You can drill down through the Modbus subVI's to see how they work. The calculation of the checksum is done in one of the subVI'.s It is not the exception code. You'd have to read up on the modbus spec from the modbus organization to figure out the meaning by exception code.
I don't recommend using the Modbus Init. I think some people have reported that it doesn't set the com port properly. I think it sets the stop bits for the port to a value that may or may not match what your device uses. I recommend using the regular Serial Configure VI to set your serial port settings.
