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Modbus - general info vacon • 5
Local contacts: http://drives.danfoss.com/danfoss-drives/local-contacts/
2
2. M
ODBUS
-
GENERAL INFO
Modbus is a communication protocol developed by Modicon systems. In simple terms, 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. 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.
Standard Modbus network contains one Master device and up to 247 Slave devices. In ModbusRTU
networks it is mandatory to define a unique Slave Address (or Unit identifier number) for the every
Slave Device. Slave Address is a number between 1 and 247. Modbus ASCII mode is not supported.
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. Because of this, Modbus gives the possibility to easily upgrade the
hardware structure of an industrial network, without the need for large changes in the software. A
device can also communicate with several Modbus nodes at once, even if they are connected with
different interface types, without the need to use a different protocol for every connection.
Figure 1.Basic structure of Modbus frame
On simple interfaces like 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 of the contents of the message—a slave takes
action and responds to it. Addressing 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 does not match their own address.
Your VACON
®
20 family AC drive is equipped with Modbus support as standard. If you need to con-
tact VACON
®
service in problems related to Modbus, send a description of the problem together
with the Drive Info File taken with VACON
®
Live to customer support. If possible, also send a "Wire-
shark" log from the situation if applicable.
11608_uk
Master´s
message
Slave
response
Start
Address
Function
Data
CRC
End
Start
Address
Function
Data
CRC
End