Analysis: PLCs vs. PACs

-- Control Engineering, 12/13/2007

Newer isn’t always better, and just because a technology is given a new name doesn’t mean that it’s better than what came before. This is particularly true in the case of industrial controllers. Control Engineering wondered if PLCs were going the way of the buggy whip and eight track tape players given that their newer counterparts, programmable automation controllers (PACs), seem to be all that vendors want to talk about. Or, is the “PLCs vs. PACs” issue just a name change, similar to some of us wearing “bell bottoms” while our children insisted they wear “flares.” We asked Craig Resnick of the ARC advisory group, who coined the term “PAC” in November 2002, and others to weigh in.

“The PLC is not outdated. A lot of applications don’t need the functionality of a PAC,” said Resnick. “A PLC is for turning things on and off and doing timing and counting functions. And [those functions] are being done in smaller and smaller packages, at cost levels where you can now put PLCs in more places.” 

Previously, to get enough memory, for example, you may have needed to get a bigger model, and hence a more expensive PLC.” The average sale price of a PLC has fallen,” said Resnick. “At what you needed for the cost of a regular PC, now you can now afford a ‘micro’ or ‘nano’ PLC. The amount of I/O or memory you need, you can now get that in a smaller model.” 

A PAC, on the other hand, can handle multiple control functions all at once, including logic, motion drive and process control. And it provides a single programming environment, common tagging and a single database in which to do it. “It may look like a PLC in terms of its physical form factor, and it even has its origins there, but it is the melding of PC functionality in an industrially hardened platform,” said Resnick. “The industrial PC market has morphed into the PAC.”

For applications run today by a distributed control system (DCS) or a motion controller, a PAC may be the right upgrade choice, said Resnick. So, too, if network connectivity or connection to the enterprise is important. Selection also might be influenced by the discipline or the vertical market. “PACs have the ability to creep into places where PLC couldn’t creep into – like Rockwell Automation getting into process verticals like pharmaceutical manufacturing. If they didn’t have a PAC, they couldn’t play there. Rockwell Automation has a strong offering in motor control centers, but they absolute needed PAC functionality,” he said.  

Control Engineering recently did a Product Research survey on PLCs, published in the December 2007 issue. Analysis of the survey data shows that control engineers are getting what they need from their PLC suppliers, with 95% satisfied with their purchases. Two comments summed up respondents opinions about the PLC selection process, however, according to Control Engineering consulting editor Dick Johnson: “To these front-line engineers, ‘standardization’ and ‘not buying more complicated PLCs than needed’ are key criteria in the selection process today.” 

In fact, programming methods may be more important than any other criteria in the selection of new controller hardware for some users. In the survey, one respondent said, “Differences between vendors on usability of the programming software are a bigger differentiator than hardware features.”  Said another: “A standardized programming language is an absolute need.”

But is the PLC today’s eight track tape deck? Not by a long shot, said Resnick. “They’ll live in on. In latest market studies, it’s a still a market that’s bullish.”

Control Engineering provides more information on PLCs.

Control Engineering provides more information on PACs. (Many of the same items are listed, as both often are mentioned in the same article.)

Or search on the desired term atop www.controleng.com.

Find the full Product Research report results in the research area of Resource Center (free with registration).

Also read: Product Research on Programmable Logic Controllers by Dick Johnson in the December 2008 Control Engineering North American edition print issue archives.

For Craig Resnick’s 2002 description of PACs, search www.arcweb.com.

—Renee Robbins, senior editor
Control Engineering News Desk
(Register here and scroll down to select your choice of eNewsletters free.)