Most major automation vendors have a Programmable Automation Controller (PAC) line, and today I'll attempt to add some clarity as to what a PAC is, and how it differs from the classic PLC.

What is a “Programmable Automation Controller?”

CPX-L1-on-WorkbenchMost people you meet in the automation industry are very comfortable describing what a Programmable Logic Control, or PLC, is.

However, many have difficulty distinguishing between the capabilities of a Programmable Logic Control, and the newer Programmable Automation Controller.

In fact, I've often been guilty myself of referring to PACs generically as PLCs just to avoid confusing a client, and even worse having to interrupt a presentation to attempt to describe the somewhat intangible difference between the two.

A Gradual Evolution

PLC-5-on-WorkbenchMuch of the confusion behind the difference between the capabilities of PLCs versus PACs is a direct result of the evolution from PLC to PAC being a gradual one.

For example, the attributes you'd most often find vendors using to describe their PACs today, like the ability to control large quantities of distributed I/O, or being capable of controlling process and batching systems, have previously been achieved with higher end PLCs like the Allen-Bradley PLC-5.

That said, there are certain features that most current generation PACs have which many legacy PLCs do not.

These include built-in support for most (if not all) IEC-61131-3 languages,  as well as enhanced instruction sets to address motion, batch, and process control applications.

5 ControlLogix-5580In contrast, most first generation PLCs produced in the early 1970s programmed exclusively in Ladder Logic, with limited instruction sets designed primarily to address discrete applications.

Most PACs also support a user generated tag database, as opposed to legacy PLCs which had rigidly named data and register files.

My PAC Definition

In conclusion, I believe the best way to describe a Programmable Automation Controller would be to first acknowledge that like a Programmable Logic Controller, it has the capability of automating discrete processes through the use of Ladder Logic.

1 CompactLogix-5380That said, most PACs also include much more extensive support of the IEC 61131 standard, including being based on a muti-tasking architecture supporting multiple programming language, as well as a user definable tag database with wide ranging data type support.

And most PACs also include extensive instruction sets to accommodate controlling a wide range of automation applications including discrete, motion, batch, and process.

Well, that's my thoughts on what a PAC is.

If you think I left anything out, or have a different take, please feel free to leave you comments below uisng the “post a comment or question” form.

And if you'd like to learn more about PACs, check out my free PAC Getting Started course at


Shawn Tierney
Automation Instructor and Blogger (post views: 1,380 views)

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6 Blog Comments

    • Good morning Cevin,

      Rockwell does classify the SLC-500, MicroLogix, and PLC-5 as PLCs, and CompactLogix and ControlLogix as PACs.


      Shawn Tierney

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      • Don’t forget DH+ and fussing with serial cables and the array of dongles needed to adapt one thing to another and messing around with speed settings/parity/etc.

        • I hear you Jeff!

          I won’t even start talking about “interchange!”

          That said, I do own a couple of used PLC-5’s and I’m very fond of them,

          I just need to find someone willing to donate a copy of RSLogix 5 to TheAutomationBlog so I can work up some how-to articles on them?


          Shawn Tierney

          Find my articles or comments helpful? Check out my courses here.
          Support our site to disable ads, get free downloads, & more here.
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