Descriptions of functions
10.8 Communication via Modbus TCP
SINAMICS DCM DC Converter
544
Operating Instructions, 12/2018, A5E34763375A
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 10- 50 Structure of a read request for device number 17, example
Value
Byte
Description
MBAP header
03 h
00 h
6D h
00 h
02 h
7
8
9
10
11
Register start address "High" (register 40110)
Register start address "Low"
Number of registers "High" (2 registers: 40110; 40111)
number of registers "Low"
The response returns the corresponding data set:
Table 10- 51 Device response to the read request, example
Value
Byte
Description
MBAP header
03 h
04 h
11 h
22 h
33 h
44 h
7
8
9
10
11
12
Number of bytes (4 bytes are returned)
Data first register "High"
Data first register "Low"
Data second register "High"
Data second register "Low"
Table 10- 52 Invalid read request
Read request
Inverter response
Invalid register address
Exception code 02 (invalid data address)
Read a write-only register
Telegram in which all values are set to 0.
Read a reserved register
Controller addresses more than 125 registers
Exception code 03 (invalid data value)
The start address and the number of registers of an
address are located outside of a defined register
block
Exception code 02 (invalid data address)
