schmersal Praxi 100, Manual

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2. Modbus - 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 Modbus Slaves (in Modbus TCP/UDP servers). 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 and ModbusUDP
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. In ModbusTCP networks, 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 messages is independent
of the type of physical interface used. The same protocol can be used regardless 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
11608_uk
Master´s
message
Slave
response
Start
Address
Function
Data
CRC
End
Start
Address
Function
Data
CRC
End
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 peerto- 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 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.