GE MM300 manual Function Code 04H, Master/slave packet format for function code 03H

COMMUNICATIONS GUIDE RS485 INTERFACE (MODBUS RTU)

MM300 implementation: Read Setpoints

For the MM300 implementation of Modbus, this function code can be used to read any setpoints (“holding registers”). Holding registers are 16 bit (two byte) values transmitted high order byte first. Thus all MM300 Setpoints 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 number of 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, consider a request for slave 17 to respond with 3 registers starting at address

006B. For this example the register data in these addresses is as follows:

Address	Data
006B	022B
006C	0000
006D	0064

The master/slave packets have the following format:

Table 1: Master/slave packet format for function code 03H

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

Function Code 04H

Modbus Implementation: Read Input Registers

MM300 implementation: Read Actual Values

For the MM300 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 MM300 Actual Values are sent as two bytes. The maximum number of registers that can be read in one transmission is 125.

MM300 MOTOR MANAGEMENT SYSTEM – COMMUNICATIONS GUIDE 5