mask_write_register command processes partial responses

[mickstevens1]

Versions

• Python: 3.10.0
• OS: Win 10
• Pymodbus: 3.0.0.dev3 (also 2.5.3)
• Modbus Hardware (if used): Custom

Pymodbus Specific

• Client: rtu - sync/async

Description

The mask_write_register command processes partial responses resulting in error:

Modbus Error: [Input/Output] No Response received from the remote unit/Unable to decode response

The issue is traced back to the class MaskWriteRegisterResponse which does not include the get_response_pdu_size method. As a result, the client does not wait for the full response and only the initial data in the receive buffer is processed. A race condition results with unpredictable failures.

Adding the following method to the class resolves the problem.

Code and Logs

def get_response_pdu_size(self):
    """
    Func_code (1 byte) + Starting Address (2 byte) + And_mask (2 Bytes) + OrMask (2 Bytes)
    :return:
    """
    return 1 + 2 + 2 + 2

[dhoomakethu]

@mickstevens1 thanks for reporting, would you mind raising a PR with your fix please?