GE 845 Instruction Manual page 138

DEVICE
4–34
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
845 general status byte:
Table 4-13: Master Packet Format for Function Code 07H
MASTER TRANSMISSION
SLAVE ADDRESS
FUNCTION CODE
CRC
Table 4-14: Slave Packet Format for Function Code 07H
SLAVE RESPONSE
SLAVE ADDRESS
FUNCTION CODE
DEVICE STATUS
(see following table)
CRC
Function code 07H includes the following bit masks:
Table 4-15: Device Table Status
Bit mask
Bit 0
Bit 1
Bit 2
Bit 3
Bit 4
Bit 5
Bit 6
Bit 7
Function Code 08H
Modbus Implementation: Loopback
845 Implementation: Loopback Test
The function is used to test the integrity of the communication link. The 845 echos the
request.
For example, consider a loopback test from slave 17:
Table 4-16: Master Packet Format for Function Code 08H
MASTER TRANSMISSION
SLAVE ADDRESS
FUNCTION CODE
DIAG CODE
DATA
CRC
Table 4-17: Slave Packet Format for Function Code 08H
SLAVE RESPONSE
SLAVE ADDRESS
FUNCTION CODE
BYTES EXAMPLE
1 11
1 07
2 4C 22
BYTES EXAMPLE
1 11
1 07
1 2C
2 22 28
Description
SP Group 3 Active
SP Group 2 Active
Pickup State
Breaker Connected
Breaker Closed
Breaker Tripped
Alarm
Trip
BYTES EXAMPLE
1 11
1 08
2 00 00
2 00 00
2 E0 0B
BYTES EXAMPLE
1 11
1 08
845 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 4: SETPOINTS
DESCRIPTION
message for slave 17
read device status
CRC error code
DESCRIPTION
message from slave 17
read device status
status = 00101100 (in binary)
CRC error code
DESCRIPTION
message for slave 17
loopback test
must be 00 00
must be 00 00
CRC error code
DESCRIPTION
message from slave 17
loopback test