GE 845 Instruction Manual page 136

DEVICE
4–32
The master/slave packets have the following format:
Table 4-5: Master Packet Format for Function Code 03H
MASTER TRANSMISSION
SLAVE ADDRESS
FUNCTION CODE
DATA STARTING ADDRESS
NUMBER OF SETPOINTS
CRC
Table 4-6: Slave Packet Format for Function Code 03H
SLAVE RESPONSE
SLAVE ADDRESS
FUNCTION CODE
BYTE COUNT
DATA 1 (see definition above)
DATA 2 (see definition above)
DATA 3 (see definition above)
CRC
Function Code 04H
Modbus Implementation: Read Input Registers
845 Implementation: Read Actual Values
For the 845 implementation of Modbus, the function code can be used to read any actual
values ("input registers"). Input registers are 16 bit (two byte) values transmitted high order
byte first. Thus all 845 Actual Values are sent as two bytes. The maximum number of
registers that can be read in one transmission is 125.
The slave response to this function code is the slave address, function code, a count of the
data bytes to follow, the data itself and the CRC. Each data item is sent as a two byte
number with the high order byte sent first.
For example, request slave 17 to respond with 1 register starting at address 0008. For this
example the value in this register (0008) is 0000.
Table 4-7: Master Packet Format for Function Code 04H
MASTER TRANSMISSION
SLAVE ADDRESS
FUNCTION CODE
DATA STARTING ADDRESS
NUMBER OF ACTUAL VALUES
CRC
Table 4-8: Slave Packet Format for Function Code 04H
SLAVE RESPONSE
SLAVE ADDRESS
FUNCTION CODE
BYTE COUNT
DATA (see definition above)
CRC
Function Code 05H
Modbus Implementation: Force Single Coil
BYTES EXAMPLE
1 11
1 03
2 00 6B
2 00 03
2 76 87
BYTES EXAMPLE
1 11
1 03
1 06
2 02 2B
2 00 00
2 00 64
2 54 83
BYTES EXAMPLE
1 11
1 04
2 00 08
2 00 01
2 B2 98
BYTES EXAMPLE
1 11
1 04
1 02
2 00 00
2 78 F3
845 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 4: SETPOINTS
DESCRIPTION
message for slave 17
read registers
data starting at 006B
3 registers = 6 bytes total
CRC error code
DESCRIPTION
message from slave 17
read registers
3 registers = 6 bytes
value in address 006B
value in address 006C
value in address 006D
CRC error code
DESCRIPTION
message for slave 17
read registers
data starting at 0008
1 register = 2 bytes
CRC error code
DESCRIPTION
message from slave 17
read registers
1 register = 2 bytes
value in address 0008
CRC error code