Joseph Doherty 55f4044a69 Task #140 — Modbus protocol-behavior knobs ...

Adds ModbusDriverOptions knobs that ≥4 of 6 surveyed vendors expose:

1. MaxCoilsPerRead (ushort, default 2000) — separate from MaxRegistersPerRead
   because coil packing (1 bit per coil) and register packing (16 bits each)
   have different spec ceilings. Coil-array reads above the cap auto-chunk
   the same way register reads have always done. New ReadBitBlockChunkedAsync
   re-assembles per-chunk LSB-first bitmaps into one logical bitmap.

2. UseFC15ForSingleCoilWrites (default false) — forces FC15 (Write Multiple
   Coils with quantity=1) for single-coil writes instead of the default FC05
   (Write Single Coil). Safety / audit PLCs that only accept the multi-write
   codes need this.

3. UseFC16ForSingleRegisterWrites (default false) — same idea for FC16 vs
   FC06 on single holding-register writes.

4. DisableFC23 (default false) — placeholder no-op for the future block-read
   coalescing (#143) work that may opt into FC23 (Read/Write Multiple
   Registers). Lets deployments pre-disable FC23 for PLCs that won't accept
   it, before we ship the optimisation that emits it.

Defaults preserve the historical wire output bit-for-bit (FC05/FC06 for
singles, no chunking under 2000 coils, no FC23). Factory DTO + JSON-binding
extended with parallel fields.

6 new ModbusProtocolOptionsTests covering: defaults, FC05→FC15 forcing,
FC06→FC16 forcing, MaxCoilsPerRead chunking math (2500 coils / 2000 cap →
2 reads of 2000 + 500). Existing 209 unit tests still green.

2026-04-24 23:59:04 -04:00

5.7 KiB

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