In the Modbus RS485 Communication,

Depending on the result of the processing two types of response can be built:

1. A positive MODBUS response:
the response function code = the request function code

2. A MODBUS Exception response:
if Function Code not supported or Invalid Address or Invalid Data or Error in Processing.

Whether it is mandatory that the Server should provide Exceptional Response if it doesn't support that function code?

What is the standard way of processing a request as per the Modbus Compliance?

 

Your question is answered in the "MODBUS APPLICATION PROTOCOL SPECIFICATION" which you can download for free from "http://www.Modbus-IDA.org". A state diagram is provided for each function showing how it is to be handled and the order in which it is to be handled.

In general though, the specification states that requesting an unsupported function code results in a response with an exception code of 1.