Modbus is a widely used industrial communication protocol originally developed by Modicon (now part of Schneider Electric) in 1979. It is open, royalty-free, and simple to implement, making it one of the most popular protocols for connecting Programmable Logic Controllers (PLCs), Remote Terminal Units (RTUs), sensors, and other industrial devices.

Modbus is primarily used for data exchange in supervisory control and data acquisition (SCADA) systems and distributed control systems (DCS). The protocol is designed for high reliability and deterministic communication, which is essential in industrial automation environments.

Modbus Protocol Overview

Modbus operates using a master-slave (or client-server) architecture, where:

• The Master (Client) initiates requests, reading from or writing to devices.
• The Slaves (Servers) respond to the requests but do not initiate communication independently.

A master device (such as an HMI or SCADA system) communicates with one or more slave devices (such as PLCs, RTUs, or field devices) over a network. Each slave is assigned a unique 8-bit address (ranging from 1 to 247 in RTU mode). Modbus RTU (Remote Terminal Unit) is usually used via RS-485 (serial network).

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