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Modbus Protocal

Modbus is a serial communication protocol that was published by Modicon since 1979 to serve their programmable logic controller (PLC). It has become a de-facto standard communication protocol in the industry. it is now one of the most common available way to link industrial electronic devices. The main reasons for the extensive use of Modbus over other communication protocols are:

It is openly published and royalty-free
Relatively easy industrial network to deploy
It moves raw bits or words without placing many restrictions on vendors
Modbus allows for communication between many devices connected to the same network, for example a system that measures temperature and humidity and communicates the results to a computer. Modbus is often used to connect a supervisory computer with a remote terminal unit (RTU) in supervisory control and data acquisition (SCADA) systems.

Communication and devices

Each device intended to communicate by Modbus is given a unique address. In serial and mobile networks, only the node which is assigned as the Master can initiate a command. In another hand,Ethernet network, any device can send out a Modbus command, although usually only one master device does so. A Modbus command contains the Modbus address of the device it is intended for. Only the intended device will act on the command, even though other devices might receive it (an exception is specific broadcast-able commands sent to node 0 which are acted upon but not acknowledged). All Modbus commands contain checking information to ensure that a command arrives to the destination correctly. The basic Modbus commands can instruct a RTU to change a value in one of its registers, control or read an I/O port; as well as commanding the device to send back one or more values contained in its registers.