Communication Over Modbus Rtu - Siemens SINAMICS G120 CU230P-2 HVAC Operating Instructions Manual

6.2.3

Communication over Modbus RTU

Overview of communication using Modbus
The Modbus protocol is a communication protocol with linear topology based on a
master/slave architecture.
Modbus offers three transmission modes:
● Modbus ASCII
● Modbus RTU
● Modbus TCP
The Control Unit supports Modbus RTU as a slave with even parity.
Communication settings
● Communication using Modbus RTU takes place over the RS485 interface with a
● The maximum cable length is 1200 m (3281 ft).
● Two 100 kΩ resistors are provided to polarize the receive and send cables.
It is not permitted to change over the units!
The "Unit changeover (Page 219)" function is not permissible with this bus system!
Frequency inverters with Control Units CU230P-2 HVAC, CU230P-2 DP, CU230P-2 CAN
Operating Instructions, 01/2011, FW 4.4, A5E02430659B AD
Data is transferred in ASCII code. The data can therefore be read directly by humans,
however, the data throughput is lower in comparison to RTU.
Modbus RTU (RTU: Remote Terminal Unit): Data is transferred in binary format and the
data throughput is greater than in ASCII code.
This type of data transmission is very similar to RTU, except that TCP/IP packages are
used to send the data. TCP port 502 is reserved for Modbus TCP. Modbus TCP is
currently undergoing definition as a standard (IEC PAS 62030 (pre-standard)).
maximum of 247 slaves.
CAUTION
Configuring the fieldbus
6.2 Communication via RS485
133