3onedata GW11 Series User Manual page 68

the client will include request data; If there is an error related to requesting a Modbus
function, the message data field will include an exception code that the server
application can use the code to determine the next operation.
Operation when there is no error associated with requesting the Modbus function:

After the host sends a data request, the slave receives the request and checks
the check code successfully, indicating that the processing is error free. Slave
operation excute the operation of the function code.
Operation when there is any error associated with requesting the Modbus

function:
After the host sends the data request, the slave receives the request and verifies
that the check code fails, indicating that the sending has an error. The slave
returns the exception function code and the exception code.
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(I)GW11XX Modbus User Manual
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