Modbus Plus: Mystery at the Museum

I like the TV show “Mysteries at the Museum”. They tell stories about odd things that are found at museums all over the country. They find some little item, in some small, backwater museum – like the two- room museum above the pharmacy in Elmore City, Oklahoma – and connect it with some part of American history.

I can imagine owning a small museum like that in an out-of-the-way town where the fishing and hunting is good. I’d have all sorts of cool, old stuff. Some of oddest ones would be my Modbus Plus devices. My visitor(s) would exclaim, “Modbus Plus! Tell us the story about that.”

I’d begin curating about the early days of automation, long before the Internet where Modbus was all the rage. Serial Modbus (RTU and ASCII) at that time just wasn’t evolutionary, it was REVOLUTIONARY. I’d describe how prior to Modbus, all we had was electrical signaling. For digital input and output devices like pushbuttons, lights, motors and the like, we wired a signal wire and a return from the controller to the device. For analog input devices like a temperature sensor or output devices like drive speed, we used 0-10 Volts, 4-20ma current loop or RTD. Everything was wired, everything was either a voltage level, a current level, or simply a contact. With thousands of inputs and outputs in a big manufacturing machine, the labor to install all those wires, check that every single one of them was terminated to the right position, and test each one took weeks and sometimes months of effort.

The Revolution

But Modbus changed all that. Modbus changed everything. Modbus introduced the concept of data on the factory floor. Modbus made it possible to connect an entire group of devices using only two wires on the controller. That alone saved a massive investment in wire, labor and installation time. Instead of miles and miles of wire connecting hundreds of devices, a simple two-wire pair could be daisy-chained from one device to the next to the next. It was revolutionary for its time.
As technology marched on, processes became faster and faster while Modbus really didn’t. The serial baud rate increased from 300 baud to 1200 to 9600 and then to 19.2K and an amazing 56K. But even that just wasn’t enough. Remember, these were pre-Ethernet days and 56K was blazing speed.Ethernet was just some obscure technology that hippies out on the west coast were playing with, but on the east coast, in manufacturing development labs where ties and sometimes suits were worn to work, we built this thing, Modbus Plus, as a faster replacement for Modbus.

Twist in the Tale

And here’s the twist in the story. You would think that if a technology had a name like Modbus Plus, it would be related to Modbus. Maybe another kind of platform or another transport layer. You’d think that maybe it would be something useful to you as you move up from serial Modbus technology.

Well, if you thought any of those thoughts, you’d be wrong. Modbus Plus wasn’t really related to Modbus at all. It used the same data representation, but it’s an entirely different protocol built around a token passing scheme. At that time, DH485, another token passing scheme from Allen-Bradley, was popular. It’s thought that Modbus Plus was created, in part, to counter that technology from Allen-Bradley.

It was an odd time, just before the emergence of Ethernet (which led to Modbus TCP), and it just never achieved much traction in the marketplace other than with a small segment of the Schneider Electric customer base. At one mega-baud, it far surpassed the networks of the day, but the specification was always kept proprietary and the units were typically built around a proprietary chip set. Looking back over the years, that was its death knell.

And as I finished curating my museum piece, my visitors would go “ooooohhhhh” and “wooooowwww” over that really cool story about a few little odd devices with the Modbus name that aren’t really Modbus.