Introduction to Modbus and Its Functionality


Resources:

The Modbus Organization Website

What is Modbus and How does it Work?

Understanding Modbus Communication Protocol

What is Modbus?

Modbus is a communication protocol developed by Modicon (now Schneider Electric) in 1979 for use with programmable logic controllers (PLCs). It is one of the most widely used automation protocols in industrial applications.

Key Features of Modbus

1. Open Protocol:

Modbus is an open protocol, meaning its specifications are publicly available and can be used freely or under license. This promotes compatibility and integration across different manufacturers and devices.

2. Master/Slave Architecture:

Modbus uses a master/slave (or client/server) architecture, where only one device (the master) can initiate communication, and the other devices (slaves) respond.

3. Physical Media:

Modbus can communicate over several types of physical media, including:

  • Serial RS-232, RS-485, RS-422

  • Ethernet (Modbus TCP)

Types of Modbus Protocols

1. Modbus RTU:

Used for serial communication, it encodes data in a compact binary format, allowing for efficient communication.

2. Modbus ASCII:

Also used for serial communication but encodes data in ASCII characters, making it easier to read but less efficient than RTU.

3. Modbus TCP:

Used for communication over Ethernet networks, it encapsulates Modbus messages in TCP/IP packets, allowing for mixed-protocol environments and greater versatility.

Communication Process

Master/Slave Communication:

The master sends a query to the slave, and the slave responds with the requested data or action. The master can address individual slaves or broadcast messages to all slaves.

Message Structure:

Modbus messages include a slave address, function code (read/write command), data, and an error-checking field to ensure message integrity.

Advantages of Modbus

Widely Supported:

Many manufacturers and software vendors support Modbus, ensuring broad compatibility.

Community Support:

Active user groups and professional societies back Modbus, providing extensive support and resources.

Future-Proof:

The open standard of Modbus allows for continuous updates and enhancements.

Applications

Modbus is commonly used in the following areas:

Process Automation:

It connects sensors, actuators, and control devices in industrial settings.

SCADA Systems:

Supervisory Control and Data Acquisition systems for monitoring and controlling industrial processes.