E.11
Date Code 20170927 Instruction Manual SEL-751 Relay
Modbus RTU Communications
Communications Protocol
The relay responses to errors in the query are shown in Tab le E.17.
08h Loopback
Diagnostic Command
The SEL-751 uses this function to allow a Modbus master to perform a
diagnostic test on the Modbus communications channel and relay. When the
subfunction field is 0000h, the relay returns a replica of the received message.
The relay responses to errors in the query are shown in Tab le E.19.
2 bytes Data
2 bytes CRC-16
Table E.17 Responses to 06h Preset Single Register Query Errors
Error Error Code Returned
Communications Counter
Increments
Illegal register address Illegal Data Address (02h) Invalid Address Illegal Write
Illegal register value Illegal Data Value (03h) Illegal Write
Format error Illegal Data Value (03h) Bad Packet Format
Table E.16 06h Preset Single Register Command (Sheet 2 of 2)
Bytes Field
Table E.18 08h Loopback Diagnostic Command
Bytes Field
Requests from the master must have the following format:
1 byte Slave Address
1 byte Function Code (08h)
2 bytes Subfunction (0000h)
2 bytes Data Field
2 bytes CRC-16
A successful response from the slave will have the following format:
1 byte Slave Address
1 byte Function Code (08h)
2 bytes Subfunction (0000h)
2 bytes Data Field (identical to data in Master request)
2 bytes CRC-16
Table E.19 Responses to 08h Loopback Diagnostic Query Errors
Error Error Code Returned
Communications Counter
Increments
Illegal subfunction code Illegal Data Value (03h) Invalid Function Code/Op
Code
Format error Illegal Data Value (03h) Bad Packet Format
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