moat-lib-modbus

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A sans-IO Modbus protocol core implementing the minimal subset used by moat.modbus: PDU encoding/decoding for function codes 1, 2, 3, 4, 5, 6, 15, 16, plus TCP (MBAP) and RTU framers with a client/server role flag, and an RTU-only monitor role for passive bus sniffing.

Stdlib only -- no pymodbus, no anyio.

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Design

The library is strictly sans-IO: it never opens a socket, touches a serial port, sleeps, waits, or spawns a task. Every component is a pure transform -- bytes in -> messages/PDUs out, messages in -> bytes out. The only mutable state permitted is a framer's internal byte accumulator. All real I/O stays in moat.modbus.

Role flag

The same function-code byte decodes to a request on a server and a response on a client. The role is therefore a constructor flag on the framer itself: FramerTCP(role) / FramerRTU(role) where role is True / "server" (decode incoming bytes as requests), False / "client" (decode as responses), or -- for RTU only -- "monitor" (passive bus sniffer; see below).

Monitor mode (RTU only)

FramerRTU("monitor") taps a live master/slave RTU line without participating in it. Sending is prohibited (buildFrame raises ModbusIOError). Frames are decoded as an alternating request/response sequence starting with a request; each decoded frame flips the expectation (queryable via expecting_request). Since the framer is sans-IO it cannot time the inter-frame gap, so the caller calls resetFrame() on an inter-frame timeout, which clears the accumulator and returns the framer to expecting a request -- covering both torn partial frames and silent slaves. Monitor mode is RTU-only; FramerTCP("monitor") is rejected.

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