Siemens SINAMICS G120 CU240E-2 - Communication over Modbus RTU; Parameters for Modbus Communication Settings

Connection to a fieldbus
6.4 Communication via RS485
Inverter with CU240B-2 and CU240E-2 Control Units
144 Operating Instructions, 07/2010, FW 4.3.2, A5E02299792B AA
6.4.3 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
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
Modbus RTU (RTU: Remote Terminal Unit): Data is transferred in binary format and the
data throughput is greater than in ASCII code.
● Modbus TCP
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)).
The Control Unit supports Modbus RTU as a slave with even parity.
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Communication settings
● Communication using Modbus RTU takes place over the RS485 interface with a
maximum of 247 slaves.
● The maximum cable length is 1200 m (3281 ft).
● Two 100 kΩ resistors are provided to polarize the receive and send cables.
6.4.3.1 Parameters for Modbus communication settings
Setting the bus address of the inverter
The Modbus address of the inverter can be set using DIP switches on the Control Unit or
using p2021. Using p2021, the address can only be set if all DIP switches for the bus
address are either set to "OFF" (0) or to "ON" (127).
If the address switches are set to a value = 1 … 127, then this address is always active and
p2021 is read-only.
Valid Modbus addressing range: 1 … 247.