Ge 489 User Manual download pdf (Page 165)

GE Multilin 489 Generator Management Relay 6-3

6 COMMUNICATIONS 6.2 MODBUS FUNCTIONS

6.2MODBUS FUNCTIONS 6.2.1 SUPPORTED 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.

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

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: BYTES EXAMPLE (HEX):

SLAVE ADDRESS 1 0B message for slave 11

FUNCTION CODE 1 03 read registers

DATA STARTING ADDRESS 2 02 35 data starting at 0235

NUMBER OF SETPOINTS 2 00 02 2 registers (4 bytes total)

CRC 2 D5 17 CRC calculated by the master

SLAVE RESPONSE: BYTES EXAMPLE (HEX):

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

1 2 ... 160 161 162 163 164 165 166 167 168 169 170 ... 258 259

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Ge 489 User Manual Page 165