RKC INSTRUMENT PG500, Communication Instruction Manual, Page: 42 / 70

4. MODBUS COMMUNICATION PROTOCOL
IMR02F04-E1
36
4.6.3 Diagnostics (Loopback test) [08H]
The master’s query message will be returned as the response message from the slave.
This function checks the communication system between the master and slave (the controller).
Example: Loopback test for slave address 1
Query message
Slave address 01H
Function code 08H
Test code High 00H
Low 00H
Data High 1FH
Low 34H
CRC-16 High E9H
Low ECH
Normal response message
Slave address 01H
Function code 08H
Test code High 00H
Low 00H
Data High 1FH
Low 34H
CRC-16 High E9H
Low ECH
Error response message
Slave address 01H
80H

Function code
88H
Error code 03H
CRC-16 High 06H
Low 01H
Contents will be the same as query message data.
Any pertinent data
Test code must be set to 00.