Function Code 04H - GE MM3 Instruction Manual

6.2 MM3 SUPPORTED FUNCTIONS
Modbus Implementation: Read Input Registers
MM3 implementation: Read Setpoints and Actual Values
For the MM3 implementation of Modbus, this command can be used to read any setpoints ("holding registers")
or actual values ("input registers"). Holding and input registers are 16 bit (two byte) values transmitted high
order byte first. Thus all MM3 Setpoints and Actual Values are sent as two bytes. The maximum number of reg-
isters that can be read in one transmission is 125. This function code is identical to function code 03.
The slave response to this function code is the slave address, function code, a count of the data bytes to fol-
low, 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 6–3: MASTER/SLAVE PACKET FORMAT FOR FUNCTION CODE 04H
MASTER TRANSMISSION
SLAVE ADDRESS
FUNCTION CODE
DATA STARTING ADDRESS
NUMBER OF ACTUAL VALUES
CRC
SLAVE RESPONSE
SLAVE ADDRESS
FUNCTION CODE
BYTE COUNT
6
DATA (see definition above)
CRC
6-6
BYTES EXAMPLE DESCRIPTION
1 11 message for slave 17
1 04 read registers
2 00 08 data starting at 0008
2 00 01 1 register = 2 bytes
2 B2 98 CRC error code
BYTES EXAMPLE DESCRIPTION
1 11 message from slave 17
1 04 read registers
1 02 1 register = 2 bytes
2 00 00 value in address 0008
2 78 F3 CRC error code
MM3 Motor Manager 3
6 COMMUNICATIONS

6.2.4 FUNCTION CODE 04H

GE Power Management