Sel 551 - Table G.12 08 H Loopback Diagnostic Command

G.8
SEL-551 Relay Instruction Manual Date Code 20110408
Modbus RTU Communications Protocol
Modbus RTU Communications Protocol
If the bit is set to 1, the following are true:
➤ Output and Alarm contacts are asserted.
➤ Relay inputs are asserted.
➤ Relay is disabled.
If the bit is set to 0, the following are true:
➤ Output and Alarm contacts are deasserted.
➤ Relay inputs are deasserted.
➤ Relay is enabled.
The relay response to errors in the query is shown below:
08h Loopback
Diagnostic Command
The SEL-551 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.
A successful response from the slave will have the following format:
1 byte Slave Address
1 byte Function Code (07h)
1 byte Status Byte
2 bytes CRC-16
The status byte is sent most significant bit first, and consists of the following
bits:
Bit 0 OUT4 Status
Bit 1 OUT3 Status
Bit 2 OUT2 Status
Bit 3 OUT1 Status
Bit 4 Alarm Output status
Bit 5 Input 2 Status
Bit 6 Input 1 Status
Bit 7 Relay Status
Error Error Code Returned
Communication
Counter Increments
Format error Illegal Data Value (03h) Bad Packet Format
Table G.11 07h Read Exception Status Command (Sheet 2 of 2)
Bytes Field
Table G.12 08h Loopback Diagnostic Command (Sheet 1 of 2)
Bytes Field
Requests from the master must have the following format:
1 byte Slave Address
1 byte Function Code (08h)
2 bytes Subfunction (0000h)