A few years ago, the term PAC (Programmable Automation Controller) started to appear. Many PLC manufacturers began to market new PAC's along with their traditional line of PLC's (see AutomationDirect.com HERE for an example). If you use AutomationDirect.com as an example, their traditional line of PLC's (DL05, DL06, and others) are still available - and still fairly inexpensive. The new Programmable Automation Controller in contrast, is predictably more expensive and has many more options for networking, expandability, and user interface. This seems to be the differentiation between the traditional PLC and the newer PAC: Relative cost, expandability, functionality, and user options.

A more telling indication of the new standard of higher-end PLC's being referred to as PAC's, is with the Rockwell/Allen-Bradley products. The Rockwell site used to list the ControlLogix and CompactLogix platforms as higher-end PLC's; however, now these systems are listed as PAC systems. Schneider/Telemecanique has seem to fallen into step and created their own PAC with the new M340 series. The new M340 series does not have more capability than the Premium or Quantum lines; however, Schneider has chosen to market this platform as a PAC.

The difference between PLC and PAC is really up to the interpretation of each user. To the OEM or integrator; however, the difference between the PLC and the PAC will be most visible with the cost of new purchase parts. PAC's will almost invariably be more expensive than platforms referred to as PLC's. Additionally, the software required to program these systems will also be more expensive than the software required to purchase their respective PLC counterparts.

To summarize, there really is no clearly defined line which differentiates a PLC from a PAC. From my own experience, a PLC system is utilized to control a few machines in a single automation cell and a PAC is used to control an entire factory floor via distributed I/O and command & control. There again, this will all depend on the experience of the user. I've integrated complete factory floors with traditional PLC systems before the PAC term was even being thrown around. I've also utilized a marketed PAC controller to run a single machine (PLC mated with an HMI and remote I/O). So...who knows. If you have a defined differentiation of PLC vs. PAC, use the form on the contact page to enlighten me and if it makes sense, I'll post your comments as an addendum to this page.