmittingAgainAbandonsTheEndingOfTheMessageBeforeIt()
+    {
+        FakeEngine engine = new();
+        using DigitalEngineSender sender = new(engine, baud: Slow);
+        sender.Transmit();
+
+        await sender.SendAsync("CQ");
+        sender.ReturnToReceiveWhenSent();
+        await WaitForAsync(() => engine.Stopped);
+
+        sender.Transmit();
+        await sender.SendAsync("TU");
+        await WaitForAsync(() => engine.Sent == "CQTU");
+
+        Assert.Equal("CQTU", engine.Sent);
+        Assert.False(engine.Released, "the old ending put the key down during the new message");
+        Assert.True(engine.IsTransmitting);
+    }
+
+    /// The same through a message alone, with no `{TX}` in front of it: text is
+    /// the operator asking for the transmitter.
+    [Fact]
+    public async Task AMessageDuringAnEndingKeysAgainAndGoesOut()
+    {
+        FakeEngine engine = new();
+        using DigitalEngineSender sender = new(engine, baud: Slow);
+        sender.Transmit();
+
+        await sender.SendAsync("CQ");
+        sender.ReturnToReceiveWhenSent();
+        await WaitForAsync(() => engine.Stopped);
+        int keyed = engine.Keyed;
+
+        await sender.SendAsync("TU");
+
+        await WaitForAsync(() => engine.Sent == "CQTU");
+        Assert.Equal("CQTU", engine.Sent);
+        Assert.True(engine.Keyed > keyed, "the transmitter was not keyed for the new message");
+        Assert.False(engine.Released);
+    }
+
+    /// The engine dropping ends the message it was ending, and nothing else.
+    [Fact]
+    public async Task TheKeyStaysDownAfterAMessageHasEnded()
+    {
+        FakeEngine engine = new();
+        using DigitalEngineSender sender = new(engine, baud: Fast);
+        sender.Transmit();
+
+        await sender.SendAsync("CQ TEST");
+        sender.ReturnToReceiveWhenSent();
+        await WaitForAsync(() => engine.Stopped);
+        engine.Drop();
+
+        await WaitForAsync(() => !sender.Buffer.IsTransmitting);
+        int keyed = engine.Keyed;
+        await Task.Delay(500);
+        Assert.Equal(keyed, engine.Keyed);
+        Assert.False(engine.IsTransmitting);
+    }
+
+    /// A macro pressed just as the last one finishes. The stop for that message
+    /// is inside the engine, waiting for its buffer to empty, and feeding an
+    /// engine with that standing left MMTTY keyed and transmitting nothing. The
+    /// stop is cleared before the new message is fed.
+    [Fact]
+    public async Task AMessageStartedWhileTheLastOneIsEndingClearsTheStopFirst()
+    {
+        FakeEngine engine = new();
+        using DigitalEngineSender sender = new(engine, baud: Fast);
+        sender.Transmit();
+        await sender.SendAsync("CQ");
+        sender.ReturnToReceiveWhenSent();
+        await WaitForAsync(() => engine.Stopped);
+
+        await sender.SendAsync("TU");
+
+        await WaitForAsync(() => engine.Sent == "CQTU");
+        Assert.Equal("CQTU", engine.Sent);
+        Assert.True(engine.Aborted, "the engine was fed with its stop still pending");
+        Assert.True(engine.IsTransmitting);
+    }
+
+    /// Two macros pressed one after the other keep the transmitter up, so the
+    /// engine never reports the drop that ends a message. The pane still starts
+    /// again on each of them. Text that has gone to the engine cannot be
+    /// edited, and keeping the whole run in the pane left none of it editable.
+    [Fact]
+    public async Task AMessageStartedAfterTheLastOneLeavesOnlyItsOwnTextInThePane()
+    {
+        FakeEngine engine = new();
+        using DigitalEngineSender sender = new(engine, baud: Fast);
+        sender.Transmit();
+        await sender.SendAsync("CQ");
+        sender.ReturnToReceiveWhenSent();
+        await WaitForAsync(() => engine.Stopped);
+
+        await sender.SendAsync("TU");
+
+        await WaitForAsync(() => engine.Sent == "CQTU");
+        Assert.Equal("TU", sender.Buffer.Sent + sender.Buffer.Pending);
+    }
+
+    private static async Task WaitForAsync(Func<bool> ready)
+    {
+        DateTime giveUp = DateTime.UtcNow + Patience;
+        while (!ready() && DateTime.UtcNow < giveUp)
+        {
+            await Task.Delay(2);
+        }
+    }
+
+    /// An engine with no buffer of its own: it takes what it is given, says
+    /// when it was told to stop, and drops the transmitter when the test says
+    /// so.
+    private sealed class FakeEngine : DigitalEngine
+    {
+        private readonly StringBuilder sent = new();
+        private readonly List<string> pushes = [];
+        private readonly Lock gate = new();
+
+        public bool IsConnected => true;
+
+        public bool IsTransmitting { get; private set; } = true;
+
+        /// True once `{RX}` told the engine to stop.
+        public bool Stopped { get; private set; }
+
+        /// True once the engine was stopped the hard way.
+        public bool Aborted { get; private set; }
+
+        /// True once the key was put down, which is what ends a message on
+        /// MMTTY.
+        public bool Released { get; private set; }
+
+        /// How many times the transmitter has been keyed.
+        public int Keyed { get; private set; }
+
+        public string Sent
+        {
+            get
+            {
+                lock (gate)
+                {
+                    return sent.ToString();
+                }
+            }
+        }
+
+        /// Every call, so a test can tell a message pushed in one piece from
+        /// the same text fed one character at a time.
+        public IReadOnlyList<string> Pushes
+        {
+            get
+            {
+                lock (gate)
+                {
+                    return [.. pushes];
+                }
+            }
+        }
+
+        // nothing here decodes or disconnects, so these two are declared to
+        // satisfy the interface and never raised
+        public event EventHandler<string>? Received
+        {
+            add { }
+            remove { }
+        }
+
+        public event EventHandler<bool>? TransmitChanged;
+
+        public event EventHandler<bool>? ConnectionChanged
+        {
+            add { }
+            remove { }
+        }
+
+        /// The engine reporting the transmitter down.
+        public void Drop()
+        {
+            IsTransmitting = false;
+            TransmitChanged?.Invoke(this, false);
+        }
+
+        public Task StartAsync(CancellationToken cancellation = default) => Task.CompletedTask;
+
+        public Task KeyAsync(CancellationToken cancellation = default)
+        {
+            Keyed++;
+            Stopped = false;
+            IsTransmitting = true;
+            return Task.CompletedTask;
+        }
+
+        public Task SendAsync(string text, CancellationToken cancellation = default)
+        {
+            lock (gate)
+            {
+                sent.Append(text);
+                pushes.Add(text);
+            }
+            return Task.CompletedTask;
+        }
+
+        public Task AbortAsync(CancellationToken cancellation = default)
+        {
+            Aborted = true;
+            return Task.CompletedTask;
+        }
+
+        public Task ReturnToReceiveAsync(CancellationToken cancellation = default)
+        {
+            Stopped = true;
+            return Task.CompletedTask;
+        }
+
+        /// MMTTY drops the transmitter as soon as the key goes down, and this
+        /// does the same.
+        public Task ReleaseKeyAsync(CancellationToken cancellation = default)
+        {
+            Released = true;
+            Drop();
+            return Task.CompletedTask;
+        }
+
+        public void Dispose()
+        {
+        }
+    }
+}

tests/Nonemm.Digital.Tests/TypeAheadTests.cs

@@ -3,20 +3,20 @@ using Nonemm.Digital;
 
 namespace Nonemm.Digital.Tests;
 
-/// The text waiting to go out, fed to the engine a few characters at a time.
-/// The pump is run at a baud rate no radio uses so the tests do not wait for
-/// RTTY.
+/// The text waiting to go out, fed to the engine one character per character
+/// time. The feeder is run at a baud rate no radio uses so the tests do not
+/// wait for RTTY.
 public class TypeAheadTests
 {
     private static readonly TimeSpan Patience = TimeSpan.FromSeconds(5);
 
     /// Fast enough that a message goes out in milliseconds, slow enough that a
-    /// test can still catch the pump partway through.
-    private const double Fast = 7500;
+    /// test can still catch the feeder partway through.
+    private const double Fast = 1500;
 
-    /// A character time of 100 ms, so a test can tell the characters sent ahead
-    /// from the ones that wait for the engine.
-    private const double Slow = 75;
+    /// A quarter of a second per character, which is long enough to read what
+    /// the engine is holding before it has transmitted it.
+    private const double Slow = 30;
 
     private readonly StringBuilder went = new();
 
@@ -31,6 +31,7 @@ public class TypeAheadTests
         }
     }
 
+    /// An engine with no count of its own, so the clock alone paces it.
     private TypeAhead Buffer(double baud = Fast) =>
         new(
             (character, _) =>
@@ -68,7 +69,7 @@ public class TypeAheadTests
     [Fact]
     public async Task WhatHasGoneOutIsNotPendingAnyMore()
     {
-        using TypeAhead buffer = Buffer(baud: 40);
+        using TypeAhead buffer = Buffer();
 
         buffer.Append("CQ TEST DE OM5M");
 
@@ -77,52 +78,72 @@ public class TypeAheadTests
         Assert.Equal("CQ TEST DE OM5M", buffer.Sent + buffer.Pending);
     }
 
+    /// The pane is edited as a whole: what has gone out and what is still to
+    /// go. What has gone out is dropped, and the rest replaces what was
+    /// waiting.
     [Fact]
     public async Task TheOperatorRewritesWhatHasNotGoneOut()
     {
-        using TypeAhead buffer = Buffer(baud: 60);
+        using TypeAhead buffer = Buffer();
         buffer.Append("OM5X 599 001");
-
         await WaitForAsync(() => buffer.Sent.Length >= 5);
-        buffer.Rewrite("599 002", TypeAhead.NoCursor);
+
+        buffer.Edit(buffer.Sent + "599 002");
 
         await WaitForAsync(() => buffer.Pending.Length == 0);
         Assert.EndsWith("599 002", Sent, StringComparison.Ordinal);
         Assert.DoesNotContain("001", Sent, StringComparison.Ordinal);
     }
 
+    /// What is typed stays off the air until the transmitter is keyed.
     [Fact]
-    public async Task NothingGoesOutFromBehindTheCursor()
+    public async Task NothingTypedGoesOutBeforeTheTransmitterIsKeyed()
     {
         using TypeAhead buffer = Buffer();
 
-        buffer.Rewrite("CQ TEST", 2);
+        buffer.Edit("CQ TEST");
 
-        await WaitForAsync(() => Sent.Length == 2);
         await Task.Delay(50);
-        Assert.Equal("CQ", Sent);
-        Assert.Equal(" TEST", buffer.Pending);
+        Assert.Equal("", Sent);
+        Assert.Equal("CQ TEST", buffer.Pending);
     }
 
     [Fact]
-    public async Task TheRestGoesOutWhenTheCursorMovesOn()
+    public async Task TheWholePaneGoesOutOnTransmit()
     {
         using TypeAhead buffer = Buffer();
-        buffer.Rewrite("CQ TEST", 2);
-        await WaitForAsync(() => Sent.Length == 2);
+        buffer.Edit("CQ TEST");
+        await Task.Delay(20);
 
-        buffer.Cursor = TypeAhead.NoCursor;
+        buffer.Transmit();
 
         await WaitForAsync(() => Sent == "CQ TEST");
         Assert.Equal("CQ TEST", Sent);
     }
 
+    /// The workflow the pane is for: keyed up, what is typed goes out behind
+    /// what is already going, with no second key to press.
     [Fact]
-    public async Task TextAddedBehindTheCursorStillGoesOut()
+    public async Task TextTypedWhileTransmittingGoesOutBehindIt()
     {
         using TypeAhead buffer = Buffer();
-        buffer.Rewrite("CQ", 2);
-        await WaitForAsync(() => Sent == "CQ");
+        buffer.Edit("CQ ");
+        buffer.Transmit();
+        await WaitForAsync(() => Sent == "CQ ");
+
+        buffer.Edit(buffer.Sent + "DE OM5M");
+
+        await WaitForAsync(() => Sent == "CQ DE OM5M");
+        Assert.Equal("CQ DE OM5M", Sent);
+    }
+
+    /// A function key pressed while the operator is typing goes on the end of
+    /// what has been typed, and keys the transmitter itself.
+    [Fact]
+    public async Task AMacroFollowsWhatWasTypedAndKeysTheTransmitter()
+    {
+        using TypeAhead buffer = Buffer();
+        buffer.Edit("CQ");
 
         buffer.Append(" TEST");
 
@@ -130,12 +151,200 @@ public class TypeAheadTests
         Assert.Equal("CQ TEST", Sent);
     }
 
+    /// Text taken out of the pane before it goes out is never transmitted.
+    [Fact]
+    public async Task TextDeletedBeforeItGoesOutIsNotSent()
+    {
+        using TypeAhead buffer = Buffer();
+        buffer.Edit("CQ TEST DE OM5X");
+        buffer.Transmit();
+        await WaitForAsync(() => buffer.Sent.Length >= 3);
+
+        buffer.Edit(buffer.Sent + "DE OM5M");
+
+        await WaitForAsync(() => buffer.Pending.Length == 0);
+        Assert.EndsWith("DE OM5M", Sent, StringComparison.Ordinal);
+        Assert.DoesNotContain("OM5X", Sent, StringComparison.Ordinal);
+    }
+
+    /// The transmitter dropping ends the message: what went out is cleared off
+    /// the pane and nothing goes out again until the transmitter is keyed.
+    [Fact]
+    public async Task TheEndOfAMessageClearsWhatWentOutAndShutsTheGate()
+    {
+        using TypeAhead buffer = Buffer();
+        buffer.Append("TU");
+        await WaitForAsync(() => Sent == "TU");
+
+        buffer.Ended();
+        buffer.Edit(" NEXT");
+
+        Assert.Equal("", buffer.Sent);
+        Assert.Equal(" NEXT", buffer.Pending);
+        await Task.Delay(50);
+        Assert.Equal("TU", Sent);
+    }
+
+    /// `Aired` is the end of the message: the engine has been given
+    /// everything and has transmitted it too.
+    [Fact]
+    public async Task DrainedComesWhenTheMessageHasGoneOut()
+    {
+        using TypeAhead buffer = Buffer();
+        int drained = 0;
+        buffer.Aired += (_, _) => Interlocked.Increment(ref drained);
+        buffer.Append("CQ TEST DE OM5M");
+
+        Assert.Equal(0, Volatile.Read(ref drained));
+        await WaitForAsync(() => Volatile.Read(ref drained) == 1);
+        Assert.Equal("CQ TEST DE OM5M", Sent);
+    }
+
+    /// The red text in the pane is `OnAir`, and it ran ahead of the
+    /// transmission because it advanced one character per symbol time. A digit
+    /// in a callsign costs a shift to figures and the letter after it a shift
+    /// back, so `OM5M` is six symbols and not four.
+    [Fact]
+    public async Task WhatIsOnTheAirIsPricedInSymbolsAndNotInCharacters()
+    {
+        using TypeAhead buffer = Buffer();
+        buffer.Append("OM5M");
+
+        // four symbol times is what the old clock allowed the whole callsign
+        await WaitForAsync(() => buffer.Sent.Length == 4);
+        await Task.Delay(buffer.SymbolTime * 4.5);
+
+        Assert.True(
+            buffer.OnAir < 4,
+            $"{buffer.OnAir} of 4 characters were called transmitted in four symbol times");
+    }
+
+    /// Letters with nothing to shift for cost one symbol each, so the two
+    /// agree there.
+    [Fact]
+    public async Task PlainLettersGoOutAtOneSymbolEach()
+    {
+        using TypeAhead buffer = Buffer();
+        buffer.Append("CQ TEST");
+
+        await WaitForAsync(() => buffer.OnAir >= 4);
+        Assert.True(buffer.OnAir <= 7);
+    }
+
+    /// The engine's count is what says how much is left, whatever the clock
+    /// thinks. An engine that says it is still holding the whole message keeps
+    /// the pane from marking any of it as gone out.
+    [Fact]
+    public async Task TheEnginesOwnCountHoldsThePaneBack()
+    {
+        FakeEngine engine = new() { Answer = 30 };
+        using TypeAhead buffer = new(engine, Fast);
+        buffer.Append("CQ TEST DE OM5M");
+
+        await WaitForAsync(() => buffer.Sent.Length == 15);
+        await Task.Delay(buffer.SymbolTime * 20);
+
+        Assert.Equal(0, buffer.OnAir);
+    }
+
+    /// And it lets go as the count falls.
+    [Fact]
+    public async Task ThePaneCatchesUpAsTheCountFalls()
+    {
+        FakeEngine engine = new() { Answer = 30 };
+        using TypeAhead buffer = new(engine, Fast);
+        buffer.Append("CQ TEST DE OM5M");
+        await WaitForAsync(() => buffer.Sent.Length == 15);
+
+        engine.Answer = 0;
+
+        await WaitForAsync(() => buffer.OnAir == 15);
+        Assert.Equal(15, buffer.OnAir);
+    }
+
+    /// The engine's count is what says how much has gone out. It answers in
+    /// symbols and they fall as they are transmitted, so what it drops between
+    /// two answers is what went on the air between them.
+    [Fact]
+    public async Task WhatTheEngineHasTransmittedComesFromItsOwnCount()
+    {
+        FakeEngine engine = new() { Answer = 20 };
+        using TypeAhead buffer = new(engine, Fast);
+        buffer.Append("CQ TEST DE OM5M");
+        await WaitForAsync(() => buffer.Sent.Length == 15);
+        await WaitForAsync(() => buffer.EngineHolds == 20);
+
+        // four symbols out of the engine, which is the first four characters
+        engine.Answer = 16;
+
+        await WaitForAsync(() => buffer.OnAir == 4);
+        Assert.Equal(4, buffer.OnAir);
+    }
+
+    /// A count that has not moved leaves the pane where it is, however long
+    /// the clock runs.
+    [Fact]
+    public async Task ACountThatDoesNotMoveHoldsThePaneStill()
+    {
+        FakeEngine engine = new() { Answer = 20 };
+        using TypeAhead buffer = new(engine, Fast);
+        buffer.Append("CQ TEST DE OM5M");
+        await WaitForAsync(() => buffer.Sent.Length == 15);
+
+        await Task.Delay(buffer.SymbolTime * 20);
+
+        Assert.Equal(0, buffer.OnAir);
+    }
+
+    /// An empty engine has transmitted everything it was given, whatever the
+    /// symbols added up to along the way. A wrong guess about the shift
+    /// corrects itself at the end of every message rather than accumulating.
+    [Fact]
+    public async Task AnEmptyEngineHasTransmittedEverything()
+    {
+        FakeEngine engine = new() { Answer = 20 };
+        using TypeAhead buffer = new(engine, Fast);
+        buffer.Append("CQ TEST DE OM5M");
+        await WaitForAsync(() => buffer.Sent.Length == 15);
+        await WaitForAsync(() => buffer.EngineHolds == 20);
+
+        engine.Answer = 0;
+
+        await WaitForAsync(() => buffer.OnAir == 15);
+        Assert.Equal(15, buffer.OnAir);
+    }
+
+    /// The count reads 0 for the first 150 ms after a push, before the engine
+    /// has caught up with what it was given. That 0 does not mean the message
+    /// is over.
+    [Fact]
+    public async Task ACountThatHasNotCaughtUpIsNotAnEmptyEngine()
+    {
+        FakeEngine engine = new() { Answer = 0 };
+        using TypeAhead buffer = new(engine, Fast);
+        buffer.Append("CQ TEST DE OM5M");
+
+        await WaitForAsync(() => buffer.Sent.Length == 15);
+        await Task.Delay(buffer.SymbolTime * 8);
+
+        Assert.Equal(0, buffer.OnAir);
+    }
+
+    /// Nothing has been given to the engine, so nothing is outstanding.
+    [Fact]
+    public void NothingIsOutstandingWhenNothingIsGoingOut()
+    {
+        using TypeAhead buffer = Buffer();
+
+        Assert.Equal(0, buffer.Outstanding);
+    }
+
     [Fact]
     public async Task TheBufferSaysWhenTheMessageHasGoneOut()
     {
         using TypeAhead buffer = Buffer();
         int drained = 0;
-        buffer.Drained += (_, _) => Interlocked.Increment(ref drained);
+        buffer.Aired += (_, _) => Interlocked.Increment(ref drained);
 
         buffer.Append("TU");
 
@@ -146,7 +355,7 @@ public class TypeAheadTests
     [Fact]
     public async Task AMessageAddedWhileOneIsGoingOutFollowsIt()
     {
-        using TypeAhead buffer = Buffer(baud: 400);
+        using TypeAhead buffer = Buffer();
         buffer.Append("CQ ");
 
         buffer.Append("DE OM5M");
@@ -158,7 +367,7 @@ public class TypeAheadTests
     [Fact]
     public async Task DroppingLeavesWhatHasAlreadyGone()
     {
-        using TypeAhead buffer = Buffer(baud: 60);
+        using TypeAhead buffer = Buffer();
         buffer.Append("CQ TEST DE OM5M");
         await WaitForAsync(() => buffer.Sent.Length >= 3);
 
@@ -174,7 +383,7 @@ public class TypeAheadTests
     [Fact]
     public void ClearingEmptiesBothHalves()
     {
-        using TypeAhead buffer = Buffer(baud: 1);
+        using TypeAhead buffer = Buffer();
         buffer.Append("CQ TEST");
 
         buffer.Clear();
@@ -183,57 +392,178 @@ public class TypeAheadTests
         Assert.Equal("", buffer.Sent);
     }
 
-    /// The engine is given the second character before it has transmitted the
-    /// first, so it has one in hand when the first is done. An engine left with
-    /// an empty buffer transmits idle instead, and that idle is added to how
-    /// long the message takes.
+    /// MMTTY answers 0 while it is still transmitting: for the first 150 ms
+    /// after a push, and for as long as it holds a word on Word out. A feeder
+    /// that read that as room fed on it and ran ahead of the air, so the clock
+    /// is the pace and the count only ever holds it back. This one runs at the
+    /// real RTTY speed, because the number that matters is how many characters
+    /// go out in a second at 45.45 baud.
     [Fact]
-    public async Task TheEngineIsKeptOneCharacterAhead()
+    public async Task AnEngineAnsweringZeroDoesNotPullThePumpForward()
     {
-        using TypeAhead buffer = Buffer(baud: Slow);
+        FakeEngine engine = new() { Answer = 0 };
+        using TypeAhead buffer = new(engine, baud: TypeAhead.DefaultBaud);
+
+        buffer.Append("CQ TEST DE OM5M OM5M K");
+
+        await Task.Delay(TimeSpan.FromSeconds(1));
+        // 45.45 baud is 6.06 characters a second, plus the `Ahead` characters
+        // the engine is primed with and one for the poll granularity
+        Assert.InRange(engine.Waiting.Length, 7, 9);
+    }
+
+    /// The engine is given the next character before it has transmitted the one
+    /// on the air, so it has one in hand when that one finishes. An engine left
+    /// with an empty buffer transmits the idle tone instead, which is audible
+    /// between the characters of a long word.
+    [Fact]
+    public async Task TheEngineIsNeverLeftWithNothingInHand()
+    {
+        FakeEngine engine = new();
+        using TypeAhead buffer = new(engine, baud: TypeAhead.DefaultBaud);
+        // transmits one character every character time, as a real engine does
+        using Timer transmitting = new(_ => engine.Transmit(1), null, 165, 165);
+
+        buffer.Append(new string('N', 30));
+
+        int emptied = 0;
+        for (int look = 0; look < 60; look++)
+        {
+            await Task.Delay(30);
+            if (engine.Waiting.Length == 0 && buffer.Pending.Length > 0)
+            {
+                emptied++;
+            }
+        }
+        Assert.Equal(0, emptied);
+    }
+
+    /// The engine transmitting makes room, and the feeder fills it. Nothing
+    /// here guesses how fast the engine is going: it says, and it is believed.
+    [Fact]
+    public async Task TheEngineIsFedAsItMakesRoom()
+    {
+        FakeEngine engine = new();
+        using TypeAhead buffer = new(engine);
+        using Timer transmitting = new(_ => engine.Transmit(1), null, 0, 10);
+
+        buffer.Append("CQ TEST DE OM5M");
+
+        await WaitForAsync(() => engine.Transmitted == "CQ TEST DE OM5M");
+        Assert.Equal("CQ TEST DE OM5M", engine.Transmitted);
+    }
+
+    /// An engine that will not say how much it holds is fed anyway: there is
+    /// nothing to pace against, so it gets the message.
+    [Fact]
+    public async Task AnEngineThatWillNotCountIsFedAnyway()
+    {
+        FakeEngine engine = new() { Counts = false };
+        using TypeAhead buffer = new(engine, baud: Fast);
 
         buffer.Append("CQ TEST");
 
-        await WaitForAsync(() => Sent.Length >= 2);
-        Assert.Equal("CQ", Sent);
-    }
-
-    /// Only the lead goes out ahead. The rest waits, which is what leaves it
-    /// where the operator can still change it.
-    [Fact]
-    public async Task NoMoreThanTheLeadGoesToTheEngineAtOnce()
-    {
-        using TypeAhead buffer = Buffer(baud: Slow);
-
-        buffer.Append("CQ TEST");
-
-        await WaitForAsync(() => Sent.Length >= 2);
-        await Task.Delay(20);
-        Assert.Equal(2, Sent.Length);
-    }
-
-    /// The estimate of what the engine still holds is only an estimate. An
-    /// engine that says it has stopped transmitting has an empty buffer, and
-    /// the next character goes to it at once rather than a character time
-    /// later.
-    [Fact]
-    public async Task AnIdleEngineIsFedWithoutWaiting()
-    {
-        using TypeAhead buffer = Buffer(baud: Slow);
-        buffer.Append("CQ TEST");
-        await WaitForAsync(() => Sent.Length >= 2);
-
-        buffer.EngineIdle();
-
-        await WaitForAsync(() => Sent.Length >= 4);
-        Assert.Equal("CQ T", Sent);
+        await WaitForAsync(() => engine.Waiting == "CQ TEST");
+        Assert.False(buffer.Counts);
+        Assert.Equal("CQ TEST", engine.Waiting);
     }
 
     [Fact]
-    public void ACharacterTakesAsLongAsTheBaudRateSays()
+    public async Task WhatTheEngineIsStillHoldingHasNotGoneOutYet()
     {
-        using TypeAhead buffer = Buffer(baud: TypeAhead.DefaultBaud);
+        using TypeAhead buffer = Buffer(Slow);
 
-        Assert.Equal(165, buffer.CharacterTime.TotalMilliseconds, 0.5);
+        buffer.Append("AB");
+
+        await WaitForAsync(() => buffer.Sent == "AB");
+        Assert.Equal(0, buffer.OnAir);
+        await WaitForAsync(() => buffer.OnAir == 2);
+        Assert.Equal(2, buffer.OnAir);
+    }
+
+    [Fact]
+    public async Task TheMessageIsNotOverUntilTheEngineHasTransmittedWhatItHolds()
+    {
+        using TypeAhead buffer = Buffer(Slow);
+        int atTheEnd = -1;
+        buffer.Aired += (_, _) => atTheEnd = buffer.OnAir;
+
+        buffer.Append("AB");
+
+        await WaitForAsync(() => atTheEnd >= 0);
+        Assert.Equal(2, atTheEnd);
+    }
+
+    /// An engine with a buffer of its own: it takes characters, holds them
+    /// until the test says they have been transmitted, and says how many it
+    /// has. `Counts` false is the engine that will not answer, and `Answer` is
+    /// the engine that answers a number of its own rather than what it holds:
+    /// MMTTY reads 0 while it is still transmitting, and reads high while it is
+    /// holding a word.
+    private sealed class FakeEngine : EngineBuffer
+    {
+        private readonly Lock gate = new();
+        private readonly StringBuilder waiting = new();
+        private readonly StringBuilder transmitted = new();
+        public bool Counts { get; init; } = true;
+
+        /// What to answer instead of what is really waiting, or -1 to answer
+        /// what is waiting. A test can move it while the buffer is running, the
+        /// way a real engine's count falls as it transmits.
+        public int Answer { get; set; } = -1;
+
+        public event EventHandler<int>? Buffered;
+
+        public string Waiting
+        {
+            get
+            {
+                lock (gate)
+                {
+                    return waiting.ToString();
+                }
+            }
+        }
+
+        public string Transmitted
+        {
+            get
+            {
+                lock (gate)
+                {
+                    return transmitted.ToString();
+                }
+            }
+        }
+
+        public Task TypeAsync(char character, CancellationToken cancellation = default)
+        {
+            lock (gate)
+            {
+                waiting.Append(character);
+            }
+            return Task.CompletedTask;
+        }
+
+        public Task AskBufferedAsync(string property = "", CancellationToken cancellation = default)
+        {
+            int left;
+            lock (gate)
+            {
+                left = Counts ? (Answer >= 0 ? Answer : waiting.Length) : -1;
+            }
+            Buffered?.Invoke(this, left);
+            return Task.CompletedTask;
+        }
+
+        public void Transmit(int count)
+        {
+            lock (gate)
+            {
+                int going = Math.Min(count, waiting.Length);
+                transmitted.Append(waiting.ToString(0, going));
+                waiting.Remove(0, going);
+            }
+        }
     }
 }

tests/Nonemm.Network.Tests/StationLinkTests.cs

@@ -0,0 +1,188 @@
+using System.Net;
+using System.Net.Sockets;
+using Nonemm.Core;
+
+namespace Nonemm.Network.Tests;
+
+/// Two links talking to each other over loopback. The stations are named by
+/// hand rather than found by a beacon: a broadcast test would depend on the
+/// machine's interfaces, and what is being tested here is what happens after
+/// two stations have found each other.
+public class StationLinkTests
+{
+    private const string Version = "1.0.11364";
+
+    private static readonly TimeSpan Patience = TimeSpan.FromSeconds(10);
+
+    [Fact]
+    public async Task AContactReachesTheOtherStation()
+    {
+        (int onePort, int twoPort) = SparePorts();
+        using StationLink one = new("RUN-PC", Version, stationNumber: 1, port: onePort);
+        using StationLink two = new("MULT-PC", Version, stationNumber: 2, port: twoPort);
+        one.Start();
+        two.Start();
+        ContactUpdate? arrived = null;
+        two.UpdateArrived += (_, update) => arrived = update;
+        one.AddStation("MULT-PC", "127.0.0.1", twoPort);
+        await WaitForAsync(() => one.Stations.Any(s => s is { IsMine: false, IsConnected: true }));
+
+        await one.SendLoggedAsync(Contact());
+
+        await WaitForAsync(() => arrived is not null);
+        ContactLogged logged = Assert.IsType<ContactLogged>(arrived);
+        Assert.Equal("G3XYZ", logged.Qso.Call.Text);
+        Assert.Equal("RUN-PC", logged.Qso.StationName);
+        Assert.Equal(1, logged.Qso.NetworkedComputerNumber);
+    }
+
+    /// The first message on a new connection says which station of the entry
+    /// the sender is, so the other end can show its number.
+    [Fact]
+    public async Task ConnectingSaysWhichStationNumberThisIs()
+    {
+        (int onePort, int twoPort) = SparePorts();
+        using StationLink one = new("RUN-PC", Version, stationNumber: 1, port: onePort);
+        using StationLink two = new("MULT-PC", Version, stationNumber: 7, port: twoPort);
+        one.Start();
+        two.Start();
+        one.AddStation("MULT-PC", "127.0.0.1", twoPort);
+        await WaitForAsync(() =>
+            two.Stations.FirstOrDefault(s => s.ComputerName == "RUN-PC")?.StationNumber == 1);
+
+        NetworkedStation? seen = two.Stations.FirstOrDefault(s => s.ComputerName == "RUN-PC");
+        Assert.Equal(1, seen?.StationNumber);
+        Assert.Equal("IAM", seen?.LastMessage);
+    }
+
+    /// The band a station is on is what the status window shows and what the
+    /// band-change rule counts.
+    [Fact]
+    public async Task WhereAStationIsReachesTheOthers()
+    {
+        (int onePort, int twoPort) = SparePorts();
+        using StationLink one = new("RUN-PC", Version, stationNumber: 1, port: onePort);
+        using StationLink two = new("MULT-PC", Version, stationNumber: 2, port: twoPort);
+        one.Start();
+        two.Start();
+        one.AddStation("MULT-PC", "127.0.0.1", twoPort);
+        await WaitForAsync(() => one.Stations.Any(s => s is { IsMine: false, IsConnected: true }));
+
+        await one.SendBandAsync(Frequency.FromKilohertz(21_025), Modes.Cw, running: true, radioNumber: 1);
+
+        await WaitForAsync(() =>
+            two.Stations.FirstOrDefault(s => s.ComputerName == "RUN-PC")?.Band?.Name == "15M");
+        NetworkedStation? seen = two.Stations.FirstOrDefault(s => s.ComputerName == "RUN-PC");
+        Assert.Equal("15M", seen?.Band?.Name);
+        Assert.Equal("CW", seen?.Mode?.Name);
+        Assert.True(seen?.IsRunning);
+    }
+
+    [Fact]
+    public async Task ChatReachesTheOtherOperator()
+    {
+        (int onePort, int twoPort) = SparePorts();
+        using StationLink one = new("RUN-PC", Version, stationNumber: 1, port: onePort);
+        using StationLink two = new("MULT-PC", Version, stationNumber: 2, port: twoPort);
+        one.Start();
+        two.Start();
+        string? said = null;
+        two.TalkArrived += (_, text) => said = text;
+        one.AddStation("MULT-PC", "127.0.0.1", twoPort);
+        await WaitForAsync(() => one.Stations.Any(s => s is { IsMine: false, IsConnected: true }));
+
+        await one.SendTalkAsync("qsy 20");
+
+        await WaitForAsync(() => said is not null);
+        Assert.Equal("[RUN-PC] qsy 20", said);
+    }
+
+    /// An echo request is answered without anybody asking, which is how N1MM
+    /// measures the round trip to each station.
+    [Fact]
+    public async Task AnEchoRequestIsAnswered()
+    {
+        (int onePort, int twoPort) = SparePorts();
+        using StationLink one = new("RUN-PC", Version, stationNumber: 1, port: onePort);
+        using StationLink two = new("MULT-PC", Version, stationNumber: 2, port: twoPort);
+        one.Start();
+        two.Start();
+        one.AddStation("MULT-PC", "127.0.0.1", twoPort);
+        two.AddStation("RUN-PC", "127.0.0.1", onePort);
+        await WaitForAsync(() =>
+            one.Stations.Any(s => s is { IsMine: false, IsConnected: true })
+            && two.Stations.Any(s => s is { IsMine: false, IsConnected: true }));
+
+        await one.SendEchoRequestAsync();
+
+        await WaitForAsync(() =>
+            one.Stations.FirstOrDefault(s => s.ComputerName == "MULT-PC")?.EchoTime is not null);
+        Assert.NotNull(one.Stations.FirstOrDefault(s => s.ComputerName == "MULT-PC")?.EchoTime);
+    }
+
+    /// This computer is in its own list, the way N1MM shows it, so an operator
+    /// can read its station number and version off the same window.
+    [Fact]
+    public void ThisComputerIsInItsOwnStationList()
+    {
+        using StationLink link = new("RUN-PC", Version, stationNumber: 3, port: SparePorts().One);
+
+        NetworkedStation mine = Assert.Single(link.Stations);
+
+        Assert.True(mine.IsMine);
+        Assert.Equal("RUN-PC", mine.ComputerName);
+        Assert.Equal(3, mine.StationNumber);
+        Assert.Equal(Version, mine.Version);
+    }
+
+    /// A message that reached nobody says so, rather than looking as though it
+    /// went out.
+    [Fact]
+    public async Task SendingWithNoStationsConnectedReachesNobody()
+    {
+        using StationLink link = new("RUN-PC", Version, port: SparePorts().One);
+        link.Start();
+
+        Assert.Equal(0, await link.SendLoggedAsync(Contact()));
+    }
+
+    private static async Task WaitForAsync(Func<bool> ready)
+    {
+        DateTime giveUp = DateTime.UtcNow + Patience;
+        while (!ready() && DateTime.UtcNow < giveUp)
+        {
+            await Task.Delay(5);
+        }
+    }
+
+    /// Two ports nothing else is on, so the tests do not fight each other or
+    /// the N1MM the machine may be running.
+    ///
+    /// Both are taken before either is let go. Asking twice in a row does not
+    /// work: the second ask often gets the port the first one has just given
+    /// back, and two links on one port leaves the second without a listener.
+    private static (int One, int Two) SparePorts()
+    {
+        TcpListener first = new(IPAddress.Loopback, 0);
+        TcpListener second = new(IPAddress.Loopback, 0);
+        first.Start();
+        second.Start();
+        int one = ((IPEndPoint)first.LocalEndpoint).Port;
+        int two = ((IPEndPoint)second.LocalEndpoint).Port;
+        first.Stop();
+        second.Stop();
+        return (one, two);
+    }
+
+    private static Qso Contact() => new()
+    {
+        Id = Qso.NewId(),
+        TimestampUtc = new DateTime(2026, 9, 3, 12, 34, 56, DateTimeKind.Utc),
+        Call = Callsign.Parse("G3XYZ"),
+        Frequency = Frequency.FromKilohertz(14_025),
+        Mode = Modes.Cw,
+        ContestName = "CQWW",
+        ContestNumber = 2,
+        SentNumber = 41,
+    };
+}

tests/Nonemm.Network.Tests/StationRecordTests.cs

@@ -0,0 +1,271 @@
+using Nonemm.Core;
+
+namespace Nonemm.Network.Tests;
+
+/// The computer-to-computer protocol on port 12070. It is N1MM's own, so the
+/// tests are about the bytes: a field in the wrong place is a contact with the
+/// callsign in the comment.
+public class StationRecordTests
+{
+    [Fact]
+    public void AMessageIsFramedTheWayN1MmFramesIt()
+    {
+        StationRecord record = new(7, "shack-pc", "IAM", ["7"]);
+
+        Assert.Equal("DATA__07%SHACK-PC%IAM%7%~__DATA", record.ToWire());
+    }
+
+    [Fact]
+    public void AMessageReadsBackTheWayItWasWritten()
+    {
+        string wire = new StationRecord(3, "SHACK-PC", "TALK", ["hello", "there"]).ToWire();
+
+        StationRecord? read = StationRecord.Read(ref wire);
+
+        Assert.NotNull(read);
+        Assert.Equal(3, read.StationNumber);
+        Assert.Equal("SHACK-PC", read.ComputerName);
+        Assert.Equal("TALK", read.Type);
+        Assert.Equal(["hello", "there"], read.Fields);
+    }
+
+    /// TCP hands over whatever has arrived, which is half a message as often as
+    /// a whole one.
+    [Fact]
+    public void HalfAMessageIsHeldUntilTheRestArrives()
+    {
+        string whole = new StationRecord(1, "PC", "IAM", ["1"]).ToWire();
+        string text = whole[..10];
+
+        Assert.Null(StationRecord.Read(ref text));
+
+        text += whole[10..];
+
+        Assert.NotNull(StationRecord.Read(ref text));
+    }
+
+    [Fact]
+    public void TwoMessagesInOneReadAreBothFound()
+    {
+        string text = new StationRecord(1, "PC", "IAM", ["1"]).ToWire()
+            + new StationRecord(2, "OTHER", "IAM", ["2"]).ToWire();
+
+        StationRecord? first = StationRecord.Read(ref text);
+        StationRecord? second = StationRecord.Read(ref text);
+
+        Assert.Equal("PC", first?.ComputerName);
+        Assert.Equal("OTHER", second?.ComputerName);
+        Assert.Null(StationRecord.Read(ref text));
+    }
+
+    /// N1MM writes `!` in place of a delimiter that turns up in a field, so a
+    /// comment with a per-cent sign in it does not split the message.
+    [Fact]
+    public void ADelimiterInsideAFieldIsReplaced()
+    {
+        StationRecord record = new(1, "PC", "TALK", ["100% sure~ok"]);
+
+        string wire = record.ToWire();
+        StationRecord? read = StationRecord.Read(ref wire);
+
+        Assert.Equal("100! sure!ok", read?.Field(0));
+    }
+
+    [Fact]
+    public void AFieldPastTheEndOfTheMessageIsEmpty()
+    {
+        StationRecord record = new(1, "PC", "IAM", ["1"]);
+
+        Assert.Equal("", record.Field(9));
+        Assert.Equal(0, record.Number(9));
+        Assert.False(record.Flag(9));
+    }
+
+    /// N1MM writes a boolean as Visual Basic prints one. Its own log holds -1
+    /// and 0 for the same thing, so both are read.
+    [Theory]
+    [InlineData("True", true)]
+    [InlineData("False", false)]
+    [InlineData("-1", true)]
+    [InlineData("1", true)]
+    [InlineData("0", false)]
+    [InlineData("", false)]
+    public void BooleansAreReadTheWayN1MmWritesThem(string field, bool expected)
+    {
+        StationRecord record = new(1, "PC", "XMIT", [field]);
+
+        Assert.Equal(expected, record.Flag(0));
+    }
+
+    [Fact]
+    public void ABeaconCarriesTheSixFieldsN1MmCounts()
+    {
+        StationBeacon beacon = new("SHACK-PC", "192.168.1.5", 12070, "1.0.11364", "OM3KFF");
+
+        Assert.Equal("SHACK-PC%192.168.1.5%12070%1.0.11364%OM3KFF%%", beacon.ToWire());
+        Assert.Equal(beacon, StationBeacon.Read(beacon.ToWire()));
+    }
+
+    /// N1MM splits the beacon on `%` and refuses anything that is not seven
+    /// long, which is how it turns away a station on an older version.
+    [Fact]
+    public void ABeaconWithTheWrongNumberOfFieldsIsNotABeacon()
+    {
+        Assert.Null(StationBeacon.Read("SHACK-PC%192.168.1.5%12070"));
+    }
+
+    /// The port-12060 contact broadcast pointed at the wrong port. N1MM says
+    /// so out loud; this drops it.
+    [Fact]
+    public void XmlOnTheBeaconPortIsNotABeacon()
+    {
+        Assert.Null(StationBeacon.Read("<?xml version=\"1.0\"?><contactinfo />"));
+    }
+
+    [Fact]
+    public void ALoggedContactRoundTripsThroughTheWire()
+    {
+        Qso qso = Contact();
+
+        string wire = StationMessages.Logged(qso, 4, "SHACK-PC").ToWire();
+        StationRecord? record = StationRecord.Read(ref wire);
+        ContactUpdate? update = StationMessages.Read(record!);
+
+        ContactLogged logged = Assert.IsType<ContactLogged>(update);
+        Assert.Equal("SHACK-PC", logged.StationName);
+        Assert.Equal(qso.Call.Text, logged.Qso.Call.Text);
+        Assert.Equal(qso.TimestampUtc, logged.Qso.TimestampUtc);
+        Assert.Equal(qso.Frequency.Hertz, logged.Qso.Frequency.Hertz);
+        Assert.Equal(qso.Mode.Name, logged.Qso.Mode.Name);
+        Assert.Equal(qso.ContestName, logged.Qso.ContestName);
+        Assert.Equal(qso.SentNumber, logged.Qso.SentNumber);
+        Assert.Equal(qso.ReceivedNumber, logged.Qso.ReceivedNumber);
+        Assert.Equal(qso.Zone, logged.Qso.Zone);
+        Assert.Equal(qso.Points, logged.Qso.Points);
+        Assert.True(logged.Qso.IsMultiplier1);
+        Assert.False(logged.Qso.IsMultiplier2);
+        Assert.Equal(qso.Operator, logged.Qso.Operator);
+        Assert.Equal(qso.ContestNumber, logged.Qso.ContestNumber);
+        Assert.Equal(qso.Continent, logged.Qso.Continent);
+        Assert.Equal(4, logged.Qso.NetworkedComputerNumber);
+    }
+
+    /// A contact that arrived over the network was made somewhere else, which
+    /// is what the log window colours a row on.
+    [Fact]
+    public void AContactFromAnotherStationIsNotOriginal()
+    {
+        string wire = StationMessages.Logged(Contact(), 4, "SHACK-PC").ToWire();
+        StationRecord? record = StationRecord.Read(ref wire);
+
+        ContactLogged logged = Assert.IsType<ContactLogged>(StationMessages.Read(record!));
+
+        Assert.False(logged.Qso.IsOriginal);
+        Assert.Equal("SHACK-PC", logged.Qso.StationName);
+    }
+
+    /// An edit carries the old callsign and the old time in front of the
+    /// contact, because that pair is what N1MM keys a contact on.
+    [Fact]
+    public void AnEditSaysWhichRowToReplace()
+    {
+        Qso qso = Contact() with { Call = Callsign.Parse("DL1ABC") };
+        DateTime was = qso.TimestampUtc.AddMinutes(-3);
+
+        string wire = StationMessages.Edited(qso, "DL1AB", was, 4, "SHACK-PC").ToWire();
+        StationRecord? record = StationRecord.Read(ref wire);
+
+        ContactReplaced replaced = Assert.IsType<ContactReplaced>(StationMessages.Read(record!));
+
+        Assert.Equal("DL1AB", replaced.OldCall);
+        Assert.Equal(was, replaced.OldTimestampUtc);
+        Assert.Equal("DL1ABC", replaced.Qso.Call.Text);
+    }
+
+    [Fact]
+    public void ADeleteNamesTheContactAndTheContest()
+    {
+        Qso qso = Contact();
+
+        string wire = StationMessages.Deleted(qso, 4, "SHACK-PC").ToWire();
+        StationRecord? record = StationRecord.Read(ref wire);
+
+        ContactDeleted deleted = Assert.IsType<ContactDeleted>(StationMessages.Read(record!));
+
+        Assert.Equal(qso.Id, deleted.Id);
+        Assert.Equal(qso.Call.Text, deleted.Call);
+        Assert.Equal(qso.TimestampUtc, deleted.TimestampUtc);
+        Assert.Equal(qso.ContestNumber, deleted.ContestNumber);
+    }
+
+    /// A resync is the same contact sent again, so it means the same thing.
+    [Fact]
+    public void AResyncedContactIsReadAsALoggedOne()
+    {
+        string wire = StationMessages.Resynced(Contact(), 4, "SHACK-PC").ToWire();
+        StationRecord? record = StationRecord.Read(ref wire);
+
+        Assert.IsType<ContactLogged>(StationMessages.Read(record!));
+    }
+
+    /// N1MM adds message types between versions. One this program does not
+    /// know is passed over rather than treated as a fault.
+    [Fact]
+    public void AMessageTypeThisProgramDoesNotKnowIsPassedOver()
+    {
+        Assert.Null(StationMessages.Read(new StationRecord(1, "PC", "SKEDD", ["something"])));
+    }
+
+    /// The frequency the other station transmits on is only kept when it is
+    /// not the one it listens on: a contact in split is the exception, not the
+    /// rule.
+    [Fact]
+    public void OneFrequencyForBothMeansNoSplit()
+    {
+        string wire = StationMessages.Logged(Contact(), 1, "PC").ToWire();
+        StationRecord? record = StationRecord.Read(ref wire);
+
+        ContactLogged logged = Assert.IsType<ContactLogged>(StationMessages.Read(record!));
+
+        Assert.Equal(0, logged.Qso.QsxFrequency.Hertz);
+    }
+
+    [Fact]
+    public void ASplitContactKeepsBothFrequencies()
+    {
+        Qso qso = Contact() with { QsxFrequency = Frequency.FromKilohertz(14_205) };
+
+        string wire = StationMessages.Logged(qso, 1, "PC").ToWire();
+        StationRecord? record = StationRecord.Read(ref wire);
+
+        ContactLogged logged = Assert.IsType<ContactLogged>(StationMessages.Read(record!));
+
+        Assert.Equal(14_205_000, logged.Qso.QsxFrequency.Hertz);
+        Assert.Equal(14_025_000, logged.Qso.Frequency.Hertz);
+    }
+
+    private static Qso Contact() => new()
+    {
+        Id = Qso.NewId(),
+        TimestampUtc = new DateTime(2026, 9, 3, 12, 34, 56, DateTimeKind.Utc),
+        Call = Callsign.Parse("G3XYZ"),
+        Frequency = Frequency.FromKilohertz(14_025),
+        Mode = Modes.Cw,
+        ContestName = "CQWW",
+        ContestNumber = 2,
+        SentReport = "599",
+        ReceivedReport = "599",
+        SentNumber = 41,
+        ReceivedNumber = 17,
+        Zone = 14,
+        Points = 3,
+        IsMultiplier1 = true,
+        Operator = "OM3KFF",
+        RadioNumber = 2,
+        Continent = "EU",
+        CountryPrefix = "G",
+        StationPrefix = "G",
+        WpxPrefix = "G3",
+        Comment = "good sig",
+    };
+}

tests/Nonemm.Session.Tests/MessageExpanderTests.cs

@@ -100,6 +100,14 @@ public class MessageExpanderTests
     public void WithOneRadioTheOtherMacrosStandForNothing() =>
         Assert.Equal("", MessageExpander.Expand("{OTHERFREQ}{OTHERMHZ}{OTHERBAND}", Session()));
 
+    /// The two carriage return macros a digital message is written with. N1MM's
+    /// own RTTY message defaults use `{ENTERLF}`.
+    [Fact]
+    public void TheCarriageReturnMacrosAreTheCharactersTheyStandFor()
+    {
+        Assert.Equal("A\rB\r\nC", MessageExpander.Expand("A{ENTER}B{ENTERLF}C", Session()));
+    }
+
     [Fact]
     public void TheNameComesFromTheCallHistoryWhenTheContestHasNoNameBox()
     {

tests/Nonemm.Session.Tests/MessagePlanTests.cs

@@ -117,8 +117,21 @@ public class MessagePlanTests
     {
         MessagePlan plan = MessagePlan.Read("{TX}CQ DE {MYCALL}{ENTER}{RX}", Session());
 
-        Assert.Equal([MessageCommand.StartTransmit, MessageCommand.ReturnToReceive],
-            plan.Before.Select(a => a.Command));
+        Assert.Equal([MessageCommand.StartTransmit], plan.Before.Select(a => a.Command));
         Assert.Equal("CQ DE DL1ABC\r", plan.Text);
+        Assert.Equal([MessageCommand.ReturnToReceive], plan.After.Select(a => a.Command));
+    }
+
+    /// `{RX}` waits for the message even when it stands in front of it, because
+    /// the transmitter cannot drop before the text has gone out. Everything
+    /// else in front of the text still runs first.
+    [Fact]
+    public void ReturnToReceiveRunsAfterTheMessageWhereverItStands()
+    {
+        MessagePlan plan = MessagePlan.Read("{TX}{RX}CQ TEST{WIPE}", Session());
+
+        Assert.Equal([MessageCommand.StartTransmit, MessageCommand.Wipe],
+            plan.Before.Select(a => a.Command));
+        Assert.Equal([MessageCommand.ReturnToReceive], plan.After.Select(a => a.Command));
     }
 }

tests/Nonemm.Session.Tests/QtcMessagesTests.cs

@@ -37,6 +37,57 @@ public class QtcMessagesTests
         Assert.Equal("TU 73", QtcMessages.Tu("TU 73", "QTC 3/10"));
     }
 
+    /// N1MM joins the three fields with hyphens on RTTY and offers no setting
+    /// for it.
+    [Fact]
+    public void ARttyLineIsTheThreeFieldsJoinedWithHyphens()
+    {
+        Assert.Equal("1234-DL1ABC-123", QtcMessages.RttyLine(" 1234 ", " DL1ABC ", " 123 "));
+    }
+
+    /// The shape is N1MM's, from `QTCWindow.cs:3609`: the heading, the spacing,
+    /// then every line with a space and the spacing behind it, then the ending.
+    /// The macros are left for the expander; `{QTC}` is the one filled in here.
+    [Fact]
+    public void SendAllReadsTheWholeSeriesOutInOneMessage()
+    {
+        string message = QtcMessages.SendAll(
+            QtcMessages.DefaultSendAllHeading,
+            QtcMessages.DefaultSendAllEnding,
+            QtcMessages.DefaultRttySpacing,
+            "QTC 3/10",
+            ["1234-DL1ABC-123", "1240-G3XYZ-124"]);
+
+        Assert.Equal(
+            "{TX}{ENTERLF}QTC 3/10 QTC 3/10{ENTER}"
+            + "1234-DL1ABC-123 {ENTER}"
+            + "1240-G3XYZ-124 {ENTER}"
+            + "{ENTERLF}QSL?? BK DE {MYCALL} K{RX}",
+            message);
+    }
+
+    [Fact]
+    public void SendAllWithNoLinesSendsNothing()
+    {
+        Assert.Equal(
+            "",
+            QtcMessages.SendAll(
+                QtcMessages.DefaultSendAllHeading,
+                QtcMessages.DefaultSendAllEnding,
+                QtcMessages.DefaultRttySpacing,
+                "QTC 3/10",
+                []));
+    }
+
+    /// One line asked for again, which N1MM wraps in the spacing on both sides.
+    [Fact]
+    public void OneLineKeysAndDropsTheTransmitterOfItsOwn()
+    {
+        Assert.Equal(
+            "{TX}{ENTER}1234-DL1ABC-123{ENTER}{RX}",
+            QtcMessages.SendOne(QtcMessages.DefaultRttySpacing, "1234-DL1ABC-123"));
+    }
+
     [Fact]
     public void WhatIsInTheBoxesIsTrimmed()
     {

tools/Nonemm.EngineProbe/EngineProbe.cs

@@ -0,0 +1,506 @@
+using System.Text;
+using Nonemm.Digital;
+
+namespace Nonemm.EngineProbe;
+
+/// Questions about MMTTY that cannot be answered without MMTTY running, asked
+/// in order and written to the report:
+///
+/// 1. Does the control answer `TxBufLen`, and is a name it does not know
+///    distinguishable from one it does?
+/// 2. While a message is going out, does that number count down at the
+///    character rate? If it does, it says exactly how much of the message is
+///    still in the engine and can still be taken back.
+/// 3. Does the engine hold a word until the space after it? That is MMTTY's
+///    Option ▸ Way to send, and Word out is what its help calls the usual
+///    setting.
+/// 4. Does `{RX}` — `SetMmttyPTT(1)`, stop once the buffer is empty — send a
+///    word the engine is holding, or drop it? A macro whose last word has no
+///    space after it hangs on the answer.
+/// 5. What comes back on the receive side while transmitting, and when? With
+///    the sound loopback off that is MMTTY echoing its transmit window; with it
+///    on it is the demodulator hearing the transmission.
+///
+/// A backspace is not on the list any more: pushed in as a character, MMTTY
+/// counted it as one more character to transmit and sent the text unchanged.
+/// It is not an edit.
+///
+/// The answers decide whether the transmit buffer can be handed to MMTTY
+/// instead of being paced from here. `docs/unfinished.md` states what each one
+/// means.
+public sealed class EngineProbe
+{
+    /// Long enough for a character at 45.45 baud, short enough to see the count
+    /// move.
+    private static readonly TimeSpan PollInterval = TimeSpan.FromMilliseconds(50);
+
+    /// A message at 45.45 baud takes a few seconds; this is well past the end
+    /// of one.
+    private static readonly TimeSpan Patience = TimeSpan.FromSeconds(30);
+
+    /// A count that has not moved for this long is not going to move.
+    private static readonly TimeSpan Still = TimeSpan.FromSeconds(5);
+
+    /// How long after a push the count is written down every time it is read,
+    /// and an empty answer is not believed. The count runs behind the engine,
+    /// and this is where the report says by how much.
+    private static readonly TimeSpan Settling = TimeSpan.FromSeconds(2);
+
+    /// How long the engine is given to key up and to drop again.
+    private static readonly TimeSpan Keying = TimeSpan.FromSeconds(3);
+
+    /// One character at 45.45 baud, which is the rate the window feeds at.
+    private static readonly TimeSpan CharacterTime = TimeSpan.FromMilliseconds(165);
+
+    private const string Message = "CQ TEST DE OM5M OM5M ";
+    private const string Word = "ABCD";
+
+    private readonly MmttyEngine engine;
+    private readonly ProbeLog log;
+    private readonly StringBuilder received = new();
+    private readonly Lock gate = new();
+    private TaskCompletionSource<int>? asking;
+    private TaskCompletionSource<bool>? keying;
+
+    public EngineProbe(MmttyEngine engine, ProbeLog log)
+    {
+        this.engine = engine;
+        this.log = log;
+        engine.Reported += (_, what) => log.Write($"bridge: {what}");
+        engine.TransmitChanged += WhenTransmitChanged;
+        engine.Buffered += WhenBuffered;
+        engine.Received += WhenReceived;
+    }
+
+    public async Task RunAsync(CancellationToken cancellation)
+    {
+        await NamesAsync(cancellation).ConfigureAwait(false);
+        await IdleAsync(cancellation).ConfigureAwait(false);
+        if (!await KeyingAsync(cancellation).ConfigureAwait(false))
+        {
+            return;
+        }
+        await MessageAsync(cancellation).ConfigureAwait(false);
+        await WordAsync(cancellation).ConfigureAwait(false);
+        await UnfinishedWordAsync(cancellation).ConfigureAwait(false);
+        await FedSlowlyAsync(cancellation).ConfigureAwait(false);
+        await PoliteStopAsync(cancellation).ConfigureAwait(false);
+        await StopCharacterAsync('\\', cancellation).ConfigureAwait(false);
+        await StopCharacterAsync('~', cancellation).ConfigureAwait(false);
+        await SentWholeAsync(cancellation).ConfigureAwait(false);
+    }
+
+    /// Question 11. N1MM hands MMTTY the whole message with `SendString` and
+    /// calls `SetMmttyPTT(1)` 400 ms later, and MMTTY ends the transmission
+    /// itself. This program hands the message over one character at a time with
+    /// `PostMmttyMessage(4, ...)` and the same stop does nothing. Is it the way
+    /// the text arrives that makes the difference?
+    private async Task SentWholeAsync(CancellationToken cancellation)
+    {
+        log.Step("11. the whole message with SendString, then SetMmttyPTT(1) as N1MM sends it");
+        TakeReceived();
+        await KeyAsync(cancellation).ConfigureAwait(false);
+        await engine.SendAsync(Message, cancellation).ConfigureAwait(false);
+        log.Write($"pushed {Message.Length} characters in one call");
+        await Task.Delay(TimeSpan.FromMilliseconds(400), cancellation).ConfigureAwait(false);
+        await engine.SetPttAsync(false, cancellation).ConfigureAwait(false);
+        log.Write("SetMmttyPTT(1) sent 400 ms after the push, which is N1MM's wait");
+        TimeSpan waited = await WatchAsync(TimeSpan.FromSeconds(10), cancellation).ConfigureAwait(false);
+        log.Write(waited >= TimeSpan.Zero
+            ? $"the transmitter dropped {waited.TotalMilliseconds:0} ms after the stop"
+            : "the transmitter stayed up");
+        log.Write($"received: \"{TakeReceived()}\"");
+        await UnkeyAsync(cancellation).ConfigureAwait(false);
+    }
+
+    /// Question 10. MMTTY's macro language ends a transmission with `\` at the
+    /// end of a macro, and `~` stops the carrier. The only way into the engine
+    /// from here is `PostMmttyMessage(4, ...)`, one typed character, so the
+    /// question is whether a character typed that way is read as a command or
+    /// transmitted as text. A stop that travels with the text is worth far more
+    /// than one timed from outside: it lands exactly at the end of the message
+    /// with nothing held on after it.
+    private async Task StopCharacterAsync(char candidate, CancellationToken cancellation)
+    {
+        log.Step($"10. \"{Word}\" and then '{candidate}' typed as a character");
+        TakeReceived();
+        await KeyAsync(cancellation).ConfigureAwait(false);
+        await TypeAsync(Word, cancellation).ConfigureAwait(false);
+        await engine.TypeAsync(candidate, cancellation).ConfigureAwait(false);
+        TimeSpan waited = await WatchAsync(TimeSpan.FromSeconds(5), cancellation).ConfigureAwait(false);
+        log.Write(waited >= TimeSpan.Zero
+            ? $"'{candidate}' dropped the transmitter after {waited.TotalMilliseconds:0} ms"
+            : $"'{candidate}' did not drop the transmitter");
+        log.Write($"received: \"{TakeReceived()}\"");
+        await engine.ReleaseKeyAsync(cancellation).ConfigureAwait(false);
+        await StoppedAsync(cancellation).ConfigureAwait(false);
+    }
+
+    /// Question 9. Does `SetMmttyPTT(1)` drop the transmitter at all? The
+    /// window sends it at the end of every message and the engine went on
+    /// transmitting, so the abort that follows it a second and a half later is
+    /// what unkeys, and it cut the last character off a message once. Nothing
+    /// is aborted here until the question is answered, and if the polite stop
+    /// does nothing the `PTT` property is put back to false to see whether that
+    /// does.
+    private async Task PoliteStopAsync(CancellationToken cancellation)
+    {
+        log.Step($"9. \"{Message}\" fed slowly, then SetMmttyPTT(1) and nothing else");
+        TakeReceived();
+        await KeyAsync(cancellation).ConfigureAwait(false);
+        foreach (char character in Message)
+        {
+            await engine.TypeAsync(character, cancellation).ConfigureAwait(false);
+            await Task.Delay(CharacterTime, cancellation).ConfigureAwait(false);
+        }
+        await engine.SetPttAsync(false, cancellation).ConfigureAwait(false);
+        log.Write("SetMmttyPTT(1) sent with the message fed");
+        TimeSpan waited = await WatchAsync(TimeSpan.FromSeconds(8), cancellation).ConfigureAwait(false);
+        if (waited >= TimeSpan.Zero)
+        {
+            log.Write($"the polite stop dropped the transmitter after {waited.TotalMilliseconds:0} ms");
+            return;
+        }
+        log.Write("the polite stop did not drop the transmitter");
+        await engine.ReleaseKeyAsync(cancellation).ConfigureAwait(false);
+        log.Write("PTT property put back to false");
+        waited = await WatchAsync(TimeSpan.FromSeconds(3), cancellation).ConfigureAwait(false);
+        log.Write(waited >= TimeSpan.Zero
+            ? $"the property dropped the transmitter after {waited.TotalMilliseconds:0} ms"
+            : "the property did not drop the transmitter either");
+        log.Write($"received: \"{TakeReceived()}\"");
+        await UnkeyAsync(cancellation).ConfigureAwait(false);
+    }
+
+    /// Watches the transmit state and the count until the engine says it has
+    /// stopped. Returns how long that took, or -1 when it never did.
+    private async Task<TimeSpan> WatchAsync(TimeSpan patience, CancellationToken cancellation)
+    {
+        DateTime from = DateTime.UtcNow;
+        DateTime giveUp = from + patience;
+        while (DateTime.UtcNow < giveUp)
+        {
+            int left = await AskAsync("", cancellation).ConfigureAwait(false);
+            if (!engine.IsTransmitting)
+            {
+                return DateTime.UtcNow - from;
+            }
+            log.Write($"{(DateTime.UtcNow - from).TotalMilliseconds,6:0} ms  transmitting, TxBufLen: {Answer(left)}");
+            await Task.Delay(TimeSpan.FromMilliseconds(250), cancellation).ConfigureAwait(false);
+        }
+        return TimeSpan.FromMilliseconds(-1);
+    }
+
+    /// Question 6. The window feeds the engine one character every character
+    /// time and keeps it nearly empty, so the engine sits with an empty buffer
+    /// between characters. Does it drop the transmitter there, and does it
+    /// hold it when the feeding stops altogether? The pane is drawn on the
+    /// answer: a transmitter that drops by itself is not the end of a message.
+    private async Task FedSlowlyAsync(CancellationToken cancellation)
+    {
+        log.Step($"7. \"{Message}\" fed one character every {CharacterTime.TotalMilliseconds:0} ms");
+        TakeReceived();
+        await KeyAsync(cancellation).ConfigureAwait(false);
+        foreach (char character in Message)
+        {
+            await engine.TypeAsync(character, cancellation).ConfigureAwait(false);
+            await Task.Delay(CharacterTime, cancellation).ConfigureAwait(false);
+        }
+        log.Write($"fed {Message.Length} characters; transmitting: {engine.IsTransmitting}");
+        log.Write("an unkey above this line is the engine dropping between two characters");
+        log.Step("8. keyed with nothing more to feed");
+        for (int look = 0; look < 12; look++)
+        {
+            await Task.Delay(TimeSpan.FromMilliseconds(250), cancellation).ConfigureAwait(false);
+            log.Write($"transmitting: {engine.IsTransmitting}, TxBufLen: {Answer(await AskAsync("", cancellation).ConfigureAwait(false))}");
+        }
+        log.Write($"received: \"{TakeReceived()}\"");
+        log.Write("still transmitting after three idle seconds means the engine holds the transmitter itself");
+        await UnkeyAsync(cancellation).ConfigureAwait(false);
+    }
+
+    /// Question 1. `DISP_E_UNKNOWNNAME` is 0x80020006; a name the control knows
+    /// answers with a number instead.
+    private async Task NamesAsync(CancellationToken cancellation)
+    {
+        log.Step("1. does the control answer TxBufLen");
+        log.Write($"TxBufLen: {Answer(await AskAsync("TxBufLen", cancellation).ConfigureAwait(false))}");
+        log.Write($"NotAProperty: {Answer(await AskAsync("NotAProperty", cancellation).ConfigureAwait(false))}");
+        log.Write("a number for the first and no answer for the second is what makes the count usable");
+    }
+
+    /// The number with nothing to transmit, which is what the count has to
+    /// start and end at.
+    private async Task IdleAsync(CancellationToken cancellation)
+    {
+        log.Step("2. TxBufLen with nothing to send");
+        for (int look = 0; look < 3; look++)
+        {
+            log.Write($"TxBufLen: {Answer(await AskAsync("", cancellation).ConfigureAwait(false))}");
+            await Task.Delay(PollInterval, cancellation).ConfigureAwait(false);
+        }
+    }
+
+    /// Nothing else in the report means anything until the engine keys, so this
+    /// stops the run rather than letting the rest measure an engine that is
+    /// sitting still.
+    private async Task<bool> KeyingAsync(CancellationToken cancellation)
+    {
+        log.Step("3. does the engine key up");
+        if (!await KeyAsync(cancellation).ConfigureAwait(false))
+        {
+            log.Write("the engine did not key: nothing below this would mean anything, so stopping");
+            log.Write("keying is the control's PTT property; SetMmttyPTT only stops a transmission");
+            return false;
+        }
+        log.Write($"TxBufLen while keyed and idle: {Answer(await AskAsync("", cancellation).ConfigureAwait(false))}");
+        await UnkeyAsync(cancellation).ConfigureAwait(false);
+        return true;
+    }
+
+    /// Questions 2 and 5. The whole message is pushed as fast as the bridge
+    /// takes it, so what the count does afterwards is the engine transmitting
+    /// rather than the probe feeding.
+    private async Task MessageAsync(CancellationToken cancellation)
+    {
+        log.Step($"4. \"{Message}\" pushed in one go, then TxBufLen every {PollInterval.TotalMilliseconds} ms");
+        TakeReceived();
+        await KeyAsync(cancellation).ConfigureAwait(false);
+        await TypeAsync(Message, cancellation).ConfigureAwait(false);
+        log.Write($"pushed {Message.Length} characters");
+        log.Write("every reading for the next two seconds: the first one that is not 0 says how far behind the count is");
+        await DrainAsync(cancellation).ConfigureAwait(false);
+        log.Write($"received while transmitting: \"{TakeReceived()}\"");
+        await UnkeyAsync(cancellation).ConfigureAwait(false);
+    }
+
+    /// Question 3. A word with no space after it is what MMTTY holds back when
+    /// it is set to Word out, and that changes what "still in the buffer"
+    /// means.
+    private async Task WordAsync(CancellationToken cancellation)
+    {
+        log.Step($"5. \"{Word}\" with no space after it, then the space");
+        TakeReceived();
+        await KeyAsync(cancellation).ConfigureAwait(false);
+        await TypeAsync(Word, cancellation).ConfigureAwait(false);
+        await Task.Delay(TimeSpan.FromSeconds(3), cancellation).ConfigureAwait(false);
+        log.Write($"TxBufLen three seconds after the word: {Answer(await AskAsync("", cancellation).ConfigureAwait(false))}");
+        log.Write($"received so far: \"{TakeReceived()}\"");
+        log.Write("nothing received here means the engine is set to Word out and is holding it");
+        await TypeAsync(" ", cancellation).ConfigureAwait(false);
+        await DrainAsync(cancellation).ConfigureAwait(false);
+        log.Write($"received after the space: \"{TakeReceived()}\"");
+        await UnkeyAsync(cancellation).ConfigureAwait(false);
+    }
+
+    /// Question 4. The word has no space after it, so an engine set to Word
+    /// out is still holding it when the transmission is told to stop.
+    private async Task UnfinishedWordAsync(CancellationToken cancellation)
+    {
+        log.Step($"6. \"{Word}\" with no space after it, then {{RX}}");
+        TakeReceived();
+        await KeyAsync(cancellation).ConfigureAwait(false);
+        await TypeAsync(Word, cancellation).ConfigureAwait(false);
+        await Task.Delay(TimeSpan.FromSeconds(2), cancellation).ConfigureAwait(false);
+        log.Write($"TxBufLen before {{RX}}: {Answer(await AskAsync("", cancellation).ConfigureAwait(false))}");
+        await UnkeyAsync(cancellation).ConfigureAwait(false);
+        log.Write($"received: \"{TakeReceived()}\"");
+        log.Write($"\"{Word}\" here means SetMmttyPTT(1) sends a held word before it stops");
+    }
+
+    /// Polls until the engine says it has nothing left, until the count stops
+    /// moving, or until the patience runs out.
+    ///
+    /// The engine counts what it has been given a moment after it is given it,
+    /// so a single empty answer straight after a push means the push has not
+    /// registered, not that the message has gone.
+    private async Task DrainAsync(CancellationToken cancellation)
+    {
+        DateTime started = DateTime.UtcNow;
+        DateTime giveUp = started + Patience;
+        DateTime moved = started;
+        int last = int.MinValue;
+        bool wasEmpty = false;
+        while (DateTime.UtcNow < giveUp)
+        {
+            int left = await AskAsync("", cancellation).ConfigureAwait(false);
+            bool settling = DateTime.UtcNow - started < Settling;
+            if (left != last || settling)
+            {
+                log.Write($"TxBufLen: {Answer(left)}");
+                moved = left != last ? DateTime.UtcNow : moved;
+                last = left;
+            }
+            if (left == 0 && wasEmpty && !settling)
+            {
+                return;
+            }
+            wasEmpty = left == 0;
+            if (DateTime.UtcNow - moved > Still)
+            {
+                log.Write($"the count has not moved for {Still.TotalSeconds} s, so it is not counting down");
+                return;
+            }
+            await Task.Delay(PollInterval, cancellation).ConfigureAwait(false);
+        }
+        log.Write($"gave up waiting after {Patience.TotalSeconds} s");
+    }
+
+    /// Waits until the engine holds no more than `wanted` characters, so the
+    /// next thing the probe does lands at a known point in the message.
+    private async Task<int> WaitForAsync(int wanted, CancellationToken cancellation)
+    {
+        DateTime giveUp = DateTime.UtcNow + Still;
+        while (DateTime.UtcNow < giveUp)
+        {
+            int left = await AskAsync("", cancellation).ConfigureAwait(false);
+            if (left <= wanted)
+            {
+                return left;
+            }
+            await Task.Delay(PollInterval, cancellation).ConfigureAwait(false);
+        }
+        log.Write($"the count never came down to {wanted}, so the backspaces go in wherever it is");
+        return await AskAsync("", cancellation).ConfigureAwait(false);
+    }
+
+    /// N1MM's `{TX}`: the control's PTT property, then wait for the engine to
+    /// say it is transmitting.
+    private async Task<bool> KeyAsync(CancellationToken cancellation)
+    {
+        if (engine.IsTransmitting)
+        {
+            return true;
+        }
+        TaskCompletionSource<bool> keyed = new(TaskCreationOptions.RunContinuationsAsynchronously);
+        lock (gate)
+        {
+            keying = keyed;
+        }
+        await engine.SetPttAsync(true, cancellation).ConfigureAwait(false);
+        try
+        {
+            await keyed.Task.WaitAsync(Keying, cancellation).ConfigureAwait(false);
+            log.Write("keyed");
+            return true;
+        }
+        catch (TimeoutException)
+        {
+            log.Write($"no transmit report {Keying.TotalSeconds} s after keying");
+            return false;
+        }
+    }
+
+    /// N1MM's `{RX}`: stop once the buffer is empty. Waits for the engine to
+    /// say it has stopped, because the next step keys again and would otherwise
+    /// see the old state and skip it.
+    private async Task UnkeyAsync(CancellationToken cancellation)
+    {
+        await engine.SetPttAsync(false, cancellation).ConfigureAwait(false);
+        if (await StoppedAsync(cancellation).ConfigureAwait(false))
+        {
+            return;
+        }
+        log.Write("still transmitting, stopping it the hard way");
+        await engine.AbortAsync(cancellation).ConfigureAwait(false);
+        if (!await StoppedAsync(cancellation).ConfigureAwait(false))
+        {
+            log.Write("the engine is still reporting a transmission after the abort");
+        }
+    }
+
+    private async Task<bool> StoppedAsync(CancellationToken cancellation)
+    {
+        DateTime giveUp = DateTime.UtcNow + Keying;
+        while (engine.IsTransmitting && DateTime.UtcNow < giveUp)
+        {
+            await Task.Delay(PollInterval, cancellation).ConfigureAwait(false);
+        }
+        return !engine.IsTransmitting;
+    }
+
+    private async Task TypeAsync(string text, CancellationToken cancellation)
+    {
+        foreach (char character in text)
+        {
+            await engine.TypeAsync(character, cancellation).ConfigureAwait(false);
+        }
+    }
+
+    /// Sends one question and waits for the answer. The bridge answers every
+    /// question, with -1 when the control would not, so this cannot hang while
+    /// the bridge is alive.
+    private async Task<int> AskAsync(string property, CancellationToken cancellation)
+    {
+        TaskCompletionSource<int> answer = new(TaskCreationOptions.RunContinuationsAsynchronously);
+        lock (gate)
+        {
+            asking = answer;
+        }
+        await engine.AskBufferedAsync(property, cancellation).ConfigureAwait(false);
+        return await answer.Task.WaitAsync(Keying, cancellation).ConfigureAwait(false);
+    }
+
+    private static string Answer(int left) => left < 0 ? "no answer" : left.ToString();
+
+    private void WhenBuffered(object? sender, int left)
+    {
+        lock (gate)
+        {
+            asking?.TrySetResult(left);
+        }
+    }
+
+    private void WhenTransmitChanged(object? sender, bool transmitting)
+    {
+        log.Write(transmitting ? "engine keyed" : "engine unkeyed");
+        if (!transmitting)
+        {
+            return;
+        }
+        lock (gate)
+        {
+            keying?.TrySetResult(true);
+        }
+    }
+
+    /// While the engine is not transmitting this is the demodulator hearing
+    /// whatever is on the band, which is noise on a quiet frequency. Only what
+    /// arrives while transmitting is about the transmission.
+    private void WhenReceived(object? sender, string text)
+    {
+        foreach (char character in text)
+        {
+            log.Write($"{(engine.IsTransmitting ? "rx" : "noise")} {Printable(character)}");
+        }
+        if (!engine.IsTransmitting)
+        {
+            return;
+        }
+        lock (received)
+        {
+            received.Append(text);
+        }
+    }
+
+    private string TakeReceived()
+    {
+        lock (received)
+        {
+            string text = received.ToString();
+            received.Clear();
+            return text;
+        }
+    }
+
+    /// Control codes matter here: a backspace coming back is the engine saying
+    /// it removed a character.
+    private static string Printable(char character) => character switch
+    {
+        '\b' => "\\b (backspace)",
+        '\r' => "\\r",
+        '\n' => "\\n",
+        _ when char.IsControl(character) => $"\\x{(int)character:x2}",
+        _ => character.ToString(),
+    };
+}

tools/Nonemm.EngineProbe/Nonemm.EngineProbe.csproj

@@ -0,0 +1,13 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <RootNamespace>Nonemm.EngineProbe</RootNamespace>
+    <IsPackable>false</IsPackable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\..\src\Nonemm.Digital\Nonemm.Digital.csproj" />
+  </ItemGroup>
+
+</Project>

tools/Nonemm.EngineProbe/ProbeLog.cs

@@ -0,0 +1,45 @@
+namespace Nonemm.EngineProbe;
+
+/// The probe's report: every line on the screen and in the file, stamped with
+/// the milliseconds since the probe started. The timing is the point of the
+/// report, so nothing is written without it.
+public sealed class ProbeLog : IDisposable
+{
+    private readonly StreamWriter file;
+    private readonly Lock gate = new();
+    private readonly long started = Environment.TickCount64;
+
+    public ProbeLog(string path)
+    {
+        Path = System.IO.Path.GetFullPath(path);
+        file = new StreamWriter(Path, append: false) { AutoFlush = true };
+    }
+
+    public string Path { get; }
+
+    public long Elapsed => Environment.TickCount64 - started;
+
+    public void Write(string text)
+    {
+        lock (gate)
+        {
+            string line = $"{Elapsed,7} ms  {text}";
+            Console.WriteLine(line);
+            file.WriteLine(line);
+        }
+    }
+
+    /// A heading, so the file can be read a step at a time.
+    public void Step(string name)
+    {
+        lock (gate)
+        {
+            Console.WriteLine();
+            Console.WriteLine($"== {name}");
+            file.WriteLine();
+            file.WriteLine($"== {name}");
+        }
+    }
+
+    public void Dispose() => file.Dispose();
+}

tools/Nonemm.EngineProbe/Program.cs

@@ -0,0 +1,125 @@
+using Nonemm.Digital;
+
+namespace Nonemm.EngineProbe;
+
+/// Starts MMTTY through the bridge, asks it the questions in `EngineProbe`, and
+/// writes what it answered to a file. It has to run where MMTTY runs: a sound
+/// card and a Wine prefix with XMMT.ocx registered.
+public static class Program
+{
+    private const string Usage = """
+        usage: engine probe --engine <path to MMTTY.EXE> [options]
+
+          --engine <path>   the engine to start, as a Linux path
+          --bridge <path>   the bridge program (default bridge/nonemm-mmtty-bridge.exe)
+          --prefix <path>   WINEPREFIX to run in (default: Wine's own)
+          --ptt <port>      serial port to key (default: none, so no radio is keyed)
+          --out <file>      where to write the report (default engine-probe.log)
+          --log <folder>    where to write the protocol log, if it is wanted
+
+        The probe transmits: MMTTY makes tones on the sound card for about half a
+        minute. It keys no serial port unless --ptt says so, but a rig listening to
+        that sound card through VOX will still go on the air.
+        """;
+
+    public static async Task<int> Main(string[] arguments)
+    {
+        Dictionary<string, string> options;
+        try
+        {
+            options = Read(arguments);
+        }
+        catch (ArgumentException problem)
+        {
+            Console.Error.WriteLine(problem.Message);
+            Console.Error.WriteLine();
+            Console.Error.WriteLine(Usage);
+            return 1;
+        }
+
+        using CancellationTokenSource stopping = new();
+        Console.CancelKeyPress += (_, e) =>
+        {
+            e.Cancel = true;
+            stopping.Cancel();
+        };
+
+        using ProbeLog log = new(Option(options, "out", "engine-probe.log"));
+        MmttyEngine engine = new(
+            new WineBridgeChannel(
+                Option(options, "bridge", Path.Combine("bridge", "nonemm-mmtty-bridge.exe")),
+                options.GetValueOrDefault("prefix"),
+                logFolder: options.GetValueOrDefault("log")),
+            new MmttyOptions
+            {
+                EnginePath = options["engine"],
+                PttPort = Option(options, "ptt", ""),
+            });
+        try
+        {
+            log.Write($"starting {options["engine"]}");
+            await engine.StartAsync(stopping.Token).ConfigureAwait(false);
+            log.Write($"MMTTY {engine.Version} is up");
+            await new EngineProbe(engine, log).RunAsync(stopping.Token).ConfigureAwait(false);
+        }
+        catch (OperationCanceledException)
+        {
+            log.Write("stopped");
+        }
+        catch (Exception problem)
+        {
+            log.Write($"failed: {problem.Message}");
+            Console.Error.WriteLine(problem);
+            return 1;
+        }
+        finally
+        {
+            await Shutdown(engine, log).ConfigureAwait(false);
+            Console.WriteLine();
+            Console.WriteLine($"the report is in {log.Path}");
+        }
+        return 0;
+    }
+
+    /// The engine is left keyed if the probe stopped partway through, so the
+    /// transmission is aborted before the engine is shut down, whatever
+    /// happened.
+    private static async Task Shutdown(MmttyEngine engine, ProbeLog log)
+    {
+        try
+        {
+            await engine.AbortAsync().ConfigureAwait(false);
+            await engine.StopAsync().ConfigureAwait(false);
+        }
+        catch (Exception problem)
+        {
+            log.Write($"the engine did not shut down cleanly: {problem.Message}");
+        }
+        engine.Dispose();
+    }
+
+    private static Dictionary<string, string> Read(string[] arguments)
+    {
+        Dictionary<string, string> options = [];
+        for (int i = 0; i < arguments.Length; i += 2)
+        {
+            if (!arguments[i].StartsWith("--", StringComparison.Ordinal))
+            {
+                throw new ArgumentException($"expected an option, found {arguments[i]}");
+            }
+            if (i + 1 >= arguments.Length)
+            {
+                throw new ArgumentException($"{arguments[i]} needs a value");
+            }
+            options[arguments[i][2..]] = arguments[i + 1];
+        }
+        if (!options.ContainsKey("engine"))
+        {
+            throw new ArgumentException("--engine is required");
+        }
+        return options;
+    }
+
+    private static string Option(Dictionary<string, string> options, string name, string fallback) =>
+        options.TryGetValue(name, out string? given) ? given : fallback;
+}