GE Power Management 750/760 Feeder Management Relay 16-
7
16 COMMUNICATIONS 16.3 MODBUS PROTOCOL
16
16.3.6 FUNCTION CODE 03H/04H: READ ACTUAL VALUES/SETPOINTS
This function code allows the master to read one or more consecutive data registers (actual values or set-
points) from an 750/760. Data registers are always 16 bit (two byte) values transmitted high order byte first.
The maximum number of registers that can be read in a single packet is 120. See the section MEMORY MAP
for exact details on the data registers.
Since some PLC implementations of Modbus only support one of function codes 03h and 04h, the 750/760
interpretation allows either function code to be used for reading one or more consecutive data registers. The
data starting address will determine the type of data being read. Function codes 03h and 04h are therefore
identical.
The following table shows the format of the master and slave packets. The example shows a master device
requesting 3 register values starting at address 200h from slave device 11; the slave device responds with the
values 555, 0, and 100 from registers 200h, 201h, and 202h respectively.
Table 16–3: MASTER/SLAVE PACKET FORMAT FOR FUNCTION CODE 03H/04H
MASTER TRANSMISSION SLAVE RESPONSE
PACKET FORMAT EXAMPLE (HEX) PACKET FORMAT EXAMPLE (HEX)
SLAVE ADDRESS 11 SLAVE ADDRESS 11
FUNCTION CODE 03 FUNCTION CODE 03
DATA STARTING ADDRESS - hi 02 BYTE COUNT 06
DATA STARTING ADDRESS - lo 00 DATA #1 - hi 02
NUMBER OF REGISTERS - hi 00 DATA #1 - lo 2B
NUMBER OF REGISTERS - lo 03 DATA #2 - hi 00
CRC - lo 06 DATA #2 - lo 00
CRC - hi E3 DATA #3 - hi 00
DATA #3 - lo 64
CRC - lo C8
CRC - hi BA
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