Module codec 

Modbus TCP wire format: MBAP header + PDU.

Reference: MODBUS Application Protocol Specification V1.1b3 and MODBUS Messaging on TCP/IP Implementation Guide V1.0b.

This module is deliberately free of any I/O, LLM or state: it turns bytes into ModbusRequest values and turns answers back into bytes. Everything that the specification determines mechanically (frame legality, quantity limits, the shape of a write echo) lives here; everything a device would decide (what a register reads as, whether a write is accepted) lives in actions.rs and is answered by the model.

There is no register file here, by design — see src/server/modbus/CLAUDE.md.

Structs

Adu

A complete Modbus/TCP Application Data Unit.

Enums

EncodeError

A value the encoder refuses to put on the wire.

FrameError

Why a byte stream could not be turned into a Modbus ADU.

ModbusRequest

A decoded, spec-legal Modbus request.

ModbusResponse

A decoded response, checked against the request it answers.

Constants

EXCEPTION_FLAG

Bit OR-ed into the function code of an exception response.

EXC_ACKNOWLEDGE

EXC_GATEWAY_PATH_UNAVAILABLE

EXC_GATEWAY_TARGET_FAILED

EXC_ILLEGAL_DATA_ADDRESS

EXC_ILLEGAL_DATA_VALUE

EXC_ILLEGAL_FUNCTION

EXC_MEMORY_PARITY_ERROR

EXC_SERVER_DEVICE_BUSY

EXC_SERVER_DEVICE_FAILURE

FC_READ_COILS

FC_READ_DISCRETE_INPUTS

FC_READ_HOLDING_REGISTERS

FC_READ_INPUT_REGISTERS

FC_WRITE_MULTIPLE_COILS

FC_WRITE_MULTIPLE_REGISTERS

FC_WRITE_SINGLE_COIL

FC_WRITE_SINGLE_REGISTER

MAX_ADU_LEN

Largest legal ADU on TCP: MBAP header + PDU.

MAX_PDU_LEN

Largest legal PDU (function code + data), per the Modbus spec.

MAX_READ_RESPONSE_DATA

Most data octets a read response can carry: the PDU ceiling less the function code and the byte-count octet. The byte count is one octet, and MAX_PDU_LEN - 2 is 251, so the PDU bound is the tighter of the two and the only one that has to be stated.

MBAP_HEADER_LEN

Length of the MBAP header: transaction id (2), protocol id (2), length (2), unit id (1).

MODBUS_PROTOCOL_ID

Protocol identifier reserved for Modbus in the MBAP header.

UNKNOWN_EXCEPTION

What exception_name returns for a code the specification does not define.

Functions

encode_adu

Wrap a PDU in an MBAP header addressed back to the requester.

encode_bits_response

PDU for a FC 1/2 read response: byte count then packed bits, least-significant bit of the first byte carrying the first requested coil.

encode_exception

PDU for an exception response: function_code | 0x80, then the exception code.

encode_registers_response

PDU for a FC 3/4 read response: byte count then big-endian registers.

encode_request

Encode a request’s PDU.

encode_write_ack

PDU acknowledging a write.

exception_code_from_name

Map a canonical exception name back to its code.

exception_name

Canonical name for an exception code, for logs and for the model’s vocabulary.

parse_request

Decode a PDU.

parse_response

Decode a response PDU against the request it answers.

try_parse_adu

Try to pull one complete ADU off the front of buf.