Communication Via Modbus; Introduction; Principle Of Data Transmission - Siemens SIWAREX WP251 Manual

Note
If execution of the function block is faulty, the variables shown do not correspond to the
actual status in the module.
11.2

Communication via Modbus

11.2.1

Introduction

The current process values and parameters can be exchanged via the RS485 interface with
Modbus RTU or the Ethernet interface with Modbus TCP/IP. It is possible to use both
interfaces for the communication.
Note
The SIWAREX WP251 is intended for operation in secure (closed) networks and does not
have any protection against unauthorized data traffic.
The following chapters describe the specifications for handling communication. The following
functions can be executed:
● Export parameters from the electronic weighing system
● Write parameters
● Export current process values
● Monitor messages
11.2.2

Principle of data transmission

This description is valid for communication via Modbus RTU and Modbus TCP/IP.
The standardized MODBUS protocol is used for communication. The master function is
always in the connected communication partner, while the SIWAREX module is always the
slave.
Data transfer is bidirectional. The master function is always in the connected module which
"controls" the communication with corresponding requests to the respective SIWAREX
module address. The SIWAREX module is always the slave and responds to the requests of
the master, provided that the address matches, with a response frame.
Each Modbus partner has its own address. The SIWAREX module has the default address
1. This address can be changed as a parameter (e.g. in SIWATOOL). This address is
without significance if the Ethernet interface is used because the connection is based on the
IP address.
SIWAREX WP251
Manual, 12/2015, A5E37203357A
Communication with SIMATIC S7-1200
11.2 Communication via Modbus
193