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>

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.