8 489 GENERATOR MANAGEMENT RELAY – COMMUNICATIONS GUIDE
CG.2.6 Function Code 08: Loopback Test
Modbus Implementation: Loopback Test
489 Implementation: Loopback Test
This function is used to test the integrity of the communication link. The 489 will echo the
request.
Message Format and Example:
Loopback test from slave 11.
CG.2.7 Function Code 16: Store Multiple Setpoints
Modbus Implementation: Preset Multiple Registers
489 Implementation: Store Multiple Setpoints
This function code allows multiple Setpoints to be stored into the 489 memory. Modbus
“registers” are 16-bit (two byte) values transmitted high order byte first. Thus all 489
setpoints are sent as two bytes. The maximum number of Setpoints that can be stored in
one transmission is dependent on the slave device. Modbus allows up to a maximum of 60
holding registers to be stored. The 489 response to this function code is to echo the slave
address, function code, starting address, the number of Setpoints stored, and the CRC.
Message Format and Example:
Request slave 11 to store the value 01F4 to Setpoint address 1180 and the value 0001 to
setpoint address 1181. After the transmission in this example is complete, 489 slave 11 will
have the following setpoints information stored:
CRC (low, high) 2 C2 08 computed CRC error code
Slave Response Bytes Example Description
Master Transmission Bytes Example Description
Slave Address 1 0B message for slave 11
Function Code 1 08 loopback test
Diagnostic Code 2 00 00 must be 0000h
Data 2 00 00 must be 0000h
CRC (low, high) 2 E0 A1 computed CRC error code
Slave Response Bytes Example Description
Slave Address 1 0B message from slave 11
Function Code 1 08 loopback test
Diagnostic Code 2 00 00 must be 0000h
Data 2 00 00 must be 0000h
CRC (low, high) 2 E0 A1 computed CRC error code
Address Data
1180 01F4
1181 0001
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