pdu
— Base Structures¶
API Documentation¶
Contains base classes for modbus request/response/error packets
-
class
pymodbus.pdu.
ModbusPDU
(**kwargs)[source]¶ 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. The value 0x00 represents the broadcast address (also 0xff).
-
check
¶ This is used for LRC/CRC in the serial modbus protocols
-
skip_encode
¶ 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.
-
-
class
pymodbus.pdu.
ModbusResponse
(**kwargs)[source]¶ Base class for a modbus response PDU
-
should_respond
¶ A flag that indicates if this response returns a result back to the client issuing the request
-
_rtu_frame_size
¶ Indicates the size of the modbus rtu response used for calculating how much to read.
-
-
class
pymodbus.pdu.
ExceptionResponse
(function_code, exception_code=None, **kwargs)[source]¶ Base class for a modbus exception PDU
-
__init__
(function_code, exception_code=None, **kwargs)[source]¶ Initializes the modbus exception response
Parameters: - function_code – The function to build an exception response for
- exception_code – The specific modbus exception to return
-
-
class
pymodbus.pdu.
IllegalFunctionRequest
(function_code, **kwargs)[source]¶ 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