Modbus Protocol is a messaging structure developed by Modicon in 1979. It is used to establish client-server communication between intelligent devices.

Modbus has become a standard communication protocol and is now a commonly available means of connecting industrial electronic device. It is popular in industrial environments because it is openly published and royalty-free.

There are mainly three variants of Modbus protocols:

• Modbus RTU (Remote Terminal Unit) – used in serial communication, and is the most common implementation available for Modbus.
• Modbus ASCII – used in serial communication and makes use of ASCII characters for protocol communication.
• Modbus TCP/IP or Modbus TCP – a Modbus variant used for communications over TCP/IP networks, connecting over port 502.

Some advantages of using Modbus Protocol are:
– It is an open-source protocol.
– Modbus messages are simple messaging structures, making it less difficult to deploy.
– Modbus moves raw words and bits and has very few restrictions.
– The driver can operate and exchange data with the PC in a few hours if the driver is already installed and you are familiar with Ethernet and TCP/IP sockets.
Some disadvantages of using Modbus Protocol are:
– Modbus is restricted to addressing 247 devices on one data link, limiting the number of field devices connected to a master station (Ethernet TCP/IP proving the exception).
– Modbus was designed in the late 1970s to communicate to programmable logic controllers, the number of data types was limited.
– Modbus/TCP is not a Secure Communication Protocol. Modbus/TCP does not provide the option to use security or encryption when communicating between devices. This creates a potential security vulnerability in an automation system.