Write And Read Access Using Function Codes; Function Codes Used; Structure Of A Read Request Via Modbus Function Code 03 (Fc 03) - Siemens SINAMICS G120XA Operating Instructions Manual

Advanced commissioning
6.2 Drive control
6.2.3.8

Write and read access using function codes

Basic structure of read and write access using function codes

Function codes used

For data exchange between the master and slave, 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 converter
FC 06: Write single register to write to individual register
FC 16: Write to multiple registers to write to several registers

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 4 and 5 of the read request.
Table 6-17
Value
11 h
03 h
00 h
6D h
00 h
02 h
xx h
xx h
160
Structure of a read request via slave number 17, example
Byte Description
0 Slave address
1 Function code
2 Register start address "High" (register 40110)
3 Register start address "Low"
4 Number of registers "High" (2 registers: 40110; 40111)
5 number of registers "Low"
6 CRC "Low"
7 CRC "High"
Operating Instructions, 03/2019, FW V1.0, A5E44751205B AB
SINAMICS G120XA converter