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Technologies - Modbus TCP/IP

Modbus is a data communications protocol commonly used for connecting industrial electronic devices. Modbus was originally published by Modicon (now Schneider Electric) in 1979 for use with its programmable logic controllers (PLCs) in industrial applications. It is often used to connect a supervisory computer with a remote terminal unit (RTU) in Supervisory Control and Data Acquisition (SCADA) systems. 


Compared to other standards, Modbus is relatively easy to deploy and maintain, and places few restrictions on the format of the data to be transmitted. Its popularity is boosted by the fact that it is openly published and royalty-free.  Modbus uses the RS485 or Ethernet as its wiring type, and supports communication to and from multiple devices connected to the same cable or Ethernet network.


Modbus TCP/IP Modbus TCP/IP (also known as just Modbus-TCP), is a Modbus RTU protocol that runs over Ethernet, using the TCP/IP (Transmission Control Protocol and Internet Protocol) networking standard. TCP/IP is just the transport protocol, so does not change the way data is stored or interpreted inside the message. A Modbus TCP/IP message is basically just a Modbus communication data message encapsulated in an Ethernet TCP/IP cover.

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Modbus TCP/IP works on clients and servers model. Modbus parent modules are referred to as clients and Modbus child modules as servers. Modbus TCP/IP has some advantages over serial Modbus. With serial Modbus, the client can send one data request at a time, but must then wait for a reply before sending a second reply. Modbus TCP/IP devices can send several requests to the same server without waiting for the reply/response. TCP connections can only be established by clients whereas the server can’t initiate the TCP transactions. The server should have open TCP connection and close it with each Modbus message. The client can close a connection when required, and also process a message from server. Some of the Modbus devices can act as both servers and clients, in this case two-way communication is possible with separate connections for client and server data flow.

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