Supported Modbus Functions; Overview; Function Codes 03/04: Read Setpoints/Actual Values - GE 489 Instruction Manual
Generator management relay
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Also See for 489:
- Instruction manual (314 pages) ,
- Communications manual (78 pages) ,
- Instruction manual (259 pages)
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6.2 SUPPORTED MODBUS FUNCTIONS
6.2 SUPPORTED MODBUS FUNCTIONS
The following functions are supported by the 489:
•
03 - Read Setpoints and Actual Values
•
04 - Read Setpoints and Actual Values
•
05 - Execute Operation
•
06 - Store Single Setpoint
•
07 - Read Device Status
•
08 - Loopback Test
•
16 - Store Multiple Setpoints
Modbus implementation:
489 Implementation:
For the 489 Modbus implementation, these commands are used to read any setpoint ("holding registers") or actual value
("input registers"). Holding and input registers are 16-bit (two byte) values transmitted high order byte first. Thus all 469 set-
points and actual values are sent as two bytes. The maximum of 125 registers can be read in one transmission. Function
codes 03 and 04 are configured to read setpoints or actual values interchangeably since 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 fol-
low, 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 0235. For this example, the register data in these
addresses is:
6
ADDRESS
DATA
0235
0064
0236
000A
MASTER TRANSMISSION:
SLAVE ADDRESS
FUNCTION CODE
DATA STARTING ADDRESS
NUMBER OF SETPOINTS
CRC
SLAVE RESPONSE:
SLAVE ADDRESS
FUNCTION CODE
BYTE COUNT
DATA 1
DATA 2
CRC
6-4
6.2.2 FUNCTION CODES 03/04: READ SETPOINTS/ACTUAL VALUES
Read Input and Holding Registers
Read Setpoints and Actual Values
BYTES
EXAMPLE (HEX):
1
0B
1
03
2
02 35
2
00 02
2
D5 17
BYTES
EXAMPLE (HEX):
1
0B
1
03
1
04
2
00 64
2
00 0A
2
EB 91
489 Generator Management Relay
message for slave 11
read registers
data starting at 0235
2 registers (4 bytes total)
CRC calculated by the master
response message from slave 11
read registers
2 registers = 4 bytes
value in address 0308
value in address 0309
CRC calculated by the slave
6 COMMUNICATIONS
6.2.1 OVERVIEW
GE Power Management
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