GE 869, Communications Manual

Page: 87 / 436

CHAPTER 10: MODBUS PROTOCOL MODBUS PROTOCOL
869 MOTOR PROTECTION SYSTEM – COMMUNICATIONS GUIDE 10-3
The master/slave packets have the following format:
Table 10-1: Master Packet Format for Function Code 03H
Table 10-2: Slave Packet Format for Function Code 03H
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
Table 10-4: Slave Packet Format for Function Code 04H
Address Data
006B 022B
006C 0000
006D 0064
MASTER TRANSMISSION BYTES EXAMPLE DESCRIPTION
SLAVE ADDRESS 1 11 message for slave 17
FUNCTION CODE 1 03 read registers
DATA STARTING ADDRESS 2 00 6B data starting at 006B
NUMBER OF SETPOINTS 2 00 03 3 registers = 6 bytes total
CRC 2 76 87 CRC error code
SLAVE RESPONSE BYTES EXAMPLE DESCRIPTION
SLAVE ADDRESS 1 11 message from slave 17
FUNCTION CODE 1 03 read registers
BYTE COUNT 1 06 3 registers = 6 bytes
DATA 1 (see definition above) 2 02 2B value in address 006B
DATA 2 (see definition above) 2 00 00 value in address 006C
DATA 3 (see definition above) 2 00 64 value in address 006D
CRC 2 54 83 CRC error code
MASTER TRANSMISSION BYTES EXAMPLE DESCRIPTION
SLAVE ADDRESS 1 11 message for slave 17
FUNCTION CODE 1 04 read registers
DATA STARTING ADDRESS 2 00 08 data starting at 0008
NUMBER OF ACTUAL VALUES 2 00 01 1 register = 2 bytes
CRC 2 B2 98 CRC error code
SLAVE RESPONSE BYTES EXAMPLE DESCRIPTION
SLAVE ADDRESS 1 11 message from slave 17
FUNCTION CODE 1 04 read registers