Siemens SINAMICS G220 - Page 393

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 Structure of a read request
 Value Byte Description
Modbus Applica‐
tion Header
- 0 … 6 -
Protocol Data Unit 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
The converter responds with the content of the register requested (bytes 8 … 11).
Table 14-28 Response of the converter to a valid read request
 Value Byte Description
Modbus Applica‐
tion Header
- 0 … 6 -
Protocol Data Unit 03h 7 Function code
04h 8 Number of bytes (four bytes are returned)
11h 9 Data from the 󹪝rst register, "High"
22h 10 Data from the 󹪝rst register, "Low"
33h 11 Data from the second register, "High"
44h 12 Data from the second register, "Low"
Table 14-29 Response of the converter to an invalid read request
 Value Byte Description
Modbus Applica‐
tion Header
- 0 … 6 -
Protocol Data Unit 83h 7 Highest bit = 1 and function code
01h
…
04h
8 Error code
Functions
14.5Modbus TCP communication
SINAMICS G220 converter
Operating Instructions, 04/2024, FW V6.2, A5E51781573B AB 391