GE MOTOR MANAGER 2, Instruction Manual

Page: 87 / 220

GE Power Management
MM2 Motor Manager 2 6-7
6 COMMUNICATIONS 6.2 MODBUS FUNCTIONS
6
6.2.3 FUNCTION CODE 04H
Modbus Implementation: Read Input Registers
MM2 implementation: Read Setpoints and Actual Values
For the MM2 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 MM2 Setpoints and Actual Values are sent as two bytes.
The maximum number of registers 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 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 6–3: MASTER/SLAVE PACKET FORMAT FOR FUNCTION CODE 04H
MASTER TRANSMISSION BYTES EXAMPLE DESCRIPTION
SLAVE ADDRESS 1 11 message for slave 17
FUNCTION CODE 1 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