Phase 3 PR 30 — Modbus integration-test project scaffold. New tests/ZB.MOM.WW.OtOpcUa.Driver.Modbus.IntegrationTests project is the harness modbus-test-plan.md called for: a skip-when-unreachable fixture that TCP-probes a Modbus simulator endpoint (MODBUS_SIM_ENDPOINT, default localhost:502) once per test session, a DL205 device profile stub (single writable holding register at address 100, probe disabled to avoid racing with assertions), and one happy-path smoke test that initializes the real ModbusDriver + real ModbusTcpTransport, writes a known Int16 value, reads it back, and asserts status=0 + value round-trip. No DL205 quirk assertions yet — those land one-per-PR as the user validates each behavior in ModbusPal (word order for 32-bit, register-zero access, coil addressing base, max registers per FC03, response framing under load, exception code on protected-bit coil write).

ModbusSimulatorFixture is a collection fixture so the 2s TCP probe runs once per run, not per test; SkipReason gets a clear operator-facing message ('start ModbusPal or override MODBUS_SIM_ENDPOINT'). Tests call Assert.Skip(sim.SkipReason) rather than silently returning — matches the test-plan convention and reads cleanly in CI logs. DL205Profile.BuildOptions deliberately disables the background probe loop since integration tests drive reads explicitly and the probe would race with assertions. Tag naming uses the DL205_ prefix so filter 'DisplayName~DL205' surfaces device-specific failures at a glance.
Project references: xunit.v3 + Shouldly + Microsoft.NET.Test.Sdk + xunit.runner.visualstudio (matches the existing Driver.Modbus.Tests unit project), project ref to src/Driver.Modbus. Registered in ZB.MOM.WW.OtOpcUa.slnx under tests/. ModbusPal/README.md documents the dev loop (install ModbusPal jar, load profile, start simulator, dotnet test), explains MODBUS_SIM_ENDPOINT override for real-PLC benchwork, and flags DL205.xmpp as the first profile to add in a follow-up PR.
dotnet test run against the scaffold (no simulator running) skips cleanly: 0 failed, 0 passed, 1 skipped, with the SkipReason surfaced. dotnet build clean (0 warnings, 0 errors). Updated docs/v2/modbus-test-plan.md to mark the scaffold PR done and renumbered future PRs from 'PR 27+' to 'PR 31+' to stay in sync with the actual PR chain.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

ZB.MOM.WW.OtOpcUa.slnx

@@ -24,6 +24,7 @@
         <Project Path="tests/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Proxy.Tests/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Proxy.Tests.csproj"/>
         <Project Path="tests/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.E2E/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.E2E.csproj"/>
         <Project Path="tests/ZB.MOM.WW.OtOpcUa.Driver.Modbus.Tests/ZB.MOM.WW.OtOpcUa.Driver.Modbus.Tests.csproj"/>
+        <Project Path="tests/ZB.MOM.WW.OtOpcUa.Driver.Modbus.IntegrationTests/ZB.MOM.WW.OtOpcUa.Driver.Modbus.IntegrationTests.csproj"/>
         <Project Path="tests/ZB.MOM.WW.OtOpcUa.Client.Shared.Tests/ZB.MOM.WW.OtOpcUa.Client.Shared.Tests.csproj"/>
         <Project Path="tests/ZB.MOM.WW.OtOpcUa.Client.CLI.Tests/ZB.MOM.WW.OtOpcUa.Client.CLI.Tests.csproj"/>
         <Project Path="tests/ZB.MOM.WW.OtOpcUa.Client.UI.Tests/ZB.MOM.WW.OtOpcUa.Client.UI.Tests.csproj"/>

docs/v2/modbus-test-plan.md

@@ -6,8 +6,10 @@ routing against a textbook Modbus server. That's necessary but not sufficient: r
 populations disagree with the spec in small, device-specific ways, and a driver that
 passes textbook tests can still misbehave against actual equipment.
 
-This doc is the harness-and-quirks playbook. It's what gets wired up in the
-`tests/Driver.Modbus.IntegrationTests` project when we ship that (PR 26 candidate).
+This doc is the harness-and-quirks playbook. The project it describes lives at
+`tests/ZB.MOM.WW.OtOpcUa.Driver.Modbus.IntegrationTests/` — scaffolded in PR 30 with
+the simulator fixture, DL205 profile stub, and one write/read smoke test. Each
+confirmed DL205 quirk lands in a follow-up PR as a named test in that project.
 
 ## Harness
 
@@ -94,10 +96,13 @@ vendors get promoted into driver defaults or opt-in options:
 
 ## Next concrete PRs
 
-- **PR 26 — Integration test project + DL205 profile scaffold**: creates
-  `tests/Driver.Modbus.IntegrationTests`, imports the ModbusPal profile (or
-  generates it from JSON), adds the fixture with skip-when-unreachable, plus
-  one smoke test that reads a register. No DL205-specific assertions yet — that
-  waits for the user to validate each quirk.
-- **PR 27+**: one PR per confirmed DL205 quirk, landing the named test + any
-  driver-side adjustment (e.g., retry on dropped TxId) needed to pass it.
+- **PR 30 — Integration test project + DL205 profile scaffold** — **DONE**.
+  Shipped `tests/ZB.MOM.WW.OtOpcUa.Driver.Modbus.IntegrationTests` with
+  `ModbusSimulatorFixture` (TCP-probe, skips with a clear `SkipReason` when the
+  endpoint is unreachable), `DL205/DL205Profile.cs` (tag map stub — one
+  writable holding register at address 100), and `DL205/DL205SmokeTests.cs`
+  (write-then-read round-trip). `ModbusPal/` directory holds the README
+  pointing at the to-be-committed `DL205.xmpp` profile.
+- **PR 31+**: one PR per confirmed DL205 quirk, landing the named test + any
+  driver-side adjustment (e.g., retry on dropped TxId) needed to pass it. Drop
+  the `DL205.xmpp` profile into `ModbusPal/` alongside the first quirk PR.

tests/ZB.MOM.WW.OtOpcUa.Driver.Modbus.IntegrationTests/DL205/DL205Profile.cs

@@ -0,0 +1,45 @@
+namespace ZB.MOM.WW.OtOpcUa.Driver.Modbus.IntegrationTests.DL205;
+
+/// <summary>
+///     Tag map for the AutomationDirect DL205 device class. Mirrors what the ModbusPal
+///     <c>.xmpp</c> profile in <c>ModbusPal/DL205.xmpp</c> exposes (or the real PLC, when
+///     <see cref="ModbusSimulatorFixture"/> is pointed at one).
+/// </summary>
+/// <remarks>
+///     This is the scaffold — each tag is deliberately generic so the smoke test has stable
+///     addresses to read. Device-specific quirk tests (word order, max-register, register-zero
+///     access, etc.) will land in their own test classes alongside this profile as the user
+///     validates each behavior in ModbusPal; see <c>docs/v2/modbus-test-plan.md</c> §per-device
+///     quirk catalog for the checklist.
+/// </remarks>
+public static class DL205Profile
+{
+    /// <summary>Holding register the smoke test reads. Address 100 sidesteps the DL205
+    /// register-zero quirk (pending confirmation) — see modbus-test-plan.md.</summary>
+    public const ushort SmokeHoldingRegister = 100;
+
+    /// <summary>Expected value the ModbusPal profile seeds into register 100. When running
+    /// against a real DL205 (or a ModbusPal profile where this register is writable), the smoke
+    /// test seeds this value first, then reads it back.</summary>
+    public const short SmokeHoldingValue = 1234;
+
+    public static ModbusDriverOptions BuildOptions(string host, int port) => new()
+    {
+        Host = host,
+        Port = port,
+        UnitId = 1,
+        Timeout = TimeSpan.FromSeconds(2),
+        Tags =
+        [
+            new ModbusTagDefinition(
+                Name: "DL205_Smoke_HReg100",
+                Region: ModbusRegion.HoldingRegisters,
+                Address: SmokeHoldingRegister,
+                DataType: ModbusDataType.Int16,
+                Writable: true),
+        ],
+        // Disable the background probe loop — integration tests drive reads explicitly and
+        // the probe would race with assertions.
+        Probe = new ModbusProbeOptions { Enabled = false },
+    };
+}

tests/ZB.MOM.WW.OtOpcUa.Driver.Modbus.IntegrationTests/DL205/DL205SmokeTests.cs

@@ -0,0 +1,53 @@
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Driver.Modbus.IntegrationTests;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Modbus.IntegrationTests.DL205;
+
+/// <summary>
+///     End-to-end smoke against the DL205 ModbusPal profile (or a real DL205 when
+///     <c>MODBUS_SIM_ENDPOINT</c> points at one). Drives the full <see cref="ModbusDriver"/>
+///     + real <see cref="ModbusTcpTransport"/> stack — no fake transport. Success proves the
+///     driver can initialize against the simulator, write a known value, and read it back
+///     with the correct status and value, which is the baseline every device-quirk test
+///     builds on.
+/// </summary>
+/// <remarks>
+///     Device-specific quirk tests (word order, max-register, register-zero access, exception
+///     code translation, etc.) land as separate test classes in this directory as each quirk
+///     is validated in ModbusPal. Keep this smoke test deliberately narrow — any deviation
+///     the driver hits beyond "happy-path FC16 + FC03 round-trip" belongs in its own named
+///     test so filtering by device class (<c>--filter DisplayName~DL205</c>) surfaces the
+///     quirk-specific failure mode.
+/// </remarks>
+[Collection(ModbusSimulatorCollection.Name)]
+[Trait("Category", "Integration")]
+[Trait("Device", "DL205")]
+public sealed class DL205SmokeTests(ModbusSimulatorFixture sim)
+{
+    [Fact]
+    public async Task DL205_roundtrip_write_then_read_of_holding_register()
+    {
+        if (sim.SkipReason is not null) Assert.Skip(sim.SkipReason);
+
+        var options = DL205Profile.BuildOptions(sim.Host, sim.Port);
+        await using var driver = new ModbusDriver(options, driverInstanceId: "dl205-smoke");
+        await driver.InitializeAsync(driverConfigJson: "{}", TestContext.Current.CancellationToken);
+
+        // Write first so the test is self-contained — ModbusPal's default register bank is
+        // zeroed at simulator start, and tests must not depend on prior-test state per the
+        // test-plan conventions.
+        var writeResults = await driver.WriteAsync(
+            [new(FullReference: "DL205_Smoke_HReg100", Value: (short)DL205Profile.SmokeHoldingValue)],
+            TestContext.Current.CancellationToken);
+        writeResults.Count.ShouldBe(1);
+        writeResults[0].StatusCode.ShouldBe(0u, "write must succeed against the ModbusPal DL205 profile");
+
+        var readResults = await driver.ReadAsync(
+            ["DL205_Smoke_HReg100"],
+            TestContext.Current.CancellationToken);
+        readResults.Count.ShouldBe(1);
+        readResults[0].StatusCode.ShouldBe(0u);
+        readResults[0].Value.ShouldBe((short)DL205Profile.SmokeHoldingValue);
+    }
+}

tests/ZB.MOM.WW.OtOpcUa.Driver.Modbus.IntegrationTests/ModbusPal/README.md

@@ -0,0 +1,30 @@
+# ModbusPal simulator profiles
+
+Drop device-specific `.xmpp` profiles here. The integration tests connect to the
+endpoint in `MODBUS_SIM_ENDPOINT` (default `localhost:502`) and expect the
+simulator to already be running — tests do not launch ModbusPal themselves,
+because its Java GUI + JRE requirement is heavier than the harness is worth.
+
+## Getting started
+
+1. Download ModbusPal from SourceForge (`modbuspal.jar`).
+2. `java -jar modbuspal.jar` to launch the GUI.
+3. Load a profile from this directory (or configure one manually) and start the
+   simulator on TCP port 502.
+4. `dotnet test tests/ZB.MOM.WW.OtOpcUa.Driver.Modbus.IntegrationTests` — tests
+   auto-skip with a clear `SkipReason` if the TCP probe at the configured
+   endpoint fails within 2 seconds.
+
+## Profile files
+
+- `DL205.xmpp` — _to be added_ — register map reflecting the AutomationDirect
+  DL205 quirks tracked in `docs/v2/modbus-test-plan.md`. The scaffolded smoke
+  test in `DL205/DL205SmokeTests.cs` needs holding register 100 writable and
+  present; a minimal ModbusPal profile with a single holding-register bank at
+  address 100 is sufficient.
+
+## Environment variables
+
+- `MODBUS_SIM_ENDPOINT` — override the simulator endpoint. Accepts `host:port`;
+  defaults to `localhost:502`. Useful when pointing the suite at a real PLC on
+  the bench.

tests/ZB.MOM.WW.OtOpcUa.Driver.Modbus.IntegrationTests/ModbusSimulatorFixture.cs

@@ -0,0 +1,66 @@
+using System.Net.Sockets;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Modbus.IntegrationTests;
+
+/// <summary>
+///     Reachability probe for a Modbus TCP simulator (ModbusPal or a real PLC). Parses
+///     <c>MODBUS_SIM_ENDPOINT</c> (default <c>localhost:502</c>) and TCP-connects once at
+///     fixture construction. Each test checks <see cref="SkipReason"/> and calls
+///     <c>Assert.Skip</c> when the endpoint was unreachable, so a dev box without a running
+///     simulator still passes `dotnet test` cleanly — matches the Galaxy live-smoke pattern in
+///     <c>GalaxyRepositoryLiveSmokeTests</c>.
+/// </summary>
+/// <remarks>
+///     <para>
+///         Do NOT keep the probe socket open for the life of the fixture. The probe is a
+///         one-shot liveness check; tests open their own transports (the real
+///         <see cref="ModbusTcpTransport"/>) against the same endpoint. Sharing a socket
+///         across tests would serialize them on a single TCP stream.
+///     </para>
+///     <para>
+///         The fixture is a collection fixture so the reachability probe runs once per test
+///         session, not per test — checking every test would waste several seconds against a
+///         firewalled endpoint that times out each attempt.
+///     </para>
+/// </remarks>
+public sealed class ModbusSimulatorFixture : IAsyncDisposable
+{
+    private const string DefaultEndpoint = "localhost:502";
+    private const string EndpointEnvVar = "MODBUS_SIM_ENDPOINT";
+
+    public string Host { get; }
+    public int Port { get; }
+    public string? SkipReason { get; }
+
+    public ModbusSimulatorFixture()
+    {
+        var raw = Environment.GetEnvironmentVariable(EndpointEnvVar) ?? DefaultEndpoint;
+        var parts = raw.Split(':', 2);
+        Host = parts[0];
+        Port = parts.Length == 2 && int.TryParse(parts[1], out var p) ? p : 502;
+
+        try
+        {
+            using var client = new TcpClient();
+            var task = client.ConnectAsync(Host, Port);
+            if (!task.Wait(TimeSpan.FromSeconds(2)) || !client.Connected)
+            {
+                SkipReason = $"Modbus simulator at {Host}:{Port} did not accept a TCP connection within 2s. " +
+                             $"Start ModbusPal (or override {EndpointEnvVar}) and re-run.";
+            }
+        }
+        catch (Exception ex)
+        {
+            SkipReason = $"Modbus simulator at {Host}:{Port} unreachable: {ex.GetType().Name}: {ex.Message}. " +
+                         $"Start ModbusPal (or override {EndpointEnvVar}) and re-run.";
+        }
+    }
+
+    public ValueTask DisposeAsync() => ValueTask.CompletedTask;
+}
+
+[Xunit.CollectionDefinition(Name)]
+public sealed class ModbusSimulatorCollection : Xunit.ICollectionFixture<ModbusSimulatorFixture>
+{
+    public const string Name = "ModbusSimulator";
+}

tests/ZB.MOM.WW.OtOpcUa.Driver.Modbus.IntegrationTests/ZB.MOM.WW.OtOpcUa.Driver.Modbus.IntegrationTests.csproj

@@ -0,0 +1,36 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+    <PropertyGroup>
+        <TargetFramework>net10.0</TargetFramework>
+        <Nullable>enable</Nullable>
+        <ImplicitUsings>enable</ImplicitUsings>
+        <IsPackable>false</IsPackable>
+        <IsTestProject>true</IsTestProject>
+        <RootNamespace>ZB.MOM.WW.OtOpcUa.Driver.Modbus.IntegrationTests</RootNamespace>
+    </PropertyGroup>
+
+    <ItemGroup>
+        <PackageReference Include="xunit.v3" Version="1.1.0"/>
+        <PackageReference Include="Shouldly" Version="4.3.0"/>
+        <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.12.0"/>
+        <PackageReference Include="xunit.runner.visualstudio" Version="3.0.2">
+            <PrivateAssets>all</PrivateAssets>
+            <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+        </PackageReference>
+    </ItemGroup>
+
+    <ItemGroup>
+        <ProjectReference Include="..\..\src\ZB.MOM.WW.OtOpcUa.Driver.Modbus\ZB.MOM.WW.OtOpcUa.Driver.Modbus.csproj"/>
+    </ItemGroup>
+
+    <ItemGroup>
+        <None Update="ModbusPal\**\*" CopyToOutputDirectory="PreserveNewest"/>
+        <None Update="DL205\**\*" CopyToOutputDirectory="PreserveNewest"/>
+    </ItemGroup>
+
+    <ItemGroup>
+        <NuGetAuditSuppress Include="https://github.com/advisories/GHSA-37gx-xxp4-5rgx"/>
+        <NuGetAuditSuppress Include="https://github.com/advisories/GHSA-w3x6-4m5h-cxqf"/>
+    </ItemGroup>
+
+</Project>