What is Modbus? A Guide to Industrial Communication

In industrial automation, seamless device communication is critical. One of the most trusted tools facilitating this connectivity is Modbus, a protocol that has stood the test of time. But what is Modbus, and why is it still widely used in modern industrial systems?

In this guide, we’ll break down the Modbus communication protocol, how it works, and where it’s commonly applied. Plus, we’ll cover how Open Automation Software makes integration easier than ever.

What is Modbus?

Modbus is a serial communication protocol developed by Modicon in 1979 to enable communication between programmable logic controllers (PLCs). It allows multiple devices connected to the same network to exchange information in a standardized format.

In simple terms, Modbus enables a Modbus Master (usually a central controller or computer) to request data from or send instructions to Modbus Slaves (sensors, actuators, or other peripheral devices). A typical setup includes one Master and up to 247 Slaves, each with a unique address.

A Brief History of Modbus

To understand what the Modbus protocol is all about, it’s helpful to look at its origins. In the late 1960s, Modicon developed the first PLC at General Motors’ request. By the late 1970s, the need for these devices to communicate became apparent. While multiple companies attempted to develop their own solutions, Modicon’s release of the Modbus protocol became a milestone in automation history.

By making Modbus an open standard, Modicon allowed developers worldwide to integrate the protocol into their hardware and software products without licensing fees. This led to its wide adoption across industries, where it remains popular due to its simplicity and reliability.

Common Uses for Modbus

So, what is a Modbus protocol used for in real-world scenarios?

Modbus is an open protocol platform, meaning manufacturers can build their equipment free of charge without needing to pay royalties. It has become a standard communications protocol in the sector, and it is a common means for connecting industrial electronic devices.

Modbus is commonly used in industrial environments to connect instruments such as temperature sensors, flow meters, and human-machine interfaces (HMIs) with control systems. Its primary function is to transmit signals from control devices to centralized controllers or data acquisition systems.

For instance, a Modbus-enabled temperature sensor might collect readings and send them to a PLC, which then makes adjustments or triggers alarms based on that data.

Understanding How Modbus Works

Modbus is generally transmitted over serial lines between devices. The most straightforward setup involves a single serial cable that is connected to serial ports on two devices: a Slave and a Master.

The data will be sent as a series of zeroes and ones called bits. Each bit is sent as a voltage. Zeroes are sent as positive voltages, and ones are sent as negatives. The bits are sent in very high quality. A standard transmission speed is typically 9600 baud (bits per second).

The Modbus Organization, Inc.

Since 2004, The Modbus Organization, a non-profit group of users and developers, has overseen the evolution of Modbus. Their goal is to promote widespread adoption of the protocol by supporting education, development, and interoperability standards.

Types of Modbus Protocols

Today, Modbus exists in several variations to support different applications and media types. The three most commonly used are:

Modbus Protocols and IoT

Despite its age, Modbus remains a cornerstone of industrial control systems. It’s used in PLCs, distributed control systems (DCS), variable frequency drives (VFDs), and more.

With the rise of IoT (Internet of Things) and IIoT (Industrial IoT), Modbus has proven adaptable. It continues to power real-time automation for everything from factory machines to smart thermostats.

Rather than being replaced by newer technologies, Modbus is increasingly integrated into them, providing a bridge between legacy systems and modern IoT platforms.

There are now several software companies providing solutions for Modbus and IoT integration, but only one of them has the experience and reliability to liberate your data with unparalleled connectivity: Open Automation Software.