Originally, Modbus was developed by Modicon and it is managed today by the Modbus-IDA User Organization. The protocol is for an open Master/Slave application where on several physical layers it can be used. The protocol is an application-layer messaging method that is positioned at level 7 of the OSI model.
The client/server communication is provided between devices connected to different types of buses or networks. Modbus -TCP is mostly used on the top of Ethernet-TCP/IP. It is an open Industrial Ethernet network that is specified as an RFC Internet standard by the Modbus-IDA User Organization in co-operation with the Internet Engineering Task Force (IETF).
For interoperability and conformance, the Modbus devices are certified by the Modbus-IDA User organization to the Modbus specification. TI is not accepted well by the critics but it is highly valued by people who used it. It’s not a new thing to program. It was majorly needed to approve Ethernet-TCP/IP as an additional data transmission technology for the Modbus Protocol, available from 1979.
This version has been adapted to TCP/IP as per the data transmission protocol. Modbus family has been extended by an additional product range which consists now the classical Modbus-RTU (asynchronous data transmission via RS-232 or RS-485), Modbus-Plus (high-speed communication via a Token Passing Network) and Modbus-TCP (Ethernet-TCP/IP-based Client/server communication). The same application protocol is shared by all these versions where a universal object module is specified for user data and communication services.