What is the format of the MRW/MRX exception buffer?
The instructions require 3 consecutive words. If a Modbus slave device does not like something in a Modbus request it receives, it will respond with an exception response. The exception response at the protocol level looks like: • Byte 1 = Address Byte • Byte 2 = Function Code of Request with Most Significant Bit Set • Byte 3 = Exception Code • Byte 4 and 5 = CRC Error Check Characters These bytes are swapped in the MRX/MWX exception response buffer. • V-Memory 1 High Byte = Function Code Byte • V-Memory 1 Low Byte = Address Byte • V-Memory 2 High Byte = One of the CRC Bytes • V-Memory 2 Low Byte = Exception Code Bytes • V-Memory 3 High Byte = 0 • V-Memory 3 Low Byte = Other CRC Byte The eight exception codes defined by the Modbus protocol are: • 01 = Illegal Function Code = This slave device does not support the requested function code • 02 = Illegal Data Address = The requested data address is not valid for this slave device • 03 = Illegal Data Value = One of the data values in a wri
