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Modbus - general info
Local contacts: http://drives.danfoss.com/danfoss-drives/local-contacts/
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 (or the Client in Modbus TCP/UDP) and the devices supplying information are Mod-
bus Slaves (in Modbus TCP/UDP servers). The Master can also write information to the Slaves. Mod-
bus 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
and ModbusUDP networks it is mandatory to define a unique Slave Address (or Unit identifier num-
ber) for the every Slave Device. Slave Address is a number between 1 and 247. In ModbusTCP net-
works, it is not mandatory to define a unique Slave Address, because the IP address identifies the
device.
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. When using more versatile network systems like
TCP/IP over Ethernet, the Modbus messages are embedded in packets with the format necessary
for the physical interface. In that case Modbus and other types of connections can co-exist at the
same physical interface at the same time. Although the main Modbus message structure is peer-
to-peer, Modbus is able to function on both point-to-point and multidrop networks.
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. There can be more masters in a Modbus network. Addressing 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.
11608_uk
Master´s
message
Slave
response
Start
Address
Function
Data
CRC
End
Start
Address
Function
Data
CRC
End
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