Write And Read Access Via Fc 03 And Fc 06 - Siemens Sinamics G120P Operating Instructions Manual

6.8.3.4

Write and read access via FC 03 and FC 06

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 Modbus function code 03, FC 03 (read holding registers) for
reading, and the Modbus function code 06, FC 06 (preset single register) for writing.
Structure of a read request via Modbus function code 03 (FC 03)
Any valid register address is permitted as the start address.
The controller can use the FC 03 to access 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- 22 Invalid read requests
Read request
Invalid register address
Read a write-only register
Read a reserved register
Controller addresses more than 125 registers
The start address and the number of registers of an address
are located outside of a defined register block
Table 6- 23
Example
11 h
03 h
00 h
6D h
00 h
02 h
xx h
xx h
Inverter cabinet units
Operating Instructions, 12/2014, A5E32923362A
Structure of a read request for slave number 17
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"
Inverter response
Exception code 02 (invalid data address)
Telegram in which all values are set to 0.
Exception code 03 (invalid data value)
Exception code 02 (invalid data address)
Operation
6.8 Communication via RS485
237