Programmable logic controllers (PLCS) have been with us for some time—the first PLCs with architecture and functionality like the ones in use now were built in the mid-1970s. They provide practical means for applying logic to automate almost any function that can be electrically controlled. One quality that separates PLCs from other types of control systems is that they are not dedicated to specific functions like fire alarms or heating, ventilating and air conditioning (HVAC) controls. They can be used to control and integrate almost any building system. That’s their beauty and difficulty.

Not your ordinary controller

A PLC is not a dedicated controller. Its function depends on the specific system it’s being used in. So you can’t just learn it as with, say, a lighting controller with certain fixed inputs outputs and functions. Rather than particular software in a particular controller, it has proprietary software that can be used to program an almost infinite range of functions. This means the user has to understand both the inner workings of the controller—the PLC—and the workings of the system it’s controlling.

Not your ordinary training

A major reason for using PLCs is that they can be programmed to directly control devices such as motors for HVAC. It’s the electrician who usually is involved in maintaining those kinds of devices. While he or she may be familiar with a standard motor controller, control by PLC is different. The input and output functions are defined as part of the design for a particular installation—usually in the form of a ladder diagram, which lays out the system logic. An advantage to being able to understand these diagrams and listings is they provide an excellent means for maintaining and troubleshooting the system.

The best training is provided by the PLC manufacturers because they know that the more people understand their product, the more hardware and software they can sell. Most major manufacturers provide training ranging from formal classes in regional locations to online courses and electronic or print textbooks Some also have a network of distributors that can provide both formal classes and one-on-one training.

It’s very important to get an overview of the basic structure of a PLC and the various kinds of inputs and outputs: discrete and analog, for example. Each manufacturer has its own programming language and each sells its own software you load into your laptop so you can talk to the PLC. In the United States, the PLC market is dominated by Allen Bradley, with Siemens second and several others, such as GE Fanuc, Mitsubishi and Schneider Electric/Telemecanique with a smaller share of the market. A good generalization is that if you learn on an Allen Bradley, you’ll be able to handle most situations.

PLC classes are given at many technical schools and are part of the curriculum at the National Joint Apprenticeship and Training Committee (NJATC). The Washington, D.C.-area JATC apprentice program has a lab with 20 stations of Allen Bradley SLC500 PLCs connected by an RS-232 communications link to laptops that use RS Logix software by Rockwell Software.

There are plenty of books that give a good introduction, which can be very useful. A search on Google or Amazon will provide long lists of them. Tom Tella’s “Simplified Guide to Programmable Logic Controllers” (www.SimpleSolvers.com) is one example. A good book or other such resource can be very helpful to keep handy. And the books provided with the software are invaluable.

Real-world experience

I had been used to working with relay control logic circuits and devices like magnetic motor controllers, but I was assigned to design a pretty complex system. It seemed that it would be an unwieldy mess if I tried the traditional approach, so I decided to jump into the deep end, so to speak. I called my local Allen Bradley distributor who came out to my office. Together, we decided what hardware and software I would need. When the PLC came in, I sat it on a table next to my desk and began my struggle. I told my mentor, the distributor, what I was trying to do; he would give me the basic approach. I would struggle with it and call him back, and he would critique my work. That’s how it went for months until the system was built and the troubleshooting began, along with more phone calls and visits. Over several years, my use of the PLCs became more sophisticated, and the business my company gave that distributor mushroomed.

The moral of the story is that, one way or another, you’ve got to get a general idea of the basics and then jump in and struggle with it. And when you’ve become fairly proficient with one make and model, you’ll probably encounter a different one, and the struggle will begin again. You might even find that the same manufacturer has completely revised its approach to programming. But as you struggle, you become more proficient and more valuable in the industry.

BROWN is an electrical engineer, technical writer and editor. For many years, he designed high-power electronics systems for industry, research laboratories and government. Reach him at [email protected] or at www.writingengineer.com, an independent professional writing service.