When something is still an industry standard 43 years after its introduction, that’s a pretty strong statement about its quality, utility, and effectiveness.
Meet Modbus, the little engine that could and can and still does. Modbus is a communication protocol, and while it was introduced 43 years ago, it’s still the standard in the world of industrial controllers.
How is that possible? To begin with, Modbus is simple and powerful. On top of that, it just works. Its flexibility allows industrial control manufacturers to add new features easily. Let’s take a moment and look at how this communications protocol works and how it is used in control systems globally.
History
Modbus was developed by Modicon®, which has long been a part of Schneider Electric, in 1979 as a means for “transmitting information over serial lines between electronic devices.” Early deployments included some of the first efforts to incorporate digital control solutions into industrial and commercial applications. Modicon invented the first programmable logic controllers (“PLCs”) and developed Modbus as a means for their PLCs to communicate with the various devices they controlled.
Modbus quickly gained popularity for several reasons. First and foremost, Modicon developed and released Modbus as an open standard anyone could use and adapt, spurring widespread adoption. Another reason driving the success of Modbus was the fact that it was written in a way that made it easy for non-programmers to work with. Open-source Modbus gave engineers for the first time a simple way to develop specialized control devices and integrate them into any Modbus-based programmable control system.
Today, Modicon’s groundbreaking PLCs can now be found in museums, and Modbus is a mainstay of the open-source community. Not content to rest on its past glory, Modbus has continued to improve and evolve. In 1999, Modbus TCP was released. This version of the communication protocol enabled Modbus to operate over an internet connection. The ease of integrating new devices and features into control systems using the Modbus protocol has improved over time, but the core practicality is the same in 2022 as it was in 1979.
Functionality
Modbus’ core design supports a system where a central computer reads data from remote devices, processes that data, and then issues commands to that or other devices. Modbus.org today refers to the PLC as the “server” and the remote devices the PLC controls as “clients.” But I think a different analogy explains more simply and clearly how Modbus works: the military.
Think of the PLC as an officer and the clients as rank-and-file soldiers. Good officers communicate with their soldiers regularly to get reports on field conditions and issue commands. They communicate with some soldiers frequently and others only occasionally. They can ask their soldiers for a full report of all data or simply ask one question or issue one command.
And when they communicate, the language they use is Modbus.
When Modbus was first released in 1979, only the PLC had processing capabilities, but today client devices can have their own processing capabilities. It’s a new world, and Modbus, like the military, keeps adapting to a world in which both the PLCs and clients can be intelligent devices.
To return to our analogy, in today’s most advanced militaries, field commanders often have the ability and authority to make their own decisions based on local intelligence. Modbus operates the same way, with the ability to delegate the same decision-making freedom to client devices. Modern client devices may have more processing power than the server they’re connected to. Modbus’ simple design makes it easy and possible for system operators to leverage that power to their advantage.
Modbus has evolved over time but still maintains its original advantages. It can communicate over low cost, low speed serial buses or the latest TCP/IP network. This flexibility allows Modbus to connect to older devices as well as those with the latest high-speed internet architecture. The Modbus serial bus design is inherently secure; the only way to hack it is to physically replace the connection from the server to the client devices. Modern TCP/IP implementations can be secured via encrypted VPN tunnels.
EV charger manufacturers offer Modbus
For EVSE manufacturers, Modbus provides an opportunity to develop and quickly release new features on their equipment. To add a new controllable feature or data source, a manufacturer simply adds a new data point and publishes a Modbus “map” which tells others how to access that feature or data. The open-source flexibility frees manufacturers from needing to fit new ideas to an existing standard use-case that may not be ideally suited to accommodate their new idea. Neither the companies that make control systems nor the EVSE manufactures need to invest manpower and money in complicated customization.
Modbus is a language everyone can speak.
This post is part of a series reviewing networking protocol standards for controlling EV charging. Previous posts examined the OCPP standard and OCPP equipped chargers. In our next post, we'll compare and contrast the advantages of OCPP and Modbus.