GE 869 Communications Manual page 87

CHAPTER 10: MODBUS PROTOCOL
869 MOTOR PROTECTION SYSTEM – COMMUNICATIONS GUIDE
Address
006B
006C
006D
The master/slave packets have the following format:
Table 10-1: Master Packet Format for Function Code 03H
MASTER TRANSMISSION
SLAVE ADDRESS
FUNCTION CODE
DATA STARTING ADDRESS
NUMBER OF SETPOINTS
CRC
Table 10-2: 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
869 Implementation: Read Actual Values
For the 869 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 869 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 10-3: Master Packet Format for Function Code 04H
MASTER TRANSMISSION
SLAVE ADDRESS
FUNCTION CODE
DATA STARTING ADDRESS
NUMBER OF ACTUAL VALUES
CRC
Table 10-4: Slave Packet Format for Function Code 04H
SLAVE RESPONSE
SLAVE ADDRESS
FUNCTION CODE
Data
022B
0000
0064
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
MODBUS PROTOCOL
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
10-3