Fieldbus Connection - Vacon 20 X Installation, Technical And Maintenance Manual

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5.4

Fieldbus connection

Modbus is a communication protocol developed by Modicon systems. In other words, it is a way
of sending information between electronic devices. The device requesting the information is
called the Modbus Master and the devices supplying information are Modbus Slaves. In a stan-
dard Modbus network, there is one Master and up to 247 Slaves, each with a unique Slave Ad-
dress from 1 to 247. The Master can also write information to the Slaves. Modbus is typically
used to transmit signals from instrumentation and control devices back to a main controller
or data gathering system.
The Modbus communication interface is built around messages. The format of these Modbus
messages is independent of the type of physical interface used. The same protocol can be used
regardless of the connection type. Thanks to this, Modbus allows for the easy upgrade of the
hardware structure of an industrial network, without the need for significant changes in the
software. A device can also communicate with several Modbus nodes at once, although they
are connected with different interface types, without the need to use a different protocol for ev-
ery connection.
Figure 33.Basic structure of Modbus frame.
On simple interfaces such as RS485, the Modbus messages are sent in plain form over the net-
work. In this case the network is dedicated to Modbus.
Each Modbus message has the same structure. Four basic elements are present in each mes-
sage. The sequence of these elements is the same for all messages, to make it easy to parse
the content of the Modbus message. A conversation is always started by a master in the Mod-
bus network. A Modbus master sends a message and—depending on the contents of the mes-
sage— a slave takes action and responds to it. There can be more masters in a Modbus
network. The address in the message header is used to define which device should respond to
a message. All other nodes on the Modbus network ignore the message if the address field
doesn't match their own address.
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