Modbus Tcp Functions - Siemens SIRIUS 3RW5 series Manual

3.2

Modbus TCP functions

Modbus TCP
Modbus TCP is a bus system according to the standard IEC 61784-2 CPF 15/1 with the
transmission protocol Modbus. Modbus TCP is based on a client/server architecture and
transmits data linearly.
As the client, the controller has the initiative in data transmission and transmits requests to
the server. The requests contain the Function Code and additional data. A server cannot
transmit information to the client autonomously. The 3RW5 soft starter works as the server
and can only respond to the requests of the client. The 3RW5 soft starter supplies the
relevant feedback messages to the bits and registers retrieved by the client or takes over the
bits and registers written by the client into the internal memory. No requests can be
transferred from server to server.
The server processes the requests of the client and answers within a certain time with one of
the following acknowledgments:
● Requested data
● Error code
You can find more information about the telegram structure in chapter "frame structure
(Page 23)".
Assignment of data to Modbus addresses with Modbus TCP
All data of the 3RW5 Soft Starter are available in datasets or in the process image.
To be addressable via Modbus, the data in these datasets or in this process image are
converted to Modbus data formats.
Data access to
Read-only bits
Readable and writable bits
Read-only data sets and words (16-bit)
Read/write data sets and words
1 Coil corresponds to 1 bit.
1 Register corresponds to 1 word (2 bytes).
You can find more information about access to the memory areas in chapter
"Function Codes (Page 51)".
SIRIUS 3RW5 Modbus TCP communication module
Manual, 03/2018, A5E43676481002A/RS-AA/001
3.2 Modbus TCP functions
Data type according to Modbus nomenclature
Discrete Inputs
Coils
Input Registers
Holding Registers
Description
21