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8 COMMUNICATIONS
The second byte of every message is the function code. Modbus defines function codes of 01h to 7Fh. The GE
Power Management SR Series Modbus protocol supports some of these functions, as summarized below.
Table 8–1: GE POWER MANAGEMENT FUNCTION CODES
FUNCTION CODE
DEFINITION
HEX
DEC
03
3
READ ACTUAL VALUES
OR SETPOINTS
04
4
05
5
EXECUTE OPERATION
06
6
STORE SINGLE
SETPOINT
10
16
STORE MULTIPLE
SETPOINTS
* Since some programmable logic controllers only support function codes 03h (or 04h) and 10h, most
of the above Modbus commands can be performed by reading from or writing to special addresses in
the 745 memory map using these function codes. See section entitled FUNCTION CODE SUBSTI-
NOTE
TUTIONS for details.
GE Power Management
DESCRIPTION
Read actual value or setpoint registers
from one or more consecutive memory
map register addresses.
Perform 745 specific operations.
Write a specific value into a single
setpoint register.
Write specific values into one or more
consecutive setpoint registers.
745 Transformer Management Relay
8.2 MODBUS PROTOCOL
8.2.8 SUPPORTED FUNCTION CODES
SUBSTITUTE
04H
03H
10H
10H
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8
8-5
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