Two driver-side filters that ≥5 of 6 surveyed vendors expose:
1. Per-tag Deadband (double?, on ModbusTagDefinition) — when set, the
PollGroupEngine onChange callback suppresses publishes whose distance
from the last-published value is below the threshold. Reduces wire
traffic to OPC UA clients on noisy analog signals (flow meters,
temperatures). Numeric scalar types only — Bool / BitInRegister / String
/ array tags publish unconditionally.
2. WriteOnChangeOnly (bool, on ModbusDriverOptions) — when true, the driver
short-circuits writes whose value matches the most recent successful
write to that tag. Saves PLC bandwidth on clients that re-publish the
same setpoint every scan. Cache invalidates on any read that returns a
different value, so HMI-side changes don't get masked.
Both default off so existing deployments see no behaviour change.
Implementation:
- ShouldPublish guard wraps the existing OnDataChange invocation. First sample
always passes through (no baseline); subsequent samples compare via
Convert.ToDouble for the cross-numeric-type math.
- IsRedundantWrite check at the top of WriteAsync; on success the cache is
populated. Object.Equals handles boxed-numeric equality; arrays are
excluded (reference-equality would never match anyway).
- ReadAsync invalidates the WriteOnChangeOnly cache when the new value
differs from the cached last-written value.
Tests (5 new ModbusSubscribeOptionsTests):
- Deadband suppresses sub-threshold changes (100 → 102 → 106 → 107 with
deadband=5 publishes 100 and 106 only).
- Deadband=null still publishes every change.
- WriteOnChangeOnly suppresses 3 identical 42 writes (only first hits wire).
- WriteOnChangeOnly default false hits the wire every time.
- Read-divergence cache invalidation: external panel write to 99, our
client's re-write of 42 must NOT be suppressed.
220/220 unit tests green; existing ProtocolOptions tests hardened against
probe-loop noise by disabling the probe in their fixtures.
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.
