Modbus Rtu - Siemens SIRIUS 3RW5 Equipment Manual

3.3.2

Modbus RTU

Modbus RTU (Remote Terminal Unit) is a bus system with the Modbus transmission
protocol. Modbus RTU is based on a master/slave architecture and transmits data serially.
As the master, the controller has the initiative in data transmission and transmits requests to
the slaves. The requests contain the Function Code and additional data. A slave cannot
transfer any information autonomously to the master. The 3RW5 soft starter works as the
slave and can only respond to the requests of the master. The 3RW5 soft starter supplies
the relevant feedback messages to the bits and registers retrieved by the master or takes
over the bits and registers written by the master into the internal memory. Requests cannot
be transferred between slaves.
The slave processes the requests of the master and answers within a certain time with one
of the following acknowledgments:
● Requested data
● Error code (Exception Code)
You can find more information about the telegram structure in chapter
Modbus RTU telegram format (Page 29).
Broadcast
The slave address "0" is reserved for broadcasting. In a broadcast telegram, the master
transmits a request with a write function to all slaves that are subordinate to the master. The
3RW5 soft starter processes the master's request as a slave, but does not respond to the
broadcast telegram.
Port configuration and communication speed
The 3RW5 Modbus RTU communication module supports different port configurations and
baud rates up to 115.2 kbit/s.
You will find the supported port configuration and baud rates in chapter Modbus RTU
communication parameters (Page 177). You will find the setting options in chapter
Port configuration and baud rate (Page 84).
SIRIUS 3RW5 Modbus communication module
Equipment Manual, 03/2020, A5E43676481002A/RS-AC/003
3.3 Functions of the 3RW5 Modbus communication module
Description
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