COMMUNICATIONS GUIDE RS485 INTERFACE (MODBUS RTU)
MM300 MOTOR MANAGEMENT SYSTEM – COMMUNICATIONS GUIDE 5
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:
The master/slave packets have the following format:
Table 1: Master/slave packet format for function code 03H
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.
Address Data
006B 022B
006C 0000
006D 0064
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