GE 745 TRANSFORMER MANAGEMENT RELAY User Manual

8-6 745 Transformer Management Relay GE Power Management
8.2 MODBUS PROTOCOL 8 COMMUNICATIONS
8
8.2.9 FUNCTION CODE 03H/04H: READ ACTUAL VALUES/SETPOINTS
Modbus implementation: Read Holding Registers
GE Power Management implementation: Read Actual Values or Setpoints
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: Example (hex):
SLAVE ADDRESS 11 query message for slave 11
FUNCTION CODE 03 read register values
DATA STARTING ADDRESS: high order byte 02 data starting at address 0200
DATA STARTING ADDRESS: low order byte 00
NUMBER OF REGISTERS: high order byte 00 3 register values = 6 bytes total
NUMBER OF REGISTERS: low order byte 03
CRC: low order byte 06
CRC: high order byte E3
Field: Example (hex):
SLAVE ADDRESS 11 response message from slave 11
FUNCTION CODE 03 read register values
BYTE COUNT 06 3 register values = 6 bytes total
DATA #1: high order byte 02 register value in address 0200 = 022B
DATA #1: low order byte 2B
DATA #2: high order byte 00 register value in address 0201 = 0000
DATA #2: low order byte 00
DATA #3: high order byte 00 register value in address 0202 = 0064
DATA #3: low order byte 64
CRC: low order byte C8
CRC: high order byte BA

GE 745 TRANSFORMER MANAGEMENT RELAY Specifications

General

Brand	GE
Model	745 TRANSFORMER MANAGEMENT RELAY
Category	Relays
Language	English