PLCs: Black Magic in a Box

Beyond the bones and bloodlines of electrical installations—the cable, conduit, relays, panel boards and the more sophisticated building automation controls—are the brains, the programmable logic controllers (PLCs). Now in their third decade of existence, PLCs are the high-end gray matter operating today’s machines and industrial processes.

Due to the many considerations involved in selecting a PLC for a particular application and the programming required for most of the devices, their installation is considered by some to be the most specialized hybrid work being performed in the industry.

“One of the unique aspects of PLC systems versus building automation systems is that the conduit and wiring are normally laid out under the Division 16 workload. It’s the systems integrator who will build the panel and terminate and program the system, but the wiring and conduit is done by the electrical contractor,” said Ron Evans, sales manager, KDC Systems, a systems integrator division of Dynalectric, Los Angeles, a wholly owned subsidiary of EMCOR Group Inc.

Few electrical contractors across the country employ system integrators, though some act as such themselves, so when contractors begin to dabble in PLC work, initial attempts typically involve some degree of subcontracting to specialists in systems and programming.

The reason is simple: complexity. Industry experts point to the technical programming as generally carrying more risk and complications than the average electrical project, therefore, they say, it is easy to get in over one’s head without a solid understanding of PLC capabilities and requirements.

Bridging contractors and integrators

Like many high-end building automation controls, PLCs bridge general electrical contracting with the world of systems integration, but proficiency with the PLC architecture and ladder logic—the fundamental programming language of PLCs—requires a very different skill set than traditional electrical work or even most building automation controls programming.

PLCs are highly engineered for volume applications, and they bring a level of mystery, according to Paul Darnbrough, engineering manager, KDC Systems.

“When you’re installing electrical and cable construction, you can see it and feel it, but programming is invisible. It’s the black magic that orchestrates the equipment,” he said.

According to Scott Tenorio, product manager, Rockwell Automation, virtually any application that requires electrical control can use a PLC, such as applications in a variety of industries including food and beverage, automotive, chemical, plastics, paper, pharmaceuticals and metals.

“PLCs were initially designed to function as electronic replacements for hardwired control devices—primarily relay coils and contacts, counters and timers. Today, these functions still comprise the majority of instructions used in micro-PLC applications,” Tenorio said.

PLCs use a programmable memory to store instructions and execute specific functions that include on/off control, timing, counting, sequencing, arithmetic and data handling. PLC systems typically consist of inputs, outputs, a central processing unit, memory, power supply, programming device and an operator interface. A number of processes use PLCs, including packaging, bottling and canning, material handling, machining, power generation and distribution, HVAC/building control systems, security systems, automated assembly, paint lines and water treatment.

Tenorio said the spectrum of logic that can be set up in PLCs ranges from basic logic, such as Boolean instructions, counters/timers or latch comparisons, to advanced instructions, such as sequencers, multistage alternators and PID loops. Several criteria are used to categorize PLCs as micro, small, medium or large. Criteria include functionality, number of inputs and outputs (I/O), cost and physical size.

“Generally speaking, nano PLCs are less than 32 I/O, micro PLCs are less than 128 I/O, small PLCs are less than 512 I/O and large PLCs are 513 plus,” Tenorio said.

Tina Crowe with Automation Direct, a Cumming, Ga.-based PLC distributor, said devices range from expandable models to small limited-purpose versions.

“A PLC can be rack mounted on a base that accommodates different modules or a smaller PLC, which is the size of a brick, containing the inputs and outputs and communications and analog capabilities, all in one little box if the application is not very complex,” she said.

Capable to code

PLCs are a mature technology, and despite advances in software development to provide more intuitive programming, the average electrician is typically not capable of coding the devices without product-specific training. Contractors who want to branch out into PLC work frequently migrate to building automation projects for HVAC or lighting. Another common application, according to Darnbrough, is basic industrial automation of conveyors and photo eyes.

“Historically, people would use relays for that, but at some point, customers want more complexity whether for their operation or even for energy savings. If you don’t have the in-house capabilities, the smartest thing for the contractor to do would be to team up with a local systems integrator, then potentially purchase or absorb that company,” Evans said.

In addition to the software component, an automation contractor may be called on to design and fabricate an industrial class of control panels to house the PLCs and interconnect to the process.

“A normal contractor might be fine doing power distribution or lighting relay panel boards. But the step up to doing an industrial automation panel can be a big one, and it’s really common for customers to specify panels as UL 508A listed,” Darnbrough said.

Installation and maintenance typically go hand-in-hand in the PLC business, Evans said, but it’s not uncommon for a qualified contractor to secure a maintenance agreement on a time-and-materials basis to support a competent on-site staff.

“Because of the processes they’re running, many times the end-user will have a very developed staff of people that can handle the maintenance of the product. They may not have people who can sit down and program the system from the ground up, but they have people who can certainly go online and look for error messages,” Darnbrough said.

New generation of functionality

Many strides have been made in functions and capabilities of PLCs and connectivity is a critical component, said Edward Brown, SECURITY + LIFE SAFETY SYSTEMS managing editor.

“One of the more used features these days is the ability to network PLCs using various modes from RS232 to Ethernet, proprietary as well as open standards. They can be addressed online via modems, and they can be used to not only control operations, but also to monitor them as a means of troubleshooting,” Brown said.

Other significant advances include the new generation of programmable automated controllers (PACs). According to the ARC Advisory Group, a PAC uses open industry standards, extends domain functionality and offers technical competence. Some of the more advanced PACs integrate discrete, process, motion, batch, safety and drive control into a single-control platform, eliminating the need for stand-alone specialty controllers, such as motion controllers and PLCs. They can be programmed in ladder logic, as well as other languages, including structured text and function block, to help ease configuration.

A number of other trends are shaping the PLC market including significant advancements in the area of flexibility, communications capabilities and scalability. According to Tenorio, today’s most advanced PACs, such as those within the Logix Control Platform of Rockwell Automation, feature the ability to handle all applications from a common control platform, adding value to manufacturer’s leveraging plant-wide control needs.

Over the years, control system networking has improved tremendously, with networks sharing a single common protocol, regardless of whether they are intended for the device level, control level, information level or specialized safety applications.

Versatile PAC-based control systems also feature scalability that allows the user to more closely match the controller to the needs of the application, without compromising functionality or learning a new control system. Tenorio said that many PLCs of the past required the end-user or OEM to learn different programming software and specify networks depending on the size and complexity of the application.

Now, Tenorio said, PAC-based control systems feature programming software that is designed for the user, with the code being reused and recognized across hardware to meet system requirements. Development environments are created to have the same look and feel regardless of which application a user is trying to control.

MCCLUNG, owner of Woodland Communications, is a construction writer from Iowa. She can be reached via e-mail at

About the Author

Debbie McClung

Freelance Writer
Debbie McClung, owner of Woodland Communications, is a construction writer from Iowa.

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