pdu — Base Structures

API Documentation

Contains base classes for modbus request/response/error packets

class pymodbus.pdu.ModbusPDU(**kwargs)

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. The value 0x00 represents the broadcast address (also 0xff).


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


This is used when the message payload has already been encoded. Generally this will occur when the PayloadBuilder is being used to create a complicated message. By setting this to True, the request will pass the currently encoded message through instead of encoding it again.


Initializes the base data for a modbus request

classmethod calculateRtuFrameSize(buffer)

Calculates the size of a PDU.

Parameters:buffer – A buffer containing the data that have been received.
Returns:The number of bytes in the PDU.

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(**kwargs)

Base class for a modbus request PDU


Proxy to the lower level initializer


Builds an error response based on the function

Parameters:exception – The exception to return
Raises:An exception response
class pymodbus.pdu.ModbusResponse(**kwargs)

Base class for a modbus response PDU


A flag that indicates if this response returns a result back to the client issuing the request


Indicates the size of the modbus rtu response used for calculating how much to read.


Proxy to the lower level initializer

class pymodbus.pdu.ModbusExceptions

An enumeration of the valid modbus exceptions

classmethod decode(code)

Given an error code, translate it to a string error name.

Parameters:code – The code number to translate
class pymodbus.pdu.ExceptionResponse(function_code, exception_code=None, **kwargs)

Base class for a modbus exception PDU

__init__(function_code, exception_code=None, **kwargs)

Initializes the modbus exception response

  • function_code – The function to build an exception response for
  • exception_code – The specific modbus exception to return

Builds a representation of an exception response

Returns:The string representation of an exception response

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, **kwargs)

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
__init__(function_code, **kwargs)

Initializes a IllegalFunctionRequest

Parameters:function_code – The function we are erroring on

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