Function Codes 03/04: Read Setpoints/Actual Values - GE 469 Manual

6.2 SUPPORTED MODBUS FUNCTIONS
Modbus implementation:
469 Implementation:
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
0308
0309
MASTER TRANSMISSION:
SLAVE ADDRESS
FUNCTION CODE
DATA STARTING ADDRESS
6
NUMBER OF SETPOINTS
CRC
SLAVE RESPONSE:
SLAVE ADDRESS
FUNCTION CODE
BYTE COUNT
DATA 1
DATA 2
CRC
6-8

6.2.3 FUNCTION CODES 03/04: READ SETPOINTS/ACTUAL VALUES

Read Input and Holding Registers
Read Setpoints and Actual Values
Data
0064
000A
BYTES
1
1
2
2
2
BYTES
1
1
1
2
2
2
469 Motor Management Relay
EXAMPLE / DESCRIPTION
0B message for slave 11
03 read registers
03 08 data starting at 0308
00 02 2 registers (4 bytes total)
45 27 CRC calculated by the master
EXAMPLE / DESCRIPTION
0B response message from slave 11
03 read registers
04 2 registers = 4 bytes
00 64 value in address 0308
00 0A value in address 0309
EB 91 CRC calculated by the slave
6 COMMUNICATIONS
GE Power Management