Modbus Rtu Technology - Siemens SITRANS F Operating Instructions Manual

Introduction

1.3 Modbus RTU technology

1.3 Modbus RTU technology
Modbus RTU is an open, serial (RS-232 or RS-485) protocol based on master/slave or
client/server architecture. The protocol interconnects field equipment such as sensors,
actuators, and controllers and is widely used in both process and manufacturing automation.
The fieldbus environment is the base level group of digital networks in the hierarchy of plant
networks.
Features
The SITRANS F Modbus RTU Communication modules comply with the MODBUS Serial
Line Protocol. Among other things this implies a Master-Slave protocol at level 2 of the OSI
model. A node (the master) issues explicit commands to one of the slave-nodes and
processes responses. Slave nodes will not transmit data without a request from the master
node, and do not communicate with other slaves.
Modbus is a mono Master system, which means that only one Master can be connected at
the time.
Communication modes
Two modes of communication are possible, Unicast and Broadcast.
● In unicast mode the Master sends a request to a specific Slave device, and waits a
● In Broadcast mode the master sends out a request to address "0", which means that the
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specified time for a response.
Master
Slave
Figure 1-1 Unicast Communication Mode
information is for all Slave devices on the network. In Broadcast mode there is no
response from the Slave devices.
Master
Slave
Figure 1-2 Broadcast Communication Mode
Slave Slave
Slave Slave
Operating Instructions, 06/2010, A5E00753974-04
Modbus RTU RS485