Modbus: So Simple Your Grandparents Could Understand It

When I was 10 years old or so, I saved up enough money to buy myself a PlayStation 2. One of my favorite and proudest memories was going to Shopko and paying $127.46 (yes, I still remember the exact price) for my brand new PS2. However, you can only imagine my disappointment when the PlayStation 3 came out just years later and Sony stopped making games for my beloved, yet outdated console.

Modbus was sort of the same way when it was introduced. Nearly every factory used Modbus to connect devices. It was the PS2 of industrial protocols, now nicknamed the granddaddy of industrial protocols. Other protocols have come along since then, making Modbus somewhat outdated. But there is still a home for Modbus inside the factory floor. Today we’re breaking down Modbus in layman’s terms so even your technologically inept granddaddy can understand it.

So, What in the World is Modbus?

To understand what Modbus is, we first need to discuss why it was necessary. At the end of the 1970s, Programmable Logic Controllers (PLCs) were becoming increasingly common in factories. PLCs are industrial computers that are adapted to control manufacturing processes from assembly lines to robotic devices and everything in between. While these PLCs were a super nice addition to the factory floor, they weren’t perfect. They couldn’t communicate with one another. This is why Modbus was created.

Modbus is an industrial communication protocol used to allow communication between electronic devices on the factory floor. In other words, Modbus is a language industrial machines use to send messages back and forth. A PLC can say to a thermometer, “Hey there, what’s the temperature of this machine? You’re not too warm, are you?” And the thermometer can respond, “this machine is currently 120 degrees Fahrenheit.” The PLC could then say, “Sounds good, carry on as you were,” or “Holy smokes! I better turn on the fans for you before you overheat.” The response to the information given by the thermometer allows the PLC to take a course of action depending on the configuration of the machine. Allowing devices to communicate prevents a lot of issues on the factory floor and creates more efficient manufacturing processes.

What’s So Great About Modbus Anyway?

Well, for starters, it’s easy to read. There are only two different data types used by Modbus: registers and coils. Coils are simply single bits. These bits are either 0s or 1s—0 for OFF and 1 for ON—simple enough! Some coils represent inputs (commands) and outputs (responses) that describe the state of a discrete input or output (e.g., Is the light on or off? Is the conveyer belt moving?). Registers, on the other hand, are 16-bit unsigned data. There are two different types of registers, input and holding registers. Input registers report the state of an external value and are quite literally a digital representation of an analog signal. For example, the voltage of an electrical current or the temperature of a machine will get assigned a value between 0 and 65535, which then tells the receiving machine exactly what is going on with the sender. Holding registers were originally used to temporarily store program data for Modbus devices. Today, they are used to store data.

Secondly, Modbus is an open standard. What this means is that when Modicon first introduced Modbus, they released it as a non-proprietary standard that could be implemented by any developer, even those competing with Modicon. As a result, Modbus became the very first widely used and accepted industrial protocol. It is so widely used that it has been adapted to be transported via different physical layers and standards. For instance, Modbus RTU uses a physical serial standard like RS-232 or RS-485 to connect devices. On the other hand, Modbus TCP is the adaptation of Modbus to be transported via Ethernet. You know, that wired internet that is way faster than dial-up.

Though many factories use newer, fancier protocols like EtherNet/IP or DeviceNet to connect their devices, Modbus still has a place in manufacturing. Today, Modbus is used to connect devices that don’t necessarily need the high speeds and greater bandwidth that other protocols can provide. So how are control engineers supposed to connect devices using Modbus to those using, say, EtherNet/IP? I mean, this would be like trying to have a conversation with somebody who only speaks Latin.

This is where we come in. We build industrial gateways that act as the Rosetta Stone of industrial communications. Our products translate messages between protocols so that devices using Modbus can be integrated easily into factories using newer protocols, like DeviceNet or EtherCAT. To learn more about how RTA can fulfill your factory floor needs, give us a ring at 1-800-249-1612 or contact us online. In the meantime, I will be playing some classic Madden on my “vintage” PS2.