# Modbus test fixture

Coverage map + gap inventory for the Modbus TCP driver's integration-test
harness backed by `pymodbus` simulator profiles per PLC family.

**TL;DR:** Modbus is the best-covered driver — a real `pymodbus` server on
localhost with per-family seed-register profiles, plus a skip-gate when the
simulator port isn't reachable. Covers DL205 / Mitsubishi MELSEC / Siemens
S7-1500 family quirks end-to-end. Gaps are mostly error-path + alarm/history
shaped (neither is a Modbus-side concept).

## What the fixture is

- **Simulator**: `pymodbus` (Python, BSD) launched as a pinned Docker
  container at
  `tests/ZB.MOM.WW.OtOpcUa.Driver.Modbus.IntegrationTests/Docker/`.
  Docker is the only supported launch path.
- **Lifecycle**: `ModbusSimulatorFixture` (collection-scoped) TCP-probes
  `localhost:5020` on first use. `MODBUS_SIM_ENDPOINT` env var overrides the
  endpoint so the same suite can target a real PLC.
- **Profiles**: `DL205Profile`, `MitsubishiProfile`, `S7_1500Profile` —
  each composes device-specific register-format + quirk-seed JSON for pymodbus.
  Profile JSONs live under `Docker/profiles/` and are baked into the image.
- **Compose services**: one per profile (`standard` / `dl205` /
  `mitsubishi` / `s7_1500`); only one binds `:5020` at a time.
- **Tests skip** via `Assert.Skip(sim.SkipReason)` when the probe fails; no
  custom FactAttribute needed because `ModbusSimulatorCollection` carries the
  skip reason.

## What it actually covers

### DL205 (Automation Direct)

- `DL205SmokeTests` — FC16 write → FC03 read round-trip on holding register
- `DL205CoilMappingTests` — Y-output / C-relay / X-input address mapping
  (octal → Modbus offset)
- `DL205ExceptionCodeTests` — Modbus exception 0x02 → OPC UA `BadOutOfRange` against the dl205 profile (natural out-of-range path)
- `ExceptionInjectionTests` — every other exception code in the mapping table (0x01 / 0x03 / 0x04 / 0x05 / 0x06 / 0x0A / 0x0B) against the `exception_injection` profile on both read + write paths
- `DL205FloatCdabQuirkTests` — CDAB word-swap float encoding
- `DL205StringQuirkTests` — packed-string V-memory layout
- `DL205VMemoryQuirkTests` — V-memory octal addressing
- `DL205XInputTests` — X-register read-only enforcement

### Mitsubishi MELSEC

- `MitsubishiSmokeTests` — read + write round-trip
- `MitsubishiQuirkTests` — word-order, device-code mapping (D/M/X/Y ranges)

### Siemens S7-1500 (Modbus gateway flavor)

- `S7_1500SmokeTests` — read + write round-trip
- `S7_ByteOrderTests` — ABCD/DCBA/BADC/CDAB byte-order matrix

### Capability surfaces hit

- `IReadable` + `IWritable` — full round-trip
- `ISubscribable` — via the shared `PollGroupEngine` (polled subscription)
- `IHostConnectivityProbe` — TCP-reach transitions

## What it does NOT cover

### 1. No `ITagDiscovery`

Modbus has no symbol table — the driver requires a static tag map from
`DriverConfig`. There is no discovery path to test + none in the fixture.

### 2. Error-path fuzzing

`pymodbus` serves the seeded values happily; the fixture can't easily inject
exception responses (code 0x01–0x0B) or malformed PDUs. The
`AbCipStatusMapper`-equivalent for exception codes is unit-tested via
`DL205ExceptionCodeTests` but the simulator itself never refuses a read.

### 3. Variant-specific quirks beyond the three profiles

- FX5U / QJ71MT91 Mitsubishi variants — profile scaffolds exist, no tests yet
- Non-S7-1500 Siemens (S7-1200 / ET200SP) — byte-order covered but
  connection-pool + fragmentation quirks untested
- DL205-family cousins (DL06, DL260) — no dedicated profile

### 4. Subscription stress

`PollGroupEngine` is unit-tested standalone but the simulator doesn't exercise
it under multi-register packing stress (FC03 with 125-register batches,
boundary splits, etc.).

### 5. Alarms / history

Not a Modbus concept. Driver doesn't implement `IAlarmSource` or
`IHistoryProvider`; no test coverage is the correct shape.

## When to trust the Modbus fixture, when to reach for a rig

| Question | Fixture | Unit tests | Real PLC |
| --- | --- | --- | --- |
| "Does FC03/FC06/FC16 work end-to-end?" | yes | - | yes |
| "Does DL205 octal addressing map correctly?" | yes | yes | yes |
| "Does float CDAB word-swap round-trip?" | yes | yes | yes |
| "Does the driver handle exception responses?" | no | yes | yes (required) |
| "Does packing 125 regs into one FC03 work?" | no | no | yes (required) |
| "Does FX5U behave like Q-series?" | no | no | yes (required) |

## Follow-up candidates

1. Add `MODBUS_SIM_ENDPOINT` override documentation to
   `docs/v2/test-data-sources.md` so operators can point the suite at a lab rig.
2. ~~Extend `pymodbus` profiles to inject exception responses~~ — **shipped**
   via the `exception_injection` compose profile + standalone
   `exception_injector.py` server. Rules in
   `Docker/profiles/exception_injection.json` map `(fc, address)` to an
   exception code; `ExceptionInjectionTests` exercises every code in
   `MapModbusExceptionToStatus` (0x01 / 0x02 / 0x03 / 0x04 / 0x05 / 0x06 /
   0x0A / 0x0B) end-to-end on both read (FC03) and write (FC06) paths.
3. Add an FX5U profile once a lab rig is available; the scaffolding is in place.

## Key fixture / config files

- `tests/ZB.MOM.WW.OtOpcUa.Driver.Modbus.IntegrationTests/ModbusSimulatorFixture.cs`
- `tests/ZB.MOM.WW.OtOpcUa.Driver.Modbus.IntegrationTests/DL205/DL205Profile.cs`
- `tests/ZB.MOM.WW.OtOpcUa.Driver.Modbus.IntegrationTests/Mitsubishi/MitsubishiProfile.cs`
- `tests/ZB.MOM.WW.OtOpcUa.Driver.Modbus.IntegrationTests/S7/S7_1500Profile.cs`
- `tests/ZB.MOM.WW.OtOpcUa.Driver.Modbus.IntegrationTests/Docker/` —
  Dockerfile + compose + per-family JSON profiles
