GE Industrial Solutions MX350 Communications Guide - Manual (Page 11)

COMMUNICATIONS GUIDE RS485 INTERFACE (MODBUS RTU)
MX350 AUTOMATIC TRANSFER CONTROL SYSTEM – COMMUNICATIONS GUIDE 7
The master/slave packets have the following format:
Table 2: MASTER/SLAVE PACKET FORMAT FOR FUNCTION CODE 03H
Function Code 04H Modbus Implementation: Read Input Registers
MX350 Implementation: Read Actual Values
For the MX350 implementation of Modbus, this 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 MX350Actual 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 3: MASTER/SLAVE PACKET FORMAT FOR FUNCTION CODE 04H
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
BYTE COUNT 1 02 1 register = 2 bytes
DATA (see definition above) 2 00 00 value in address
0008
CRC 2 78 F3 CRC error code