11-8 8 SERIES PROTECTIVE RELAY PLATFORM – COMMUNICATIONS GUIDE
FUNCTION CODE 08H (LOOPBACK TEST) CHAPTER 11: MODBUS PROTOCOL
Function Code 08H (Loopback Test)
Modbus Implementation: Loopback
8 Series relay Implementation: Loopback Test
The function is used to test the integrity of the communication link. The
8 Series relay echos the request.
For example, consider a loopback test from slave 254:
Table 11-12: Master Packet Format for Function Code 08H
Table 11-13: Slave Packet Format for Function Code 08H
ASE 2000 V2 Example
--> 00 00 00 00 00 06 FE 08 00 00 00 00
--> 00 00 00 00 00 06 FE 08 00 00 00 00
MASTER TRANSMISSION BYTES EXAMPLE DESCRIPTION
SLAVE ADDRESS 1 FE message for slave 254
FUNCTION CODE 1 08 loopback test
DIAGNOSTICS CODE 2 00 00 must be 00 00
DATA 2 00 00 must be 00 00
CRC (low, high) 2 F4 04 CRC error code
SLAVE RESPONSE BYTES EXAMPLE DESCRIPTION
SLAVE ADDRESS 1 FE message from slave 17
FUNCTION CODE 1 08 loopback test
DIAGNOSTICS CODE 2 00 00 must be 00 00
DATA 2 00 00 must be 00 00
CRC (low, high) 2 F4 04 CRC error code
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