The device counterpart to warden-modbus's master/scanner: request-frame in,
response-frame out, in host memory. CRC16 is byte-identical to the master
(poly 0xA001, low-first; known vector 01 03 00 00 00 01 -> 84 0A verified).
Implements the data plane — read/write holding & input registers, coils,
discrete inputs (FC 0x01-0x06, 0x0F, 0x10) + Report Slave ID (0x11) — with
exception replies (illegal function/address/value) and the two real-world
faults the master must survive: a device that silently ignores a request
(drop_next) and one that NAKs everything (force_exception). This is what the
modbus-master MC/DC harness drives against; MEI (0x2B/0x0E) is a follow-up.
11 tests, sim crate 25/25 green.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017wB8KB3MMQztRDXCMCkPrf
