6-8 469 Motor Management Relay GE Power Management
6.2 SUPPORTED MODBUS FUNCTIONS 6 COMMUNICATIONS
6
6.2.3 FUNCTION CODES 03/04: READ SETPOINTS/ACTUAL VALUES
Modbus implementation: Read Input and Holding Registers
469 Implementation: Read Setpoints and Actual Values
For the 469 implementation of Modbus, these commands can be used to read any Setpoint ("holding regis-
ters") or Actual Value ("input registers"). Holding and input registers are 16 bit (two byte) values transmitted
high order byte first. Thus all 469 Setpoints and Actual Values are sent as two bytes. The maximum number of
registers that can be read in one transmission is 125. Function codes 03 and 04 are configured to read set-
points or actual values interchangeably because some PLCs do not support both function codes.
The slave response to these function codes 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. The CRC is sent as a two byte number with the low order byte sent first.
MESSAGE FORMAT AND EXAMPLE
Request slave 11 to respond with 2 registers starting at address 0308. For this example the register data in
these addresses is:
Address Data
0308 0064
0309 000A
MASTER TRANSMISSION: BYTES EXAMPLE / DESCRIPTION
SLAVE ADDRESS 1 0B message for slave 11
FUNCTION CODE 1 03 read registers
DATA STARTING ADDRESS 2 03 08 data starting at 0308
NUMBER OF SETPOINTS 2 00 02 2 registers (4 bytes total)
CRC 2 45 27 CRC calculated by the master
SLAVE RESPONSE: BYTES EXAMPLE / DESCRIPTION
SLAVE ADDRESS 1 0B response message from slave 11
FUNCTION CODE 1 03 read registers
BYTE COUNT 1 04 2 registers = 4 bytes
DATA 1 2 00 64 value in address 0308
DATA 2 2 00 0A value in address 0309
CRC 2 EB 91 CRC calculated by the slave
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