Siemens SINAMICS G120 CU230P-2, Operating Instructions Manual, Page: 142 / 346

Configuring the fieldbus
6.2 Communication via RS485
Frequency inverters with Control Units CU230P-2 HVAC, CU230P-2 DP, CU230P-2 CAN
140
Operating Instructions, 01/2011, FW 4.4, A5E02430659B AD
Table 6- 31 Structure of a read request for slave number 17
Example
Byte Description
11 h
03 h
00 h
6D h
00 h
02 h
xx h
xx h
0
1
2
3
4
5
6
7
Slave address
Function code
Register start address "High" (register 40110)
Register start address "Low"
No. of registers "High" (2 registers: 40110; 40111)
Number of registers "Low"
CRC "Low"
CRC "High"
The response returns the corresponding data set:
Table 6- 32 Slave response to the read request
Example
Byte Description
11 h
03 h
04 h
11 h
22 h
33 h
44 h
xx h
xx h
0
1
2
3
4
5
6
7
8
Slave address
Function code
No. of bytes (4 bytes are returned)
Data of first register "High"
Data of first register "Low"
Data of second register "High"
Data of second register "Low"
CRC "Low"
CRC "High"
Structure of a write request via Modbus function code 06 (FC 06)
The start address is the holding register address. If an incorrect address is entered (a
holding register address does not exist), exception code 02 (invalid data address) is
returned. An attempt to write to a "read-only" register or a reserved register is replied to with
a Modbus error telegram (Exception Code 4 - device failure). In this instance, the detailed
internal error code that occurred on the last parameter access via the holding registers can
be read out via holding register 40499.
Using FC 06, precisely one register can always be addressed with one request. The value
which is to be written to the addressed register is contained in bytes 4 and 5 of the write
request.