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What is Modbus and How does it Work?


By Unblocked Hub

August 3, 2022

Modbus is a communication protocol for use with PLC published by Modicon in 1979. Modicon is now owned by Schneider Electric. Modbus is a common language for devices and equipment to communicate with each other.

Modbus communication protocol is the oldest and most popular automation protocol in the field of process automation and SCADA (central control and data acquisition). Knowing how to create Modbus-based networks is essential for any electrician or engineer working in a related field. The ability to integrate devices from different manufacturers is a skill in demand in the industry. This skill will make the person more valuable in the industry.


The development and updating of Modbus protocols is managed by the Modbus organization. Modbus organization is a supplier and user association. Various versions of the Modbus protocol are available for the serial port. These are Ethernet, Modbus RTU, Modbus ASCII, Modbus TCP and Modbus Plus. Modicon has released the Modbus interface for multipoint networking based on master/slave architecture. Communication between nodes is achieved through send request and read response type messages.


  Modbus serial communicates over various types of physical media such as RS-232, RS-422, RS-485 and ethernet. The original modbus interface worked on serial RS-232. Most later applications use RS-485. Because RS-485 allowed for faster speeds, longer distances and multiple devices on a single network. On simple interfaces such as RS-232 or RS-485, Modbus messages are sent straight across the network. And the network is dedicated to Modbus communication only. However, if the network contains multiple heterogeneous devices, a versatile network system such as TCP/IP over Ethernet can be used. In this case, Modbus and other protocol types coexist on the same physical interface. The main Modbus message structure is peer-to-peer.


Modbus can operate on both point-to-point and multi-network connections. Modbus devices communicate using the master-slave technique. Only one device can initiate an action. Other devices respond by sending the requested data to the host or performing the requested action. The master can send messages to the slaves individually or in bulk. Slaves cannot send messages on their own. There is an error checking area for messages. In this error check, it is checked whether the message content is correct or not.