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Description of function
The structure of the protocol data unit (PDU) in the case of read access using function code FC 03
is as follows:
• Byte 7: FCode 03
• Byte 8 and Byte 9: Register
• Byte 10 and Byte 11: Number of registers the controller is reading
Example
Table 14-27
Modbus Applica‐
tion Header
Protocol Data Unit
The converter responds with the content of the register requested (bytes 8 ... 11).
Table 14-28
Modbus Applica‐
tion Header
Protocol Data Unit
Table 14-29
Modbus Applica‐
tion Header
Protocol Data Unit
SINAMICS G220 converter
Operating Instructions, 04/2024, FW V6.2, A5E51781573B AB
Structure of a read request
Value Byte
Description
-
0 ... 6
-
03 h
7
Function code
00 h
8
"High" register (40110)
6D h
9
"Low" register
00 h
10
Number of "High" registers (two registers: 40110; 40111)
02 h
11
Number of "Low" registers
Response of the converter to a valid read request
Value
Byte
Description
-
0 ... 6 -
03 h
7
Function code
04 h
8
Number of bytes (four bytes are returned)
11 h
9
Data from the first register, "High"
22 h
10
Data from the first register, "Low"
33 h
11
Data from the second register, "High"
44 h
12
Data from the second register, "Low"
Response of the converter to an invalid read request
Value
Byte
Description
-
0 ... 6 -
83 h
7
Highest bit = 1 and function code
01 h
8
Error code
...
04 h
14.5 Modbus TCP communication
Functions
391
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