
CHAPTER 3: MODBUS FUNCTIONS
FUNCTION CODE 05H: EXECUTE OPERATION
750/760 FEEDER MANAGEMENT RELAY – COMMUNICATIONS GUIDE
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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.
Function Code 05h: Execute Operation
This function code allows the master to perform various operations in the 750/760.
Operations are shown in the
Summary of Operation Codes for Function 05h
table.
The following table shows the format of the master and slave packets. The example shows
a master device requesting the slave device 11 to perform a reset. The ‘high’ and ‘low’
Code Value bytes always have the values FFh and 00h, respectively and are a remnant of
the original Modbus definition of this function code.
Master Transmission
Bytes
Example
Description
Slave Address
1
11
message for slave 11
Function Code
1
03
read register values
Data Starting Address
2
02 00
data starting at 0200h
Number of Registers
2
00 03
3 registers = 6 bytes total
CRC (low, high)
2
06 E3
computed CRC error code
Slave Response
Bytes
Example
Description
Slave Address
1
11
message from slave 11
Function Code
1
03
read register values
Byte Count
1
06
3 values = 6 bytes total
Data #1 (high, low)
2
02 2B
value in address 0200h
Data #2 (high, low)
2
00 00
value in address 0201h
Data #3 (high, low)
2
00 64
value in address 0202h
CRC (low, high)
2
C8 BA
computed CRC error code
Master Transmission
Bytes
Example
Description
Slave Address
1
11
message for slave 11
Function Code
1
05
execute operation
Operation Code
2
00 01
remote reset
Code Value
2
FF 00
perform operation
CRC (low, high)
2
DF 6A
computed CRC error code
Slave Response
Bytes
Example
Description
Slave Address
1
11
message from slave 11
Function Code
1
05
execute operation