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Comparing PLCs and PACs

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Designed to replace timers, relays and counters previously used in automation systems, programmable logic controllers (PLCs) have been in use in a wide variety of industrial applications for several decades, giving engineers the ability to carry out an extraordinary range of automated processes. These typically range from manufacturing and assembly tasks to transportation and infrastructure control.

Programmable automation controllers (PACs) are a more recent take on the PLC, offering a wide range of improvements over the PLC while carrying out many of its functions. Nevertheless, the two technologies are constantly confused for one another based on their similarities. The following explores the noticeable differences between PLCs and PACs, as well as the benefits and drawbacks that both offer in the automation arena.

Understanding the Basic Differences

There are several differences that make both PLCs and PACs stand out from one another:

  • A typical PLC uses a single processor module with 1 processor chip per module. A PAC is able to host multiple processor modules on a single rack, with 2 or more processor chips in each module.
  • Most PLCs rely on ladder logic, whereas a PAC is capable of utilizing structured text and functional block diagrams for its programming needs.
  • Whereas PLCs are mainly attached to proprietary terminals, PACs utilize PC-based control systems, allowing for more flexible control options.
  • PACs typically offer a much larger number of input/output (I/O) points than PLCs. PACs also enjoy greater memory sizes than their PLC counterparts.
  • PACs can be configured in a modular manner with little to no need for wiring or sensor changes, whereas an older PLC may require significantly more modifications.

Pros and Cons of PLCs

PLCs excel at relatively simple applications, such as those that call for basic equipment control. PLCs are also the automation solution of choice for applications where high execution speeds are required. The programming itself is also kept simple, in most cases, given the use of the ladder logic programming format.

While PLCs offer excellent control capabilities for simple and high-speed automation processes, expanding its capabilities beyond its basic functions often proves difficult. Modern PLCs have managed to expand their PID and I/O handling features, but the modular design of a typical PAC makes it easier to configure it for a broad array of specialized duties.  

Pros and Cons of PACs

When it comes to complex automation tasks requiring extensive programming, PACs are the preferred choice among many companies. The PC-based system offers a broader array of communications options than traditional PLCs, including USB data logging, web-based data storage and LCD monitoring. Thanks to the open nature of the automation architecture, a typical PAC is capable of supporting a variety of network technologies and standard protocols, including SQL and Ethernet.

Unlike most PLCs, PACs utilize tag-based memory structures that allow users to define data types during programming. Tag-based memory offers greater flexibility and expansion capability than the fixed memory mapping typically used by PLCs. In addition, most PACs feature greater memory and I/O capabilities than their PLC counterparts.

The main downside to a PAC involves cost. According to PLC Engineers, most PAC platforms carry a higher price tag than their PLC counterparts. Given a choice between a low-end PAC and a high-end PLC platform, a company in need of an affordable automation control solution may be tempted to choose the PLC based on setup costs, as well as support availability, prior company and vendor relationships and options for future expansion.

Differentiating modern PLCs from PACs involves more than just differences in jargon and packaging. It also means examining each and every benefit and drawback that both controllers offer, as well as examining your own company's needs for programmable automation hardware and software. For more information about your options, visit