Phase 8A (1/2): de-Span the serial layer, swap JSON to Newtonsoft

Prepares RioJoy.Core for the net40 (Windows XP) target, which has no
System.Memory, ValueTask, or System.Text.Json:
- IRioTransport and the whole protocol/framing layer now use byte[] +
  Task (RioPacket.Payload, PacketParser/Builder, RioChecksum, replies,
  AnalogReport, RioHidReport). At 9600 baud Span bought nothing; the
  SerialPortTransport bridge copies disappear entirely.
- ConfigStore/OverlayTemplateStore switch to Newtonsoft 13 with the
  same conventions (indented, PascalCase, string enums, null-skipping);
  verified against the real STJ-written config.json and regions.json
  (load + round-trip). System.Memory and System.Text.Json packages
  dropped.

275 tests green.

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

src/RioJoy.Core/Hid/RioHidReport.cs

@@ -43,10 +43,7 @@ public sealed class RioHidReport
         SetHat(RioHat.Centered);
     }
 
-    /// <summary>The current report bytes (length <see cref="Size"/>).</summary>
-    public ReadOnlySpan<byte> Bytes => _buffer;
-
-    /// <summary>Copy of the current report bytes.</summary>
+    /// <summary>Copy of the current report bytes (length <see cref="Size"/>).</summary>
     public byte[] ToArray() => (byte[])_buffer.Clone();
 
     /// <summary>Set an axis value (clamped to 0..<see cref="AxisMax"/>), little-endian.</summary>

src/RioJoy.Core/Overlay/OverlayTemplateStore.cs

@@ -1,5 +1,5 @@
-using System.Text.Json;
-using System.Text.Json.Serialization;
+using Newtonsoft.Json;
+using Newtonsoft.Json.Converters;
 
 namespace RioJoy.Core.Overlay;
 
@@ -11,24 +11,24 @@ namespace RioJoy.Core.Overlay;
 /// </summary>
 public static class OverlayTemplateStore
 {
-    private static readonly JsonSerializerOptions Options = new()
+    private static readonly JsonSerializerSettings Options = new()
     {
-        WriteIndented = true,
-        DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull,
-        Converters = { new JsonStringEnumConverter() },
+        Formatting = Formatting.Indented,
+        NullValueHandling = NullValueHandling.Ignore,
+        Converters = { new StringEnumConverter() },
     };
 
     public static string Serialize(OverlayTemplate template)
     {
         if (template is null) throw new ArgumentNullException(nameof(template));
-        return JsonSerializer.Serialize(template, Options);
+        return JsonConvert.SerializeObject(template, Options);
     }
 
     public static OverlayTemplate Deserialize(string json)
     {
         if (string.IsNullOrWhiteSpace(json)) throw new ArgumentException("Value cannot be null or whitespace.", nameof(json));
-        return JsonSerializer.Deserialize<OverlayTemplate>(json, Options)
-               ?? throw new JsonException("Overlay template JSON deserialized to null.");
+        return JsonConvert.DeserializeObject<OverlayTemplate>(json, Options)
+               ?? throw new JsonSerializationException("Overlay template JSON deserialized to null.");
     }
 
     public static void Save(OverlayTemplate template, string path)

src/RioJoy.Core/Plasma/PlasmaDisplay.cs

@@ -48,5 +48,5 @@ public sealed class PlasmaDisplay
     }
 
     private Task WriteAsync(byte[] data, CancellationToken ct) =>
-        _transport.WriteAsync(data, ct).AsTask();
+        _transport.WriteAsync(data, ct);
 }

src/RioJoy.Core/Profiles/ConfigStore.cs

@@ -1,32 +1,36 @@
-using System.Text.Json;
-using System.Text.Json.Serialization;
+using Newtonsoft.Json;
+using Newtonsoft.Json.Converters;
 
 namespace RioJoy.Core.Profiles;
 
 /// <summary>
 /// Loads and saves <see cref="AppConfig"/> as JSON. Replaces the legacy
 /// SimpleIni single-file config with a profile library (see docs/PLAN.md).
+/// Newtonsoft rather than System.Text.Json so the net40 (Windows XP) flavor
+/// shares one serializer — and one on-disk format — with net48 (PLAN.md §8A).
+/// Conventions match the original STJ settings: indented, PascalCase names,
+/// enums as strings, nulls skipped — existing config.json files parse as-is.
 /// </summary>
 public static class ConfigStore
 {
-    private static readonly JsonSerializerOptions Options = new()
+    private static readonly JsonSerializerSettings Options = new()
     {
-        WriteIndented = true,
-        DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull,
-        Converters = { new JsonStringEnumConverter() },
+        Formatting = Formatting.Indented,
+        NullValueHandling = NullValueHandling.Ignore,
+        Converters = { new StringEnumConverter() },
     };
 
     public static string Serialize(AppConfig config)
     {
         if (config is null) throw new ArgumentNullException(nameof(config));
-        return JsonSerializer.Serialize(config, Options);
+        return JsonConvert.SerializeObject(config, Options);
     }
 
     public static AppConfig Deserialize(string json)
     {
         if (string.IsNullOrWhiteSpace(json)) throw new ArgumentException("Value cannot be null or whitespace.", nameof(json));
-        return JsonSerializer.Deserialize<AppConfig>(json, Options)
-               ?? throw new JsonException("Config JSON deserialized to null.");
+        return JsonConvert.DeserializeObject<AppConfig>(json, Options)
+               ?? throw new JsonSerializationException("Config JSON deserialized to null.");
     }
 
     /// <summary>Save the config to <paramref name="path"/> (creating directories).</summary>

src/RioJoy.Core/Protocol/AnalogReport.cs

@@ -44,8 +44,9 @@ public readonly struct AnalogReport
     /// <c>0xFE</c> (<see cref="RioControl.Restart"/>), which the RIO uses as an
     /// "invalid sample" sentinel — the legacy code ignores such replies.
     /// </summary>
-    public static bool TryParse(ReadOnlySpan<byte> payload, out AnalogReport report)
+    public static bool TryParse(byte[] payload, out AnalogReport report)
     {
+        if (payload is null) throw new ArgumentNullException(nameof(payload));
         if (payload.Length != RioCommandTable.PayloadLength(RioCommand.AnalogReply))
             throw new ArgumentException(
                 $"AnalogReply payload must be {RioCommandTable.PayloadLength(RioCommand.AnalogReply)} bytes.",

src/RioJoy.Core/Protocol/PacketBuilder.cs

@@ -13,8 +13,9 @@ public static class PacketBuilder
     /// <paramref name="payload"/>. The payload length must match the command's
     /// entry in the length table.
     /// </summary>
-    public static byte[] Build(RioCommand command, ReadOnlySpan<byte> payload)
+    public static byte[] Build(RioCommand command, byte[] payload)
     {
+        if (payload is null) throw new ArgumentNullException(nameof(payload));
         int expected = RioCommandTable.PayloadLength(command);
         if (payload.Length != expected)
             throw new ArgumentException(
@@ -23,13 +24,15 @@ public static class PacketBuilder
 
         var packet = new byte[1 + expected + 1];
         packet[0] = (byte)command;
-        payload.CopyTo(packet.AsSpan(1));
-        packet[^1] = RioChecksum.Compute(packet.AsSpan(0, 1 + expected));
+        Array.Copy(payload, 0, packet, 1, expected);
+        packet[packet.Length - 1] = RioChecksum.Compute(packet, 0, 1 + expected);
         return packet;
     }
 
     /// <summary>Build a zero-payload command packet (CheckRequest etc.).</summary>
-    public static byte[] Build(RioCommand command) => Build(command, ReadOnlySpan<byte>.Empty);
+    public static byte[] Build(RioCommand command) => Build(command, EmptyPayload);
+
+    private static readonly byte[] EmptyPayload = new byte[0];
 
     // --- Convenience builders for the PC → RIO commands ---------------------
 
@@ -40,12 +43,12 @@ public static class PacketBuilder
     public static byte[] AnalogRequest() => Build(RioCommand.AnalogRequest);
 
     public static byte[] ResetRequest(RioResetTarget target) =>
-        Build(RioCommand.ResetRequest, stackalloc byte[] { (byte)target });
+        Build(RioCommand.ResetRequest, new[] { (byte)target });
 
     /// <summary>
     /// LampRequest: payload is <c>[lamp#, state]</c>. The state byte is a
     /// 7-bit lamp-state value (see <see cref="RioLampState"/>).
     /// </summary>
     public static byte[] LampRequest(byte lampNumber, byte state) =>
-        Build(RioCommand.LampRequest, stackalloc byte[] { lampNumber, state });
+        Build(RioCommand.LampRequest, new[] { lampNumber, state });
 }

src/RioJoy.Core/Protocol/PacketParser.cs

@@ -92,15 +92,17 @@ public sealed class PacketParser
     }
 
     /// <summary>
-    /// Feed a chunk of received bytes, invoking <paramref name="onEvent"/> for
-    /// each event produced, in order.
+    /// Feed the first <paramref name="count"/> received bytes of
+    /// <paramref name="data"/>, invoking <paramref name="onEvent"/> for each
+    /// event produced, in order.
     /// </summary>
-    public void Feed(ReadOnlySpan<byte> data, Action<RioRxEvent> onEvent)
+    public void Feed(byte[] data, int count, Action<RioRxEvent> onEvent)
     {
+        if (data is null) throw new ArgumentNullException(nameof(data));
         if (onEvent is null) throw new ArgumentNullException(nameof(onEvent));
-        foreach (byte ch in data)
+        for (int i = 0; i < count; i++)
         {
-            if (Feed(ch, out RioRxEvent ev))
+            if (Feed(data[i], out RioRxEvent ev))
                 onEvent(ev);
         }
     }
@@ -110,11 +112,12 @@ public sealed class PacketParser
         // _buffer = [command][payload…][checksum]; _count includes all of them.
         int bodyLen = _count - 1;             // command + payload (exclude checksum)
         byte receivedChecksum = _buffer[bodyLen];
-        byte computed = RioChecksum.Compute(_buffer.AsSpan(0, bodyLen));
+        byte computed = RioChecksum.Compute(_buffer, 0, bodyLen);
         bool checksumValid = computed == receivedChecksum;
 
         var command = (RioCommand)_buffer[0];
-        var payload = _buffer.AsSpan(1, bodyLen - 1).ToArray(); // copy out; buffer is reused
+        var payload = new byte[bodyLen - 1];  // copy out; buffer is reused
+        Array.Copy(_buffer, 1, payload, 0, payload.Length);
         var packet = new RioPacket(command, payload);
 
         Reset();

src/RioJoy.Core/Protocol/RioChecksum.cs

@@ -9,15 +9,22 @@ namespace RioJoy.Core.Protocol;
 public static class RioChecksum
 {
     /// <summary>
-    /// Compute the 7-bit checksum over <paramref name="commandAndPayload"/>
-    /// (the command byte followed by its payload bytes — not the checksum byte).
+    /// Compute the 7-bit checksum over the command byte followed by its payload
+    /// bytes (not the checksum byte): <paramref name="count"/> bytes of
+    /// <paramref name="commandAndPayload"/> starting at <paramref name="offset"/>.
     /// </summary>
-    public static byte Compute(ReadOnlySpan<byte> commandAndPayload)
+    public static byte Compute(byte[] commandAndPayload, int offset, int count)
     {
+        if (commandAndPayload is null) throw new ArgumentNullException(nameof(commandAndPayload));
+
         byte sum = 0;
-        foreach (byte b in commandAndPayload)
-            sum += (byte)(b & 0x7F);
+        for (int i = offset; i < offset + count; i++)
+            sum += (byte)(commandAndPayload[i] & 0x7F);
 
         return (byte)(sum & 0x7F);
     }
+
+    /// <summary>Compute the checksum over all of <paramref name="commandAndPayload"/>.</summary>
+    public static byte Compute(byte[] commandAndPayload) =>
+        Compute(commandAndPayload, 0, commandAndPayload?.Length ?? 0);
 }

src/RioJoy.Core/Protocol/RioPacket.cs

@@ -11,13 +11,14 @@ public readonly struct RioPacket
     public RioCommand Command { get; }
 
     /// <summary>
-    /// The payload bytes (high bit always clear). Length matches
-    /// <see cref="RioCommandTable.PayloadLength(RioCommand)"/>.
+    /// The payload bytes (high bit always clear; treat as read-only). Length
+    /// matches <see cref="RioCommandTable.PayloadLength(RioCommand)"/>.
     /// </summary>
-    public ReadOnlyMemory<byte> Payload { get; }
+    public byte[] Payload { get; }
 
-    public RioPacket(RioCommand command, ReadOnlyMemory<byte> payload)
+    public RioPacket(RioCommand command, byte[] payload)
     {
+        if (payload is null) throw new ArgumentNullException(nameof(payload));
         int expected = RioCommandTable.PayloadLength(command);
         if (payload.Length != expected)
             throw new ArgumentException(
@@ -30,7 +31,7 @@ public readonly struct RioPacket
 
     public override string ToString()
     {
-        var hex = BitConverter.ToString(Payload.ToArray()).Replace("-", string.Empty);
-        return Payload.IsEmpty ? Command.ToString() : $"{Command} [{hex}]";
+        var hex = BitConverter.ToString(Payload).Replace("-", string.Empty);
+        return Payload.Length == 0 ? Command.ToString() : $"{Command} [{hex}]";
     }
 }

src/RioJoy.Core/Protocol/RioReplies.cs

@@ -16,7 +16,7 @@ public readonly struct VersionInfo
     }
 
     /// <summary>Decode a <see cref="RioCommand.VersionReply"/> payload (2 bytes).</summary>
-    public static VersionInfo Parse(ReadOnlySpan<byte> payload)
+    public static VersionInfo Parse(byte[] payload)
     {
         Require(payload, RioCommand.VersionReply);
         return new VersionInfo(payload[0], payload[1]);
@@ -24,8 +24,9 @@ public readonly struct VersionInfo
 
     public override string ToString() => $"{Major}.{Minor}";
 
-    internal static void Require(ReadOnlySpan<byte> payload, RioCommand command)
+    internal static void Require(byte[] payload, RioCommand command)
     {
+        if (payload is null) throw new ArgumentNullException(nameof(payload));
         int expected = RioCommandTable.PayloadLength(command);
         if (payload.Length != expected)
             throw new ArgumentException(
@@ -51,7 +52,7 @@ public readonly struct CheckStatus
     }
 
     /// <summary>Decode a <see cref="RioCommand.CheckReply"/> payload (2 bytes).</summary>
-    public static CheckStatus Parse(ReadOnlySpan<byte> payload)
+    public static CheckStatus Parse(byte[] payload)
     {
         VersionInfo.Require(payload, RioCommand.CheckReply);
         return new CheckStatus((RioStatusType)payload[0], payload[1]);

src/RioJoy.Core/RioJoy.Core.csproj

@@ -15,8 +15,7 @@
   <ItemGroup>
     <PackageReference Include="Nefarius.ViGEm.Client" Version="1.21.256" />
     <PackageReference Include="System.IO.Ports" Version="8.0.0" />
-    <PackageReference Include="System.Memory" Version="4.5.5" />
-    <PackageReference Include="System.Text.Json" Version="8.0.5" />
+    <PackageReference Include="Newtonsoft.Json" Version="13.0.3" />
     <PackageReference Include="Microsoft.Bcl.HashCode" Version="1.1.1" />
     <PackageReference Include="PolySharp" Version="1.14.1">
       <PrivateAssets>all</PrivateAssets>

src/RioJoy.Core/RioRuntime.cs

@@ -144,7 +144,7 @@ public sealed class RioRuntime : IRioCommandSink, IDisposable
     private void OnPacket(RioPacket packet)
     {
         EnsureLampsInitialized();
-        ReadOnlySpan<byte> p = packet.Payload.Span;
+        byte[] p = packet.Payload;
         switch (packet.Command)
         {
             case RioCommand.ButtonPressed when p[0] < RioAddress.ButtonCount:
@@ -185,7 +185,7 @@ public sealed class RioRuntime : IRioCommandSink, IDisposable
         ButtonActivity?.Invoke(address, pressed);
     }
 
-    private static bool IsKeypad(ReadOnlySpan<byte> p) => p[0] is 0 or 1 && p[1] <= 0x0F;
+    private static bool IsKeypad(byte[] p) => p[0] is 0 or 1 && p[1] <= 0x0F;
 
     // IRioCommandSink: RIO commands routed from a button (RIOcmd port).
     void IRioCommandSink.Execute(RioCommandCode command)

src/RioJoy.Core/Serial/IRioTransport.cs

@@ -15,8 +15,13 @@ public interface IRioTransport : IDisposable
     /// Read available bytes into <paramref name="buffer"/>. Returns the number of
     /// bytes read; a return of 0 indicates the transport has closed.
     /// </summary>
-    ValueTask<int> ReadAsync(Memory<byte> buffer, CancellationToken cancellationToken);
+    /// <remarks>
+    /// Plain <c>byte[]</c>/<see cref="Task"/> rather than Memory/ValueTask: the
+    /// link runs at 9600 baud, and the net40 (Windows XP) flavor has neither
+    /// System.Memory nor ValueTask (see docs/PLAN.md §Phase 8).
+    /// </remarks>
+    Task<int> ReadAsync(byte[] buffer, CancellationToken cancellationToken);
 
     /// <summary>Write all of <paramref name="data"/> to the transport.</summary>
-    ValueTask WriteAsync(ReadOnlyMemory<byte> data, CancellationToken cancellationToken);
+    Task WriteAsync(byte[] data, CancellationToken cancellationToken);
 }

src/RioJoy.Core/Serial/RioSerialLink.cs

@@ -79,7 +79,7 @@ public sealed class RioSerialLink
     }
 
     /// <summary>Send a pre-built packet (see <see cref="PacketBuilder"/>) to the RIO.</summary>
-    public async Task SendAsync(ReadOnlyMemory<byte> packet, CancellationToken cancellationToken = default)
+    public async Task SendAsync(byte[] packet, CancellationToken cancellationToken = default)
     {
         await _writeLock.WaitAsync(cancellationToken).ConfigureAwait(false);
         try
@@ -155,7 +155,7 @@ public sealed class RioSerialLink
         switch (packet.Command)
         {
             case RioCommand.AnalogReply:
-                if (AnalogReport.TryParse(packet.Payload.Span, out AnalogReport report))
+                if (AnalogReport.TryParse(packet.Payload, out AnalogReport report))
                 {
                     _sinceAnalog.Restart();
                     AnalogReceived?.Invoke(report);
@@ -163,11 +163,11 @@ public sealed class RioSerialLink
                 break;
 
             case RioCommand.VersionReply:
-                VersionReceived?.Invoke(VersionInfo.Parse(packet.Payload.Span));
+                VersionReceived?.Invoke(VersionInfo.Parse(packet.Payload));
                 break;
 
             case RioCommand.CheckReply:
-                CheckReceived?.Invoke(CheckStatus.Parse(packet.Payload.Span));
+                CheckReceived?.Invoke(CheckStatus.Parse(packet.Payload));
                 break;
         }
     }

src/RioJoy.Core/Serial/SerialPortTransport.cs

@@ -47,21 +47,11 @@ public sealed class SerialPortTransport : IRioTransport
 
     public string Description => $"{_port.PortName} @ {BaudRate} 8N1";
 
-    // net48's Stream has no Memory-based ReadAsync/WriteAsync overloads, so bridge
-    // through a pooled array and copy into/out of the caller's Memory<byte>.
-    public async ValueTask<int> ReadAsync(Memory<byte> buffer, CancellationToken cancellationToken)
-    {
-        byte[] tmp = new byte[buffer.Length];
-        int read = await _stream.ReadAsync(tmp, 0, tmp.Length, cancellationToken).ConfigureAwait(false);
-        new ReadOnlySpan<byte>(tmp, 0, read).CopyTo(buffer.Span);
-        return read;
-    }
+    public Task<int> ReadAsync(byte[] buffer, CancellationToken cancellationToken) =>
+        _stream.ReadAsync(buffer, 0, buffer.Length, cancellationToken);
 
-    public async ValueTask WriteAsync(ReadOnlyMemory<byte> data, CancellationToken cancellationToken)
-    {
-        byte[] tmp = data.ToArray();
-        await _stream.WriteAsync(tmp, 0, tmp.Length, cancellationToken).ConfigureAwait(false);
-    }
+    public Task WriteAsync(byte[] data, CancellationToken cancellationToken) =>
+        _stream.WriteAsync(data, 0, data.Length, cancellationToken);
 
     public void Dispose()
     {

tests/RioJoy.Core.Tests/Hid/RioHidReportTests.cs

@@ -29,12 +29,12 @@ public class RioHidReportTests
     {
         var report = new RioHidReport();
         report.SetAxis(JoyAxis.Z, 0x1234);
-        Assert.Equal(0x34, report.Bytes[4]);
-        Assert.Equal(0x12, report.Bytes[5]);
+        Assert.Equal(0x34, report.ToArray()[4]);
+        Assert.Equal(0x12, report.ToArray()[5]);
 
         report.SetAxis(JoyAxis.Z, 70000); // over max → clamped to 32767
-        Assert.Equal(0xFF, report.Bytes[4]);
-        Assert.Equal(0x7F, report.Bytes[5]);
+        Assert.Equal(0xFF, report.ToArray()[4]);
+        Assert.Equal(0x7F, report.ToArray()[5]);
     }
 
     [Theory]
@@ -47,7 +47,7 @@ public class RioHidReportTests
     {
         var report = new RioHidReport();
         report.SetHat(hat);
-        Assert.Equal(expected, report.Bytes[12] & 0x0F);
+        Assert.Equal(expected, report.ToArray()[12] & 0x0F);
     }
 
     [Theory]
@@ -59,10 +59,10 @@ public class RioHidReportTests
     {
         var report = new RioHidReport();
         report.SetButton(button, pressed: true);
-        Assert.Equal(mask, report.Bytes[byteIndex]);
+        Assert.Equal(mask, report.ToArray()[byteIndex]);
 
         report.SetButton(button, pressed: false);
-        Assert.Equal(0, report.Bytes[byteIndex]);
+        Assert.Equal(0, report.ToArray()[byteIndex]);
     }
 
     [Fact]
@@ -80,6 +80,6 @@ public class RioHidReportTests
         report.SetButton(1, true);
         report.SetButton(2, true);
         report.SetButton(1, false);
-        Assert.Equal(0x02, report.Bytes[13]); // only button 2 remains
+        Assert.Equal(0x02, report.ToArray()[13]); // only button 2 remains
     }
 }

tests/RioJoy.Core.Tests/Protocol/PacketBuilderTests.cs

@@ -41,7 +41,7 @@ public class PacketBuilderTests
         byte[] packet = PacketBuilder.Build(RioCommand.AnalogReply, payload);
 
         Assert.Equal((byte)RioCommand.AnalogReply, packet[0]);
-        Assert.Equal(payload, packet.AsSpan(1, packet.Length - 2).ToArray());
-        Assert.Equal(RioChecksum.Compute(packet.AsSpan(0, packet.Length - 1)), packet[^1]);
+        Assert.Equal(payload, packet.Skip(1).Take(packet.Length - 2).ToArray());
+        Assert.Equal(RioChecksum.Compute(packet, 0, packet.Length - 1), packet[packet.Length - 1]);
     }
 }

tests/RioJoy.Core.Tests/Protocol/PacketParserTests.cs

@@ -5,7 +5,7 @@ namespace RioJoy.Core.Tests.Protocol;
 
 public class PacketParserTests
 {
-    private static List<RioRxEvent> FeedAll(PacketParser parser, ReadOnlySpan<byte> data)
+    private static List<RioRxEvent> FeedAll(PacketParser parser, byte[] data)
     {
         var events = new List<RioRxEvent>();
         foreach (byte b in data)

tests/RioJoy.Core.Tests/Protocol/RioChecksumTests.cs

@@ -8,7 +8,7 @@ public class RioChecksumTests
     [Fact]
     public void Empty_IsZero()
     {
-        Assert.Equal(0, RioChecksum.Compute(ReadOnlySpan<byte>.Empty));
+        Assert.Equal(0, RioChecksum.Compute(new byte[0]));
     }
 
     [Fact]

tests/RioJoy.Core.Tests/Serial/FakeTransport.cs

@@ -23,13 +23,13 @@ internal sealed class FakeTransport : IRioTransport
     /// <summary>Signal that no more inbound data will arrive (transport closed).</summary>
     public void CompleteIncoming() => _incoming.Writer.TryComplete();
 
-    public async ValueTask<int> ReadAsync(Memory<byte> buffer, CancellationToken cancellationToken)
+    public async Task<int> ReadAsync(byte[] buffer, CancellationToken cancellationToken)
     {
         while (await _incoming.Reader.WaitToReadAsync(cancellationToken).ConfigureAwait(false))
         {
             if (_incoming.Reader.TryRead(out byte[]? chunk))
             {
-                chunk.AsSpan().CopyTo(buffer.Span);
+                Array.Copy(chunk, buffer, chunk.Length);
                 return chunk.Length;
             }
         }
@@ -37,10 +37,10 @@ internal sealed class FakeTransport : IRioTransport
         return 0; // completed
     }
 
-    public ValueTask WriteAsync(ReadOnlyMemory<byte> data, CancellationToken cancellationToken)
+    public Task WriteAsync(byte[] data, CancellationToken cancellationToken)
     {
-        _writes.Writer.TryWrite(data.ToArray());
-        return default; // net48 has no ValueTask.CompletedTask; default(ValueTask) is the completed task
+        _writes.Writer.TryWrite((byte[])data.Clone());
+        return Task.CompletedTask;
     }
 
     /// <summary>Read the next outbound write, failing if none arrives in time.</summary>

tools/RioSerialMonitor/Program.cs

@@ -68,7 +68,7 @@ bool Changed(AnalogReport a, AnalogReport b) =>
 link.PacketReceived += p =>
 {
     packets++;
-    ReadOnlySpan<byte> pl = p.Payload.Span;
+    byte[] pl = p.Payload;
     switch (p.Command)
     {
         case RioCommand.ButtonPressed when pl[0] < RioAddress.ButtonCount: