Exception Codes
A normal response from a slave is the echoing of the function code and the supplying of the data that is requested.
Exceptions are reported in the response from a slave node by setting the msb of the function code and inserting the applicable error code in the data field. Only the last example below will result in an exception being reported to the master node.

• If the slave does not receive the query due to a communication error, no response is returned.The master node will eventually process a time-out condition for the query.

• If the slave receives the query, but detects a communication error (parity or CRC), no response is returned. The master program will eventually process a time-out condition for the query.

• If the slave receives the query without a communication error but cannot handle it (e.g. a request to read a non-existent register), the slave will return an exception response informing the master of the nature of the error.

The error codes that could be returned in the data field are:
01 - Illegal or Unknown Function
02 - Illegal Data Address
03 - Illegal Data value