Modbus Functions; Supported Modbus Functions - GE TRANSFORMER MANAGEMENT RELAY 745 Instruction Manual

7 COMMUNICATIONS

7.3MODBUS FUNCTIONS

a) DESCRIPTION
The second byte of every message is the function code. Modbus defines function codes of 01h to 7Fh. The GE Multilin SR
Series Modbus protocol supports some of these functions, as summarized below.
Table 7–1: GE MULTILIN MODBUS FUNCTION CODES
FUNCTION CODE
HEX DEC
03 3
04 4
05 5
06 6
10 16
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 7.3.6: Function Code Substitutions on page 7–11 for details.
NOTE
b) FUNCTION CODE 03/04H: READ ACTUAL VALUES OR SETPOINTS
Modbus implementation:
GE Multilin implementation:
Since some PLC implementations of Modbus only support one of function codes 03h and 04h, the 745 interpretation allows
either function code to be used for reading one or more consecutive setpoints or actual values. The data starting address
will determine the type of data being read. Function codes 03h and 04h are therefore identical.
The GE Multilin implementation of Modbus views "holding registers" as any setpoint or actual value register in the 745
memory map. Registers are 16 bit (two byte) values transmitted high order byte first. Thus all GE Multilin setpoints and
actual values in the memory map are sent as two byte registers. This function code allows the master to read one or more
consecutive setpoints or actual values from the addressed slave device.
The maximum number of values that can be read in a single message is 120.
MESSAGE FORMAT AND EXAMPLE:
Request to read 3 register values starting from address 0200 from slave device 11.
MASTER QUERY MESSAGE:
SLAVE ADDRESS
FUNCTION CODE
DATA STARTING ADDRESS: high order byte, low order byte
NUMBER OF REGISTERS: high order byte, low order byte
CRC: low order byte, high order byte
SLAVE RESPONSE:
SLAVE ADDRESS
FUNCTION CODE
BYTE COUNT
DATA #1: high order byte, low order byte
DATA #2: high order byte, low order byte
DATA #3: high order byte, low order byte
CRC: low order byte, high order byte
GE Multilin
Courtesy of NationalSwitchgear.com
DEFINITION
READ ACTUAL VALUES
OR SETPOINTS
EXECUTE OPERATION
STORE SINGLE
SETPOINT
STORE MULTIPLE
SETPOINTS
Read Holding Registers
Read Actual Values or Setpoints
745 Transformer Management Relay

7.3.1 SUPPORTED MODBUS FUNCTIONS

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.
EXAMPLE (HEX):
11 query message for slave 11
03 read register values
02 00 data starting at address 0200
00 03 3 register values = 6 bytes total
06 E3 computed cyclic redundancy check
EXAMPLE (HEX):
11 response message from slave 11
03 read register values
06 3 register values = 6 bytes total
02 2B register value in address 0200 = 022B
00 00 register value in address 0201 = 0000
00 64 register value in address 0202 = 0064
C8 BA computed cyclic redundancy check code
7.3 MODBUS FUNCTIONS
SUBSTITUTE
04H
03H
10H
10H
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