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5. Modbus Applications
The tWF include a Modbus port that allows you to access terminals data via Wi-Fi
and communicates using a master-slave technique in which only one device (the master)
can initiate transactions (called queries). The other devices (slaves) respond by supplying
the requested data to the master, or by taking the action requested in the query.
Most SCADA (Supervisor Control And Data Acquisition) and HMI software can
easily integrate serial devices via the Modbus protocol, such as Citect, ICONICS, iFIX,
InduSoft, Intouch, Entivity Studio, Entivity Live, Entivity VLC, Trace Mode, Wizcon,
Wonderware, etc.
For Android Device, a freeware HMI Modbus is easy to use.
5.1 What is Modbus TCP/IP?
Modbus is a communication protocol developed by Modicon in 1979.
Different versions of Modbus used today include Modbus RTU (based on serial
communication like RS485 and RS232), Modbus ASCII and Modbus TCP, which is the
Modbus RTU protocol embedded into TCP packets.
Modbus TCP is an internet protocol. The protocol embeds a Modbus frame into a
TCP frame so that a connection oriented approach is obtained thereby making it reliable.
The master query's the slave and the slave responds with the reply. The protocol is open
and hence highly scalable.
5.2 Protocol Description
The Modbus protocol defines a simple protocol data unit independent of the
underlying communication layers. The mapping of Modbus protocol on network can
introduce some additional fields on the application data unit.
Transaction ID
(2 bytes)
tWF-DA1PD2R1
tWF-DA1PD2R1 User's Manual
Modbus/TCP Application Data Unit
Protocol ID
Length
(2 bytes)
(2 bytes)
MBAP Header
Figure 5-1: Modbus/TCP Application Data Unit
User's Manual (Rev1.0, APR./2019) ------------- 26
Unit ID
FCode
(1 bytes)
(1 bytes)
Protocol Data Unit
Data
(0 to 252 bytes)
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