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.
