Function Code 03H - GE MM3 Instruction Manual

6 COMMUNICATIONS
Modbus implementation: Read Holding 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 04.
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 6–2: MASTER/SLAVE PACKET FORMAT FOR FUNCTION CODE 03H
MASTER TRANSMISSION
SLAVE ADDRESS
FUNCTION CODE
DATA STARTING ADDRESS
NUMBER OF SETPOINTS
CRC
SLAVE RESPONSE
SLAVE ADDRESS
FUNCTION CODE
BYTE COUNT
DATA 1 (see definition above)
DATA 2 (see definition above)
DATA 3 (see definition above)
CRC
GE Power Management
BYTES EXAMPLE DESCRIPTION
1 11 message for slave 17
1 03 read registers
2 00 6B data starting at 006B
2 00 03 3 registers = 6 bytes total
2 76 87 CRC error code
BYTES EXAMPLE DESCRIPTION
1 11 message from slave 17
1 03 read registers
1 06 3 registers = 6 bytes
2 02 2B value in address 006B
2 00 00 value in address 006C
2 00 64 value in address 006D
2 54 83 CRC error code
MM3 Motor Manager 3
6.2 MM3 SUPPORTED FUNCTIONS

6.2.3 FUNCTION CODE 03H

6
6-5