pdu
— Base Structures¶
API Documentation¶
Contains base classes for modbus request/response/error packets
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class
pymodbus.pdu.
ModbusPDU
(**kwargs)¶ Base class for all Modbus mesages
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transaction_id
¶ This value is used to uniquely identify a request response pair. It can be implemented as a simple counter
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protocol_id
¶ This is a constant set at 0 to indicate Modbus. It is put here for ease of expansion.
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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).
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check
¶ This is used for LRC/CRC in the serial modbus protocols
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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.
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__init__
(**kwargs)¶ Initializes the base data for a modbus request
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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.
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decode
(data)¶ Decodes data part of the message.
Parameters: data – is a string object Raises: A not implemented exception
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encode
()¶ Encodes the message
Raises: A not implemented exception
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class
pymodbus.pdu.
ModbusRequest
(**kwargs)¶ Base class for a modbus request PDU
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__init__
(**kwargs)¶ Proxy to the lower level initializer
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doException
(exception)¶ Builds an error response based on the function
Parameters: exception – The exception to return Raises: An exception response
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class
pymodbus.pdu.
ModbusResponse
(**kwargs)¶ Base class for a modbus response PDU
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should_respond
¶ A flag that indicates if this response returns a result back to the client issuing the request
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_rtu_frame_size
¶ Indicates the size of the modbus rtu response used for calculating how much to read.
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__init__
(**kwargs)¶ Proxy to the lower level initializer
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class
pymodbus.pdu.
ModbusExceptions
¶ An enumeration of the valid modbus exceptions
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classmethod
decode
(code)¶ Given an error code, translate it to a string error name.
Parameters: code – The code number to translate
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classmethod
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class
pymodbus.pdu.
ExceptionResponse
(function_code, exception_code=None, **kwargs)¶ Base class for a modbus exception PDU
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__init__
(function_code, exception_code=None, **kwargs)¶ Initializes the modbus exception response
Parameters: - function_code – The function to build an exception response for
- exception_code – The specific modbus exception to return
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__str__
()¶ Builds a representation of an exception response
Returns: The string representation of an exception response
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decode
(data)¶ Decodes a modbus exception response
Parameters: data – The packet data to decode
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encode
()¶ Encodes a modbus exception response
Returns: The encoded exception packet
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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
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__init__
(function_code, **kwargs)¶ Initializes a IllegalFunctionRequest
Parameters: function_code – The function we are erroring on
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decode
(data)¶ This is here so this failure will run correctly
Parameters: data – Not used
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execute
(context)¶ Builds an illegal function request error response
Parameters: context – The current context for the message Returns: The error response packet
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