Question

What is the format of the MWX/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 so:

• 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).

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FAQ Subcategory: None

This item was last updated on 2011-04-29
Document ID: 1084

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