DL06 Frequently Asked Questions
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 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 2021-03-23
Document ID: 1083
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