Write And Read Access Using Function Codes - Siemens SINAMICS S120 Series Function Manual

Communication via Modbus TCP

6.5 Write and read access using function codes

6.5 Write and read access using function codes
Function codes used
For data exchange between the controller and device, predefined function codes are used
for communication via Modbus.
The Control Unit uses the following Modbus function codes:
● FC 03: Holding register to read data from the inverter
● FC 06: Write single register to write to individual register
● FC 16: Write to multiple registers to write to several registers
Structure of a Modbus TCP message
Figure 6-1
Structure of a read request via Modbus function code 03 (FC 03)
Any valid register address is permitted as the start address.
Via FC 03, the control can address more than one register with one request. The number of
addressed registers is contained in bytes 10 and 11 of the read request.
Table 6- 4
Value
MBAP header
03 h
00 h
6D h
00 h
02 h
172
Individual components, including Modbus Application Header (MBAP) and function code
Structure of a read request for device number 17, example
Byte Description
7
8 Register start address "High" (register 40110)
9 Register start address "Low"
10 Number of registers "High" (2 registers: 40110; 40111)
11 number of registers "Low"
Function Manual, 12/2018, 6SL3097-5BD00-0BP0
Communication