136 Rockwell Automation Publication 825-UM004D-EN-P - November 2012
Chapter 10 Modbus RTU Communications
Modbus Exception Responses
The 825-P sends an exception code under the conditions described in
Table 67.
Table 67 - 825-P Modbus Exception Codes
In the event that any of the errors listed in Table 67 occur, the relay assembles a
response message that includes the exception code in the data field. The relay sets
the most significant bit in the function code field to indicate to the master that
the data field contains an error code, instead of the requested data.
Cyclical Redundancy Check
The 825-P calculates a 2-byte CRC value using the device address, function code,
and data region. It appends this value to the end of every Modbus response.
When the master device receives the response, it recalculates the CRC. If the
calculated CRC matches the CRC sent by the 825-P, the master device uses the
data received. If there is not a match, the check fails and the message is ignored.
The devices use a similar process when the master sends queries.
03h Read Holding Register
Command
Use function code 03h to read directly from the Modbus Register Map shown
in Appendix B.
You may read a maximum of 125 registers at once with this function code. Most
masters use 4X references with this function code. If you are accustomed to 4X
references with this function code, for five-digit addressing, add 40001 to the
standard database address.
Exception
Code
Error Type Description
1 Illegal Function Code The received function code is either undefined or unsupported.
2 Illegal Data Address The received command contains an unsupported address in the data field.
3 Illegal Data Value The received command contains a value that is out of range.
4 Device Error The 825-P is in the wrong state for the requested function.
This also stands for Service Failure for DeviceNet interface
applications. The relay is unable to perform the requested action
(i.e., cannot write to a read-only register).
6 Busy The 825-P is unable to process the
command at this time a resource is busy.
8 Memory Error Checksum error on stored data.
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