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8.2 MODBUS PROTOCOL
Modbus implementation:
GE Power Management implementation:
Since some PLC implementations of Modbus only support one of function codes 03h and 04h, the 745 inter-
pretation 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 Power Management 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 Power Management setpoints and actual values in the memory map are sent as two byte regis-
ters. 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
DATA STARTING ADDRESS: low order byte
NUMBER OF REGISTERS: high order byte
NUMBER OF REGISTERS: low order byte
CRC: low order byte
CRC: high order byte
Field:
SLAVE ADDRESS
FUNCTION CODE
BYTE COUNT
DATA #1: high order byte
DATA #1: low order byte
DATA #2: high order byte
DATA #2: low order byte
DATA #3: high order byte
8
DATA #3: low order byte
CRC: low order byte
CRC: high order byte
8-6
8.2.9 FUNCTION CODE 03H/04H: READ ACTUAL VALUES/SETPOINTS
Read Holding Registers
Read Actual Values or Setpoints
Example (hex):
11
query message for slave 11
03
read register values
02
data starting at address 0200
00
00
3 register values = 6 bytes total
03
06
E3
Example (hex):
11
response message from slave 11
03
read register values
06
3 register values = 6 bytes total
02
register value in address 0200 = 022B
2B
00
register value in address 0201 = 0000
00
00
register value in address 0202 = 0064
64
C8
BA
745 Transformer Management Relay
8 COMMUNICATIONS
GE Power Management
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