Lifts the previous "one driver = one slave" assumption so a single Modbus
driver instance can front N RTU slaves behind one Ethernet gateway (Anybus,
ProSoft, Lantronix style). Each tag carries an optional UnitId that drives
the MBAP unit-id byte per-PDU, and the IPerCallHostResolver contract surfaces
per-slave host strings so per-PLC circuit breakers fire per-slave (matches
the AB CIP template documented in docs/v2/multi-host-dispatch.md).
Changes:
- ModbusTagDefinition gains optional UnitId (byte?). Null = use driver-level
ModbusDriverOptions.UnitId (preserves single-slave deployments verbatim).
- ResolveUnitId(tag) helper computed once per ReadOneAsync / WriteOneAsync
call; passed through ReadRegisterBlockAsync / ReadBitBlockAsync /
ReadRegisterBlockChunkedAsync / ReadBitBlockChunkedAsync explicitly. The
probe loop continues using driver-level UnitId (the probe is a
connection-health check, not slave-specific).
- ModbusDriver implements IPerCallHostResolver. ResolveHost(fullReference)
returns "host:port/unitN" — distinct strings per slave so the resilience
pipeline keys breakers on the right granularity. Unknown references fall
back to the bare HostName (single-slave behaviour).
- BitInRegister RMW path also threads the per-tag UnitId through both the
read and write halves so a multi-slave deployment stays correct under bit-
level writes.
- Factory DTO + JSON binding extended with the per-tag UnitId field.
Tests (4 new ModbusMultiUnitTests):
- Per-tag UnitId routes to the correct slave in the MBAP header (driver-level
UnitId=99 must NOT appear when both tags override).
- Tag without override falls back to driver-level UnitId.
- IPerCallHostResolver returns distinct "host:port/unitN" strings per slave.
- Unknown reference returns the bare HostName fallback.
Existing 220 unit tests + 107 addressing tests still green. Per-PLC breaker
isolation under simulated dead slaves is verifiable via the existing AB CIP
test infra; live coverage lands as an integration test in the #138 docs/e2e
refresh.
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