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Modbus RTU Protocol Exception Codes

Modbus Exception Codes are error codes that are returned by a Modbus Slave device when it has detected an problem with a command that it received. Exception codes can be generated for several reasons, including bad data values, invalid Modbus function codes, invalid register values and more. Below is a list of standardized Exception codes supported by the Modbus RTU serial protocol.

Exception Code   Description
01 - Illegal Function   The Function value in the Modbus command query is not supported by the slave device.
02 - Illegal Data Address The Register Address value in the Modbus command query is not a valid register in the slave device. There is no supported register of that value in the Slave device.
03 - Illegal Data Value The Data value in the Modbus command query is not supported by the slave device register that is being written to.
04 - Slave Device Failure An error occurred in the slave device while attempting to perform a requested Modbus function query.
05 - Acknowledge The Slave device may return an Acknowledge response after receiving a function query that typically takes a long time to execute. The Acknowledge response may be returned before the function is finished performing the request. 
06 - Slave Device Busy The Slave device is busy processing a function or task. The Modbus Master should resend the command query at a later time.
Finally, when specifying Modbus hardware or software, take a moment to review the documentation for each item and ensure that both the hardware and software will support the necessary functions to operate properly. It is a rare event that there will be problems but this review process will eliminate the possibility of surprises later.

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Modbus Exception/Error Codes

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For more information about the Modbus RTU protocol visit Modbus.Org for more details.
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