Modbus Functions; Supported Functions; Function Codes 03/04: Read Setpoints / Actual Values - GE 489 Instruction Manual

6 COMMUNICATIONS

6.2MODBUS FUNCTIONS

The following functions are supported by the 489:
• Function Code 03: Read Setpoints and Actual Values
• Function Code 04: Read Setpoints and Actual Values
• Function Code 05: Execute Operation
• Function Code 06: Store Single Setpoint
• Function Code 07: Read Device Status
• Function Code 08: Loopback Test
• Function Code 16: Store Multiple Setpoints
A detailed explanation of how the 489 implements these function codes is shown in the following sections.
Modbus implementation: Read Input and Holding Registers
489 Implementation: Read Setpoints and Actual Values
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 489 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:
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
GE Multilin

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

BYTES EXAMPLE (HEX):
1 0B message for slave 11
1 03 read registers
2 02 35 data starting at 0235
2 00 02 2 registers (4 bytes total)
2 D5 17 CRC calculated by the master
BYTES EXAMPLE (HEX):
1 0B response message from slave 11
1 03 read registers
1 04 2 registers = 4 bytes
2 00 64 value in address 0308
2 00 0A value in address 0309
2 EB 91 CRC calculated by the slave
489 Generator Management Relay
6.2 MODBUS FUNCTIONS

6.2.1 SUPPORTED FUNCTIONS

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