
CHAPTER 7: MODBUS COMMUNICATIONS
MODBUS FUNCTIONS
PQM POWER QUALITY METER – INSTRUCTION MANUAL
7–11
7.2.7
Function Code
08 – Loopback
Test
Modbus Implementation
: Loopback Test
PQM Implementation
: Loopback Test
This function is used to test the integrity of the communication link. The PQM will echo the
request.
MESSAGE FORMAT AND EXAMPLE:
Loopback test from slave 17.
Table 7–6: Master/Slave Packet Format for Function Code 08h
MASTER
TRANSMISSION
BYTES
EXAMPLE
DESCRIPTION
SLAVE ADDRESS
1
11
message for slave 17
FUNCTION CODE
1
08
loopback test
DIAG CODE
2
00 00
must be 00 00
DATA
2
00 00
must be 00 00
CRC
2
E0 0B
CRC error code
SLAVE RESPONSE
BYTES
EXAMPLE
DESCRIPTION
SLAVE ADDRESS
1
11
message from slave 17
FUNCTION CODE
1
08
loopback test
DIAG CODE
2
00 00
must be 00 00
DATA
2
00 00
must be 00 00
CRC
2
E0 0B
CRC error code