
7–10
PQM POWER QUALITY METER – INSTRUCTION MANUAL
MODBUS FUNCTIONS
CHAPTER 7: MODBUS COMMUNICATIONS
7.2.6
Function Code
07 – Read
Device Status
Modbus Implementation
: Read Exception Status
PQM Implementation
: Read Device Status
This is a function used to quickly read the status of a selected device. A short message
length allows for rapid reading of status. The status byte returned will have individual bits
set to 1 or 0 depending on the status of the slave device.
PQM General Status Byte:
LSBit
B0: Alarm condition = 1
B1: Self test failure = 1
B2: Alarm relay energized = 1
B3: Aux 1 relay energized = 1
B4: Aux 2 relay energized = 1
B5: Aux 3 relay energized = 1
B6: Not used
MSBit
B7: Not used
MESSAGE FORMAT AND EXAMPLE:
Request status from slave 17.
Table 7–5: Master/Slave Packet Format for Function Code 07h
MASTER
TRANSMISSION
BYTES
EXAMPLE
DESCRIPTION
SLAVE ADDRESS
1
11
message for slave 17
FUNCTION CODE
1
07
read device status
CRC
2
4C 22
CRC error code
SLAVE RESPONSE
BYTES
EXAMPLE
DESCRIPTION
SLAVE ADDRESS
1
11
message from slave 17
FUNCTION CODE
1
07
read device status
DEVICE STATUS (see
definition above)
1
2C
status = 00101100 (in
binary)
CRC
2
22 28
CRC error code