Named-pipe transport for the DOSBox-X fork's namedpipe backend

Both devices now serve \.\pipe\vrio and \.\pipe\vplasma for the whole
app lifetime alongside the COM rows — the com0com-free path. Framing per
the pinned contract (0x00 len data / 0x01 DTR+RTS lines, one lines frame
on connect, unknown type = log + drop): PipeFraming/PipeFrameDecoder and
VRioPipeService (TX paced at the wire rate, peer DTR edges feed
HostHandshake) in VRio.Core, listener-only VPlasmaPipeService twin in
VPlasma.Core. Busy pipe names retry, so vRIO's built-in glass and the
standalone vPLASMA coexist. README documents the transport and the fork
conf lines.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/VPlasma.App/MainForm.cs

@@ -7,6 +7,9 @@ namespace VPlasma.App;
 /// device end is hardwired to <see cref="PortName"/> (the plasma side of the
 /// second com0com pair) and opened automatically — a retry timer keeps
 /// trying while the port is missing or busy, and reopens it if it dies. The
+/// DOSBox-X fork's named-pipe transport (<c>\\.\pipe\vplasma</c>) is served
+/// permanently alongside it (if vRIO's built-in glass already holds the
+/// pipe name, the server just retries until it frees up). The
 /// title bar carries the port status; double-clicking the glass cycles the
 /// self-test pages (banner, charset, graphics) through the wire parser, and
 /// a right-click resets the display to its power-on state.
@@ -18,6 +21,7 @@ internal sealed class MainForm : Form
 
     private readonly VPlasmaDevice _device = new();
     private readonly VPlasmaSerialService _service;
+    private readonly VPlasmaPipeService _pipeService;
     private readonly PlasmaCanvas _canvas = new();
 
     private readonly System.Windows.Forms.Timer _reconnectTimer = new() { Interval = 3000 };
@@ -32,6 +36,7 @@ internal sealed class MainForm : Form
         StartPosition = FormStartPosition.CenterScreen;
 
         _service = new VPlasmaSerialService(_device);
+        _pipeService = new VPlasmaPipeService(_device);
 
         _canvas.Location = new Point(0, 0);
         Controls.Add(_canvas);
@@ -39,6 +44,7 @@ internal sealed class MainForm : Form
         // Device / service events arrive on the serial reader thread; marshal to the UI.
         _device.Updated += () => RunOnUi(() => _canvas.UpdateFrom(_device));
         _service.ConnectionChanged += _ => RunOnUi(UpdateTitle);
+        _pipeService.ClientChanged += _ => RunOnUi(UpdateTitle);
 
         _canvas.DoubleClick += (_, _) => RunSelfTest();
         _canvas.MouseClick += (_, e) =>
@@ -55,10 +61,12 @@ internal sealed class MainForm : Form
         {
             _reconnectTimer.Dispose();
             _service.Dispose();
+            _pipeService.Dispose();
         };
 
         _canvas.UpdateFrom(_device);
         EnsureOpen();
+        _pipeService.Start();
     }
 
     private void EnsureOpen()
@@ -82,6 +90,8 @@ internal sealed class MainForm : Form
         string status = _service.IsOpen
             ? $"{PortName} @ 9600 8N1"
             : $"{PortName} unavailable — retrying";
+        if (_pipeService.IsClientConnected)
+            status += "  •  pipe host connected";
         Text = $"vPLASMA v{Application.ProductVersion} — {status}";
     }
 

src/VPlasma.Core/Device/VPlasmaPipeService.cs

@@ -0,0 +1,237 @@
+using System.IO.Pipes;
+
+namespace VPlasma.Core.Device;
+
+/// <summary>
+/// Serves a <see cref="VPlasmaDevice"/> over the named pipe that the
+/// DOSBox-X fork's <c>serial2=namedpipe pipe:vplasma</c> backend connects
+/// to — the com0com-free transport. We are the pipe <em>server</em> (the
+/// display is always present); DOSBox-X is the client and background-retries
+/// while the pipe is missing, so this service listens for the whole app
+/// lifetime alongside the COM path.
+///
+/// <para>Framing is the contract shared with the fork (see VRio.Core's
+/// <c>PipeFraming</c>, and <c>serialnamedpipe.h</c> on the fork side):
+/// <c>0x00 len bytes</c> carries serial data, <c>0x01 bits</c> carries the
+/// sender's DTR/RTS. Like the serial twin, the plasma is a pure listener —
+/// we send one lines frame on connect (DTR+RTS high, "display present") and
+/// then only ever read. The game may toggle its own lines around writes
+/// (DOS console redirection flips RTS per byte); the display doesn't care,
+/// so inbound lines frames are consumed silently.</para>
+/// </summary>
+public sealed class VPlasmaPipeService : IDisposable
+{
+    /// <summary>The contract pipe name: DOSBox-X connects to <c>\\.\pipe\vplasma</c>.</summary>
+    public const string DefaultPipeName = "vplasma";
+
+    // Frame types and line bits, per the shared contract.
+    private const byte DataType = 0x00;
+    private const byte LinesType = 0x01;
+    private const byte LinesPresent = 0x03; // DTR + RTS
+
+    private readonly VPlasmaDevice _device;
+    private readonly string _pipeName;
+
+    private NamedPipeServerStream? _pipe;
+    private Thread? _server;
+    private volatile bool _running;
+    private volatile bool _clientConnected;
+
+    public VPlasmaPipeService(VPlasmaDevice device, string pipeName = DefaultPipeName)
+    {
+        _device = device ?? throw new ArgumentNullException(nameof(device));
+        if (string.IsNullOrWhiteSpace(pipeName))
+            throw new ArgumentException("Pipe name is required.", nameof(pipeName));
+        _pipeName = pipeName;
+    }
+
+    /// <summary>The served pipe name (without the <c>\\.\pipe\</c> prefix).</summary>
+    public string PipeName => _pipeName;
+
+    /// <summary>True while a client is connected.</summary>
+    public bool IsClientConnected => _clientConnected;
+
+    /// <summary>A client connected (true) or went away (false).</summary>
+    public event Action<bool>? ClientChanged;
+
+    /// <summary>Pipe-level log lines (listen/connect/errors).</summary>
+    public event Action<string>? Logged;
+
+    /// <summary>Start listening (idempotent). Clients may come and go forever.</summary>
+    public void Start()
+    {
+        if (_running)
+            return;
+
+        _running = true;
+        _server = new Thread(ServerLoop) { IsBackground = true, Name = $"vPLASMA pipe server ({_pipeName})" };
+        _server.Start();
+
+        Logged?.Invoke($@"Listening on \\.\pipe\{_pipeName} — DOSBox-X connects when it boots");
+    }
+
+    /// <summary>Stop listening and drop any client (idempotent).</summary>
+    public void Stop()
+    {
+        if (!_running)
+            return;
+
+        _running = false;
+        NamedPipeServerStream? pipe = _pipe;
+        _pipe = null;
+        try { pipe?.Dispose(); }
+        catch (IOException) { }
+
+        // A WaitForConnection pending on the disposed stream can survive the
+        // Dispose on net48; a throwaway client connect releases it either way.
+        try
+        {
+            using var poke = new NamedPipeClientStream(".", _pipeName, PipeDirection.Out);
+            poke.Connect(100);
+        }
+        catch (Exception ex) when (ex is IOException or TimeoutException or UnauthorizedAccessException) { }
+
+        _server?.Join(1000);
+        _server = null;
+        _clientConnected = false;
+
+        Logged?.Invoke("Pipe server stopped");
+    }
+
+    private void ServerLoop()
+    {
+        bool busyLogged = false; // log the name collision once, not per retry
+
+        while (_running)
+        {
+            NamedPipeServerStream pipe;
+            try
+            {
+                pipe = new NamedPipeServerStream(_pipeName, PipeDirection.InOut, 1,
+                    PipeTransmissionMode.Byte, PipeOptions.Asynchronous);
+            }
+            catch (Exception ex) when (ex is IOException or UnauthorizedAccessException)
+            {
+                // Name already served — e.g. vRIO's built-in glass and the
+                // standalone vPLASMA both running. Whoever loses just waits.
+                if (!busyLogged)
+                {
+                    busyLogged = true;
+                    Logged?.Invoke($@"\\.\pipe\{_pipeName} is busy ({ex.Message.TrimEnd('.')}) — retrying until it frees up");
+                }
+                for (int i = 0; i < 20 && _running; i++)
+                    Thread.Sleep(100);
+                continue;
+            }
+            busyLogged = false;
+
+            _pipe = pipe;
+            try
+            {
+                pipe.WaitForConnection();
+
+                if (_running)
+                {
+                    // Our lines frame goes first: display present.
+                    pipe.Write(new byte[] { LinesType, LinesPresent }, 0, 2);
+                    pipe.Flush();
+
+                    _clientConnected = true;
+                    Logged?.Invoke("Pipe client connected — sent lines DTR+RTS (display present)");
+                    ClientChanged?.Invoke(true);
+
+                    ReadUntilDisconnect(pipe);
+                }
+            }
+            catch (Exception ex) when (ex is IOException or ObjectDisposedException or InvalidOperationException)
+            {
+                // Disposed by Stop(), or a client vanished mid-connect.
+            }
+
+            bool wasConnected = _clientConnected;
+            _clientConnected = false;
+            _pipe = null;
+            try { pipe.Dispose(); }
+            catch (IOException) { }
+
+            if (wasConnected && _running)
+            {
+                Logged?.Invoke("Pipe client disconnected");
+                ClientChanged?.Invoke(false);
+            }
+        }
+    }
+
+    private void ReadUntilDisconnect(NamedPipeServerStream pipe)
+    {
+        // Inline deframer (the listener-only subset of VRio.Core's
+        // PipeFrameDecoder): data payloads go to the device, lines frames are
+        // consumed silently, anything else is a protocol bug — log and drop
+        // the connection rather than resync.
+        const int StateType = 0, StateLength = 1, StatePayload = 2, StateLines = 3;
+        int state = StateType, remaining = 0;
+
+        var buffer = new byte[512];
+        var payload = new byte[byte.MaxValue];
+        int fill = 0;
+
+        while (_running)
+        {
+            int n;
+            try
+            {
+                n = pipe.Read(buffer, 0, buffer.Length);
+            }
+            catch (Exception ex) when (ex is IOException or ObjectDisposedException or InvalidOperationException or OperationCanceledException)
+            {
+                if (_running)
+                    Logged?.Invoke($"Pipe error: {ex.Message}");
+                return;
+            }
+
+            if (n == 0)
+                return; // client closed its end
+
+            for (int i = 0; i < n; i++)
+            {
+                byte b = buffer[i];
+                switch (state)
+                {
+                    case StateType when b == DataType:
+                        state = StateLength;
+                        break;
+                    case StateType when b == LinesType:
+                        state = StateLines;
+                        break;
+                    case StateType:
+                        Logged?.Invoke($"Pipe protocol violation: unknown frame type 0x{b:X2} — dropping the connection");
+                        return;
+
+                    case StateLength when b == 0:
+                        Logged?.Invoke("Pipe protocol violation: zero-length data frame — dropping the connection");
+                        return;
+                    case StateLength:
+                        remaining = b;
+                        fill = 0;
+                        state = StatePayload;
+                        break;
+
+                    case StatePayload:
+                        payload[fill++] = b;
+                        if (fill == remaining)
+                        {
+                            state = StateType;
+                            _device.OnReceived(payload, remaining);
+                        }
+                        break;
+
+                    case StateLines:
+                        state = StateType; // the display ignores host line state
+                        break;
+                }
+            }
+        }
+    }
+
+    public void Dispose() => Stop();
+}

src/VRio.App/MainForm.cs

@@ -18,6 +18,9 @@ namespace VRio.App;
 /// RIOJoy at the other end of the null-modem pair, and every click here
 /// arrives at RIOJoy exactly as if the physical cockpit sent it; point the
 /// game's COM2 passthrough at the plasma pair and the glass lights up.
+/// Both devices also serve the DOSBox-X fork's named-pipe transport
+/// (<c>\\.\pipe\vrio</c>, <c>\\.\pipe\vplasma</c>) for the whole app
+/// lifetime — the com0com-free path.
 /// </summary>
 internal sealed class MainForm : Form
 {
@@ -36,8 +39,10 @@ internal sealed class MainForm : Form
 
     private readonly VRioDevice _device = new();
     private readonly VRioSerialService _service;
+    private readonly VRioPipeService _pipeService;
     private readonly VPlasmaDevice _plasmaDevice = new();
     private readonly VPlasmaSerialService _plasmaService;
+    private readonly VPlasmaPipeService _plasmaPipeService;
     private readonly PanelCanvas _canvas = new();
     private readonly PlasmaCanvas _plasmaCanvas = new(PanelCanvas.PlasmaDotPitch)
     {
@@ -190,6 +195,11 @@ internal sealed class MainForm : Form
 
         _service = new VRioSerialService(_device);
         _plasmaService = new VPlasmaSerialService(_plasmaDevice);
+        // The named-pipe endpoints for DOSBox-X's namedpipe serial backend.
+        // Unlike a COM port, a listening pipe costs nothing and conflicts with
+        // nothing, so both servers run for the whole app lifetime.
+        _pipeService = new VRioPipeService(_device);
+        _plasmaPipeService = new VPlasmaPipeService(_plasmaDevice);
         _router = new InputRouter(_device);
         _lampMirror = new KeyboardLampMirror(_device);
 
@@ -224,6 +234,17 @@ internal sealed class MainForm : Form
         _service.ConnectionChanged += open => RunOnUi(() => OnConnectionChanged(open));
         _service.HostHandshake += high => RunOnUi(() =>
             PrependLog(high ? "Host raised DTR (board-reset handshake)" : "Host dropped DTR"));
+        _pipeService.Logged += line => RunOnUi(() => PrependLog($"pipe: {line}"));
+        _pipeService.HostHandshake += high => RunOnUi(() =>
+            PrependLog(high ? "Pipe host raised DTR (board-reset handshake)" : "Pipe host dropped DTR"));
+        _pipeService.ClientChanged += connected => RunOnUi(() =>
+        {
+            // The RIO is a single-host device; two live transports means two
+            // hosts fighting over one board.
+            if (connected && _service.IsOpen)
+                PrependLog($"Warning: a pipe client connected while {_service.PortName} is open — close one transport.");
+        });
+        _plasmaPipeService.Logged += line => RunOnUi(() => PrependLog($"vPLASMA pipe: {line}"));
 
         // Built-in plasma glass: a pure listener on its own port, same gestures
         // as the standalone vPLASMA — double-click cycles the self-test pages,
@@ -305,6 +326,8 @@ internal sealed class MainForm : Form
             _rawKeyboard.Dispose();
             _service.Dispose();
             _plasmaService.Dispose();
+            _pipeService.Dispose();
+            _plasmaPipeService.Dispose();
         };
 
         RefreshPorts();
@@ -313,6 +336,8 @@ internal sealed class MainForm : Form
         LoadBindings();
         _plasmaCanvas.UpdateFrom(_plasmaDevice); // paint the power-on frame
         AutoOpenPreferredPorts();
+        _pipeService.Start();
+        _plasmaPipeService.Start();
     }
 
     private Panel BuildControlStrip()

src/VRio.Core/Device/PipeFraming.cs

@@ -0,0 +1,134 @@
+namespace VRio.Core.Device;
+
+/// <summary>
+/// The framing shared with the DOSBox-X fork's <c>namedpipe</c> serial
+/// backend (its <c>serialnamedpipe.h</c> header comment is the contract's
+/// source of truth on that side). A pipe is a plain byte stream, so serial
+/// data and modem control lines are multiplexed as typed frames:
+///
+/// <code>
+///   0x00 &lt;len:u8&gt; &lt;len bytes&gt;   serial data, len ≥ 1 (batching allowed)
+///   0x01 &lt;lines:u8&gt;             the sender's OWN output lines (bit0 DTR,
+///                                bit1 RTS); the receiver applies the
+///                                null-modem cross: peer DTR → local DSR,
+///                                peer RTS → local CTS
+/// </code>
+///
+/// Each side sends one lines frame immediately on connect; until it arrives
+/// the peer's lines are assumed low, and a disconnect drops them low again.
+/// Any other frame type is a protocol bug, not line noise — pipes don't drop
+/// bytes — so the receiver logs it and drops the connection instead of
+/// trying to resync.
+/// </summary>
+public static class PipeFraming
+{
+    public const byte DataType = 0x00;
+    public const byte LinesType = 0x01;
+
+    /// <summary>Lines-frame bit: the sender's DTR output.</summary>
+    public const byte LineDtr = 0x01;
+
+    /// <summary>Lines-frame bit: the sender's RTS output.</summary>
+    public const byte LineRts = 0x02;
+
+    /// <summary>Both outputs asserted — what a present, powered device reports.</summary>
+    public const byte LinesPresent = LineDtr | LineRts;
+
+    /// <summary>Build a lines frame carrying <paramref name="lines"/>.</summary>
+    public static byte[] EncodeLines(byte lines) => new[] { LinesType, lines };
+}
+
+/// <summary>
+/// Incremental decoder for <see cref="PipeFraming"/>: feed it raw pipe reads,
+/// get one <see cref="Data"/> event per complete data frame and one
+/// <see cref="Lines"/> event per lines frame. Frames may split across reads
+/// at any byte boundary. A malformed stream (unknown type, zero-length data
+/// frame) poisons the decoder: <see cref="Feed"/> returns false and
+/// <see cref="Violation"/> says why — drop the connection and
+/// <see cref="Reset"/> before serving the next one.
+/// </summary>
+public sealed class PipeFrameDecoder
+{
+    private enum State { Type, Length, Payload, Lines }
+
+    private readonly byte[] _payload = new byte[byte.MaxValue];
+    private State _state;
+    private int _fill, _length;
+
+    /// <summary>
+    /// A complete data frame's payload as (buffer, count). The buffer is
+    /// reused across frames — consume it synchronously.
+    /// </summary>
+    public event Action<byte[], int>? Data;
+
+    /// <summary>A lines frame's bits (see <see cref="PipeFraming.LineDtr"/>).</summary>
+    public event Action<byte>? Lines;
+
+    /// <summary>Why the stream was rejected, or null while it is healthy.</summary>
+    public string? Violation { get; private set; }
+
+    /// <summary>Forget any partial frame and clear a violation (new connection).</summary>
+    public void Reset()
+    {
+        _state = State.Type;
+        _fill = _length = 0;
+        Violation = null;
+    }
+
+    /// <summary>
+    /// Consume <paramref name="count"/> received bytes from
+    /// <paramref name="buffer"/>. Returns false when the stream violates the
+    /// framing contract (see <see cref="Violation"/>); a poisoned decoder
+    /// keeps returning false until <see cref="Reset"/>.
+    /// </summary>
+    public bool Feed(byte[] buffer, int count)
+    {
+        if (Violation is not null)
+            return false;
+
+        for (int i = 0; i < count; i++)
+        {
+            byte b = buffer[i];
+            switch (_state)
+            {
+                case State.Type when b == PipeFraming.DataType:
+                    _state = State.Length;
+                    break;
+
+                case State.Type when b == PipeFraming.LinesType:
+                    _state = State.Lines;
+                    break;
+
+                case State.Type:
+                    Violation = $"unknown frame type 0x{b:X2}";
+                    return false;
+
+                case State.Length when b == 0:
+                    Violation = "zero-length data frame";
+                    return false;
+
+                case State.Length:
+                    _length = b;
+                    _fill = 0;
+                    _state = State.Payload;
+                    break;
+
+                case State.Payload:
+                    _payload[_fill++] = b;
+                    if (_fill == _length)
+                    {
+                        _state = State.Type;
+                        Data?.Invoke(_payload, _length);
+                    }
+                    break;
+
+                case State.Lines:
+                    _state = State.Type;
+                    Lines?.Invoke(b);
+                    break;
+            }
+        }
+
+        return true;
+    }
+}

src/VRio.Core/Device/VRioPipeService.cs

@@ -0,0 +1,389 @@
+using System.Diagnostics;
+using System.IO.Pipes;
+using System.Runtime.InteropServices;
+
+namespace VRio.Core.Device;
+
+/// <summary>
+/// Serves a <see cref="VRioDevice"/> over the named pipe that the DOSBox-X
+/// fork's <c>serial1=namedpipe pipe:vrio</c> backend connects to — the
+/// com0com-free transport. vRIO is the pipe <em>server</em> (the device is
+/// always present); DOSBox-X is the client and background-retries while the
+/// pipe is missing, so this service listens for the whole app lifetime
+/// alongside the COM path at no cost. An unconnected pipe is an unplugged
+/// cable: the device keeps running, transmit bytes fall on the floor.
+///
+/// <para>Framing per <see cref="PipeFraming"/>. On connect we immediately
+/// send our lines frame (DTR+RTS high — "board present", the same lines the
+/// serial path asserts via DtrEnable/RtsEnable); the peer's DTR edges surface
+/// through <see cref="HostHandshake"/> exactly like the DSR blips through a
+/// null modem, and a disconnect drops all lines low. The game hardware-resets
+/// the RIO by pulsing DTR, and the in-band lines frames keep that edge's
+/// position in the byte stream — the reason the contract multiplexes control
+/// onto the data pipe instead of using a second one.</para>
+///
+/// <para>Outbound bytes are paced one per 10-bit frame time exactly like
+/// <see cref="VRioSerialService"/> (see its remarks for the full rationale).
+/// With the 9600-baud wire gone, this pacer is the only thing standing
+/// between the host and an impossible burst, so each paced byte travels as
+/// its own 3-byte data frame and the inter-byte timing survives the pipe.</para>
+/// </summary>
+public sealed class VRioPipeService : IDisposable
+{
+    /// <summary>The contract pipe name: DOSBox-X connects to <c>\\.\pipe\vrio</c>.</summary>
+    public const string DefaultPipeName = "vrio";
+
+    // Pacing mirrors VRioSerialService: one byte per 10-bit frame at 9600
+    // baud; sleep until ~1.8 ms from the slot, then spin the remainder.
+    private static readonly long BytePeriodTicks = Stopwatch.Frequency * 10 / VRioSerialService.BaudRate;
+    private static readonly long SpinThresholdTicks = Stopwatch.Frequency * 18 / 10_000;
+
+    private readonly VRioDevice _device;
+    private readonly string _pipeName;
+    private readonly object _txGate = new();
+    private readonly Queue<byte> _txQueue = new();
+    private readonly PipeFrameDecoder _decoder = new();
+
+    private NamedPipeServerStream? _pipe;
+    private Thread? _server;
+    private Thread? _writer;
+    private volatile bool _running;
+    private volatile bool _clientConnected;
+    private bool _peerDtr, _peerRts;
+    private bool _timerResolutionRaised;
+
+    public VRioPipeService(VRioDevice device, string pipeName = DefaultPipeName)
+    {
+        _device = device ?? throw new ArgumentNullException(nameof(device));
+        if (string.IsNullOrWhiteSpace(pipeName))
+            throw new ArgumentException("Pipe name is required.", nameof(pipeName));
+        _pipeName = pipeName;
+
+        _device.Transmit += Write;
+        _decoder.Data += (buffer, count) => _device.OnReceived(buffer, count);
+        _decoder.Lines += OnPeerLines;
+    }
+
+    /// <summary>The served pipe name (without the <c>\\.\pipe\</c> prefix).</summary>
+    public string PipeName => _pipeName;
+
+    /// <summary>True while a client is connected.</summary>
+    public bool IsClientConnected => _clientConnected;
+
+    /// <summary>A client connected (true) or went away (false).</summary>
+    public event Action<bool>? ClientChanged;
+
+    /// <summary>
+    /// The host's DTR line changed (the game pulses it to hardware-reset the
+    /// board). Same semantics as <see cref="VRioSerialService.HostHandshake"/>:
+    /// the argument is the new line state as seen on our DSR pin.
+    /// </summary>
+    public event Action<bool>? HostHandshake;
+
+    /// <summary>Pipe-level log lines (listen/connect/errors).</summary>
+    public event Action<string>? Logged;
+
+    /// <summary>Start listening (idempotent). Clients may come and go forever.</summary>
+    public void Start()
+    {
+        if (_running)
+            return;
+
+        _running = true;
+        lock (_txGate) _txQueue.Clear();
+
+        _server = new Thread(ServerLoop) { IsBackground = true, Name = $"vRIO pipe server ({_pipeName})" };
+        _server.Start();
+        _writer = new Thread(WriteLoop) { IsBackground = true, Name = $"vRIO pipe writer ({_pipeName})" };
+        _writer.Start();
+
+        Logged?.Invoke($@"Listening on \\.\pipe\{_pipeName} (TX paced at the wire rate) — DOSBox-X connects when it boots");
+    }
+
+    /// <summary>Stop listening and drop any client (idempotent).</summary>
+    public void Stop()
+    {
+        if (!_running)
+            return;
+
+        _running = false;
+        lock (_txGate)
+        {
+            _txQueue.Clear();
+            Monitor.PulseAll(_txGate); // wake the writer so it can exit
+        }
+
+        NamedPipeServerStream? pipe = _pipe;
+        _pipe = null;
+        try { pipe?.Dispose(); }
+        catch (IOException) { }
+
+        // A WaitForConnection pending on the disposed stream can survive the
+        // Dispose on net48; a throwaway client connect releases it either way.
+        try
+        {
+            using var poke = new NamedPipeClientStream(".", _pipeName, PipeDirection.Out);
+            poke.Connect(100);
+        }
+        catch (Exception ex) when (ex is IOException or TimeoutException or UnauthorizedAccessException) { }
+
+        _server?.Join(1000);
+        _server = null;
+        _writer?.Join(1000);
+        _writer = null;
+
+        if (_timerResolutionRaised)
+        {
+            timeEndPeriod(1);
+            _timerResolutionRaised = false;
+        }
+        _clientConnected = false;
+
+        Logged?.Invoke("Pipe server stopped");
+    }
+
+    private void ServerLoop()
+    {
+        bool busyLogged = false; // log the name collision once, not per retry
+
+        while (_running)
+        {
+            NamedPipeServerStream pipe;
+            try
+            {
+                pipe = new NamedPipeServerStream(_pipeName, PipeDirection.InOut, 1,
+                    PipeTransmissionMode.Byte, PipeOptions.Asynchronous);
+            }
+            catch (Exception ex) when (ex is IOException or UnauthorizedAccessException)
+            {
+                // Name already served — most likely a second vRIO instance.
+                if (!busyLogged)
+                {
+                    busyLogged = true;
+                    Logged?.Invoke($@"\\.\pipe\{_pipeName} is busy ({ex.Message.TrimEnd('.')}) — retrying until it frees up");
+                }
+                for (int i = 0; i < 20 && _running; i++)
+                    Thread.Sleep(100);
+                continue;
+            }
+            busyLogged = false;
+
+            _pipe = pipe;
+            try
+            {
+                pipe.WaitForConnection();
+            }
+            catch (Exception ex) when (ex is IOException or ObjectDisposedException or InvalidOperationException)
+            {
+                // Disposed by Stop(), or a client vanished mid-connect.
+                _pipe = null;
+                try { pipe.Dispose(); }
+                catch (IOException) { }
+                continue;
+            }
+
+            if (!_running || !OnClientConnected(pipe))
+            {
+                _pipe = null;
+                try { pipe.Dispose(); }
+                catch (IOException) { }
+                continue;
+            }
+
+            ReadUntilDisconnect(pipe);
+
+            OnClientDisconnected();
+            _pipe = null;
+            try { pipe.Dispose(); }
+            catch (IOException) { }
+        }
+    }
+
+    /// <summary>Per-connection setup; false if the client died before it finished.</summary>
+    private bool OnClientConnected(NamedPipeServerStream pipe)
+    {
+        _decoder.Reset();
+        _peerDtr = _peerRts = false;
+        lock (_txGate) _txQueue.Clear();
+
+        // Our lines frame goes first, before the writer can interleave data
+        // (it only writes once _clientConnected flips below): DTR+RTS high,
+        // board present — the peer maps them to its DSR/CTS.
+        try
+        {
+            byte[] lines = PipeFraming.EncodeLines(PipeFraming.LinesPresent);
+            pipe.Write(lines, 0, lines.Length);
+            pipe.Flush();
+        }
+        catch (Exception ex) when (ex is IOException or ObjectDisposedException or InvalidOperationException)
+        {
+            return false;
+        }
+
+        // 1 ms system timer resolution while a client is connected, so the
+        // pacer's Thread.Sleep(1) actually sleeps ~1 ms.
+        _timerResolutionRaised = timeBeginPeriod(1) == 0;
+        _clientConnected = true;
+
+        Logged?.Invoke("Pipe client connected — sent lines DTR+RTS (board present)");
+        ClientChanged?.Invoke(true);
+        return true;
+    }
+
+    private void OnClientDisconnected()
+    {
+        _clientConnected = false;
+        lock (_txGate) _txQueue.Clear();
+
+        if (_timerResolutionRaised)
+        {
+            timeEndPeriod(1);
+            _timerResolutionRaised = false;
+        }
+
+        // Contract: a disconnect drops all lines low on the surviving side.
+        if (_peerDtr)
+        {
+            _peerDtr = false;
+            HostHandshake?.Invoke(false);
+        }
+        _peerRts = false;
+
+        if (_running)
+        {
+            Logged?.Invoke("Pipe client disconnected");
+            ClientChanged?.Invoke(false);
+        }
+    }
+
+    private void ReadUntilDisconnect(NamedPipeServerStream pipe)
+    {
+        var buffer = new byte[512];
+        while (_running)
+        {
+            int n;
+            try
+            {
+                n = pipe.Read(buffer, 0, buffer.Length);
+            }
+            catch (Exception ex) when (ex is IOException or ObjectDisposedException or InvalidOperationException or OperationCanceledException)
+            {
+                if (_running)
+                    Logged?.Invoke($"Pipe error: {ex.Message}");
+                return;
+            }
+
+            if (n == 0)
+                return; // client closed its end
+
+            if (!_decoder.Feed(buffer, n))
+            {
+                Logged?.Invoke($"Pipe protocol violation: {_decoder.Violation} — dropping the connection");
+                return;
+            }
+        }
+    }
+
+    private void OnPeerLines(byte lines)
+    {
+        _peerRts = (lines & PipeFraming.LineRts) != 0; // tracked; nothing consumes CTS today
+        bool dtr = (lines & PipeFraming.LineDtr) != 0;
+        if (dtr == _peerDtr)
+            return;
+
+        _peerDtr = dtr;
+        HostHandshake?.Invoke(dtr);
+    }
+
+    // The device's Transmit handler: queue the frame for the paced writer so
+    // the caller (UI click, reader thread mid-reply) never blocks on the pipe.
+    private void Write(byte[] data)
+    {
+        if (!_running || !_clientConnected)
+            return; // no host on the pipe — a real UART shifts into an unterminated line
+
+        lock (_txGate)
+        {
+            foreach (byte b in data)
+                _txQueue.Enqueue(b);
+            Monitor.Pulse(_txGate);
+        }
+    }
+
+    private void WriteLoop()
+    {
+        // One data frame per paced byte, so the pipe carries the same
+        // inter-byte spacing a 9600-baud wire would.
+        var frame = new byte[] { PipeFraming.DataType, 1, 0 };
+        long slot = Stopwatch.GetTimestamp();
+
+        while (_running)
+        {
+            lock (_txGate)
+            {
+                while (_txQueue.Count == 0)
+                {
+                    if (!_running)
+                        return;
+                    Monitor.Wait(_txGate, 200); // timed, so a missed pulse can't wedge shutdown
+                }
+                frame[2] = _txQueue.Dequeue();
+            }
+
+            // This byte's wire slot: one frame after the previous byte, or now
+            // if the line has been idle (no burst "catch-up" debt).
+            slot = Math.Max(slot + BytePeriodTicks, Stopwatch.GetTimestamp());
+            PaceUntil(slot);
+
+            NamedPipeServerStream? pipe = _pipe;
+            if (pipe is null || !_clientConnected)
+                continue; // client left while this byte waited its slot — it's stale, drop it
+
+            try
+            {
+                pipe.Write(frame, 0, frame.Length);
+                pipe.Flush();
+            }
+            catch (Exception ex) when (ex is IOException or ObjectDisposedException or InvalidOperationException)
+            {
+                // The client died mid-write; the backlog is already stale.
+                // Drop it and keep serving — the server loop notices the EOF
+                // and cycles back to WaitForConnection.
+                lock (_txGate) _txQueue.Clear();
+                continue;
+            }
+
+            // If the wait overshot its slot, pace the next byte from the
+            // actual emission instead — a stall must not cause a catch-up burst.
+            long now = Stopwatch.GetTimestamp();
+            if (now > slot)
+                slot = now;
+        }
+    }
+
+    private static void PaceUntil(long slotTicks)
+    {
+        while (true)
+        {
+            long remaining = slotTicks - Stopwatch.GetTimestamp();
+            if (remaining <= 0)
+                return;
+            if (remaining > SpinThresholdTicks)
+                Thread.Sleep(1);
+            else
+                Thread.SpinWait(64);
+        }
+    }
+
+    [DllImport("winmm.dll")]
+    private static extern uint timeBeginPeriod(uint uMilliseconds);
+
+    [DllImport("winmm.dll")]
+    private static extern uint timeEndPeriod(uint uMilliseconds);
+
+    public void Dispose()
+    {
+        _device.Transmit -= Write;
+        Stop();
+    }
+}