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


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


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


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.


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


Decodes data part of the message.

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

Encodes the message

Raises :A not implemented exception
class pymodbus.pdu.ModbusRequest

Base class for a modbus request PDU


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


Decodes a modbus exception response

Parameters:data – The packet data to decode

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

This is here so this failure will run correctly

Parameters:data – Not used

Builds an illegal function request error response

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

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