CHAPTER 10: MODBUS PROTOCOL MODBUS PROTOCOL
869 MOTOR PROTECTION SYSTEM – COMMUNICATIONS GUIDE 10-5
Table 10-8: Slave Packet Format for Function Code 06H
Function Code 07H
Modbus Implementation: Read Exception Status
869 Implementation: Read Device Status
The 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 has individual bits set to 1 or 0
depending on the status of the slave device. For this example, consider the following
869
general status byte:
Table 10-9: Master Packet Format for Function Code 07H
Table 10-10: Slave Packet Format for Function Code 07H
Function code 07H includes the following bit masks:
Table 10-11: Device Table Status
Function Code 08H
Modbus Implementation: Loopback
869 Implementation: Loopback Test
The function is used to test the integrity of the communication link. The 869 echos the
request.
For example, consider a loopback test from slave 17:
SLAVE RESPONSE BYTES EXAMPLE DESCRIPTION
SLAVE ADDRESS 1 11 message from slave 17
FUNCTION CODE 1 06 store single setpoint
DATA STARTING ADDRESS 2 04 5C setpoint address 04 5C
DATA 2 00 02 data for setpoint address 04 5C
CRC 2 CB B9 CRC error code
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 following table)
1 2C status = 00101100 (in binary)
CRC 2 22 28 CRC error code
Bit mask Description
Bit 0 SP Group 3 Active
Bit 1 SP Group 2 Active
Bit 2 Pickup State
Bit 3 Breaker Connected
Bit 4 Breaker Closed
Bit 5 Breaker Tripped
Bit 6 Alarm
Bit 7 Trip
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