Control unit
vacon • 53
Local contacts: http://drives.danfoss.com/danfoss-drives/local-contacts/
5
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 standard Modbus
network, there is one Master and up to 247 Slaves, each with a unique Slave Address 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 mes-
sages is independent of the type of physical interface used. The same protocol can be used regard-
less 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 differ-
ent interface types, without the need to use a different protocol for every connection.
Figure 39.Basic structure of Modbus frame.
On simple interfaces such as RS485, the Modbus messages are sent in plain form over the network.
In this case the network is dedicated to Modbus.
Each Modbus message has the same structure. Four basic elements are present in each message.
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 Modbus network. A
Modbus master sends a message and—depending on the contents of the message— a slave takes
action and responds to it. There can be more masters in a Modbus network. The address in the mes-
sage 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 does not match their own address.
