The FC15, FC16, BitInRegister[] RMW FC16 and FC23 builders allocated one byte
more than they filled (6 + 1 + n / 10 + 1 + n, data copied at offset 6 / 10), so
every multi-write request ended in a stray zero byte beyond the length its
byte-count field declares. A strict device or gateway that checks the frame
length against the byte count refuses it. Each PDU is now exactly 6 + n
(FC15/FC16) or 10 + n (FC23).
Tests: ModbusWritePduFramingTests asserts the exact length and byte-count field
for FC15 (1/8/9 coils), FC16 (array, Float64 array, scalar multi-register), the
BitInRegister RMW FC16 and FC23 (all red before the fix); the parity suite's
conformant device now refuses a frame-length mismatch on FC15/FC16/FC23.
Live: Driver.Modbus.IntegrationTests against 10.100.0.35:5020 (standard
profile) passes, including the FC16 and FC23 multi-register round trips.
Refs #741
Claude-Session: https://claude.ai/code/session_01Ub9hGmYLQvn9XETzw7CMsT
Joseph Doherty
0f305f1827