pdu — Base Structures

Module author: Galen Collins <bashwork@gmail.com>

Section author: Galen Collins <bashwork@gmail.com>

API Documentation

Contains base classes for modbus request/response/error packets

class pymodbus.pdu.ModbusPDU

Base class for all Modbus mesages

transaction_id

This value is used to uniquely identify a request response pair. It can be implemented as a simple counter

protocol_id

This is a constant set at 0 to indicate Modbus. It is put here for ease of expansion.

unit_id

This is used to route the request to the correct child. In the TCP modbus, it is used for routing (or not used at all. However, for the serial versions, it is used to specify which child to perform the requests against.

check

This is used for LRC/CRC in the serial modbus protocols

decode(data)

Decodes data part of the message.

Parameters:data – is a string object Raises :A not implemented exception

encode()

Encodes the message

Raises :A not implemented exception

class pymodbus.pdu.ModbusRequest

Base class for a modbus request PDU

doException(exception)

Builds an error response based on the function

Parameters:exception – The exception to return Raises :An exception response

class pymodbus.pdu.ModbusResponse

Base class for a modbus response PDU

class pymodbus.pdu.ModbusExceptions

An enumeration of the valid modbus exceptions

class pymodbus.pdu.ExceptionResponse(function_code, exception_code=None)

Base class for a modbus exception PDU

decode(data)

Decodes a modbus exception response

Parameters:data – The packet data to decode

encode()

Encodes a modbus exception response

Returns:The encoded exception packet

class pymodbus.pdu.IllegalFunctionRequest(function_code)

Defines the Modbus slave exception type ‘Illegal Function’ This exception code is returned if the slave:

- does not implement the function code **or**
- is not in a state that allows it to process the function

decode(data)

This is here so this failure will run correctly

Parameters:data – Not used

execute(context)

Builds an illegal function request error response

Parameters:context – The current context for the message Returns:The error response packet