AutomationDirect Logo DL06 Frequently Asked Questions

Question What is the format of the MRW/MRX exception buffer?
Answer 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 write request is invalid for this slave device
  • 04 = Slave Device Failure = Unrecoverable error in the slave device
  • 05 = Acknowledge
  • 06 = Slave Device Busy
  • 07 = Negative Acknowledge
  • 08 = Memory Parity Error

The Exception Response format and error code information comes from this document:
www.modbus.org/docs/PI_MBUS_300.pdf
Starting at page 93 (100 of 121).

FAQ Subcategory: None

This item was last updated on 08-25-2009
Document ID: 1083

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