G.7
Date Code 20110408 Instruction Manual SEL-551 Relay
Modbus RTU Communications Protocol
Modbus RTU Communications Protocol
The relay responses to errors in the query are shown below:
06h Preset Single
Register Command
The SEL-551 uses this function to allow a Modbus master to write directly to
a database register. If you are accustomed to 4X references with this function
code, for 6-digit addressing, add 400001 to the standard database addresses.
The command response is identical to the command request.
The relay responses to errors in the query are shown below:
07h Read Exception
Status Command
The SEL-551 uses this function to allow a Modbus master to read the present
status of the relay and protected circuit.
19 Pulse RB6
20 Pulse RB7
21 Pulse RB8
Error Error Code Returned
Communication Counter
Increments
Invalid bit (coil) number Illegal Data Address (02h) Invalid Address
Illegal bit state requested Illegal Data Value (03h) Illegal Function Code/Op Code
Format error Illegal Data Value (03h) Bad Packet Format
Table G.9 SEL-551 Command Coils (Sheet 2 of 2)
Coil Field
Table G.10 06h Preset Single Register Command
Bytes Field
Requests from the master must have the following format:
1 byte Slave Address
1 byte Function Code (06h)
2 bytes Register Address
2 bytes Data
2 bytes CRC-16
Error Error Code Returned
Communication 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 G.11 07h Read Exception Status Command (Sheet 1 of 2)
Bytes Field
Requests from the master must have the following format:
1 byte Slave Address
1 byte Function Code (07h)
0 bytes No Data Fields Are Sent
2 bytes CRC-16
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