New Years, Old Modbus

The New Year is a nostalgic time of year, one that converges with an end-of-year propensity to reflect on our recent past and planning for the future. So I thought I’d look back at some history of Modbus – the most pervasive communications protocol in industrial and building automation and the most commonly available means of connecting automated electronic devices.

I’m sure that for anyone under 30, raised in the age of Google and smartphones, the history I am going to describe here will sound more like the Flintstones than real history. (And if you don’t know who the Flintstones are, you really do need to read this!)

You might call the Modbus protocol the grandfather of industrial networking. It truly is as old as the hills and has the whiskers to prove it. In today’s age of Internet connectivity and Web Services, Modbus’ unconnected message and simple request-response communication structure is almost quaint. It’s almost as old as the first Programmable Logic Controller, the Modicon 084, which in those days was called a PC for “Programmable Controller.”

Modbus is an open standard, meaning that manufacturers can build it into their equipment without having to pay royalties. Modbus is used widely by many manufacturers throughout many industries. Modbus is typically used to transmit data from control instrumentation to a logic controller or a system for archiving data. In building automation, for example, temperature and humidity are often communicated to a computer for long term storage. Modbus is often used to connect a supervisory computer with a remote terminal unit (RTU) in supervisory control and data acquisition (SCADA) systems.

Before Programmable Controllers, Control Engineers did relay control – hardwired relays on the wall acted as the machine logic. There were rooms with walls full of relays, terminal blocks and more wire than you could easily measure. The walls came to be organized with power lines down the sides connecting various control inputs to various kinds of output relays. The structures began to look like ladders, so the term “ladder logic” came to represent that kind of control logic.

This wasn’t, as you can imagine, optimum. The problems were many, including:

•A massive amount of time required to change the control logic. To do it properly, you moved the control inputs and output relays around. To do it quickly, you just rewired it, creating a control system that couldn’t be easily understood.

•These control rooms weren’t ventilated properly. Control Engineers at the time didn’t have the know-how to make good terminations. Contacts often failed as they became worn or dirty and machine downtime from loose wires was common.

•There was a complete lack of systems to document the control system.

It was common to spend hours tracking down a problem that could be fixed in ten seconds by wiping off a contact. This was the era that birthed the PLC. Richard Morley and several associates founded the Modicon Corporation in 1968. They introduced the first Programmable Controller, the “084,” so named because it was the 84th project at Bedford and Associates, the company they had left to found Modicon.

Ten years after that first PLC and after a few successors to the 084, Modicon introduced Modbus, the world’s first industrial communication network, and arguably the most successful. Modbus connected those Modicon PLCs to each other and to remote devices, starting a whole new era in factory floor connectivity.

Modbus, in that stone-age era, was all about serial communications: some RS232, but mostly multi-dropped RS485. That multi-dropped RS485 served our needs for many years, and it will always be remembered as the very first automation protocol.

In future blogs I’ll have more to say about Modbus RTU and Modbus TCP. I’ll be regularly publishing information that you’ll need to continue your Modbus education. Watch for more in RTA blog posts, articles in the RTA “Best Darn Newsletter” and the RTA Page on LinkedIn.
John