PLC Vs. PAC: Understanding The Differences

For over four decades, programmable logic controllers have been a mainstay in the industrial manufacturing industry. These highly advanced controllers gave engineers the tools needed to safely and successfully carry out a broad range of automated processes, from assembly lines and printing presses, to amusement parks and water treatment systems.

But PLCs aren't the only option available to industrial customers, these days. Programmable automation controllers are making headway in the automation market, offering their own array of unique capabilities and user options.

What are the Differences?

Surprisingly, most of the differences between PLCs and PACs are physical and technological:

  • PLCs tend to use a single processor and occupy an entire rack, whereas PAC racks not only host multiple PAC modules, but those modules also have multiple processors.
  • PACs are also commonly linked to PC-based control systems for enhanced functionality. Older PLCs, on the other hand, were often linked to proprietary terminals. Newer PLCs take advantage of PC-based software for programming and controlling PLC functions.

In terms of actual function, PLCs and PACs are remarkably similar to one another – so much so that people tend to use the terms PLC and PAC interchangeably, as Business Industrial Network's Don Fitchett notes. In fact, many of the features shared by PLCs and PACs are themselves interchangeable.

However, such assumptions are difficult to set aside when it comes to training new operators, as both PLCs and PACs offer differences in available features, as well as how they handle analog input/output (I/O) and other function controls.

The Advantages of PLCs

PLCs are ideal for simple and high speed equipment control, as well as other applications where simplicity and reliability are paramount. The ladder logic programming language is also simple enough for maintenance personnel to operate with few problems. As a mature technology, PLCs have also proven cost-effective for operations in search of low cost outlay and a favorable economic return on investment.

However, there are a few disadvantages to using PLC architecture. For instance, ladder logic lacks the flexibility needed for applications reliant on interoperability and complex controls. PLCs are also relatively limited in their expansion capabilities, often requiring hardware-based changes and upgrades to match the features and functionality of their PAC counterparts.

Where PACs Excel

PACs offer a more flexible and user-friendly method of automation process control. In addition to an expanded array of compatible programming languages, PACs offer greater memory and I/O capacity. This allows the average PAC to better handle complex automation tasks that require extensive data handling and large amounts of analog I/O. For this reason, PACs are excellent at multitasking duties.

While PLCs have a number of options for handling data and communications, PACs expand those options using the latest technologies. For example, USB data logging, LCD monitoring and system data storage via web server are all available on most modern PACs. PACs also share compatibility with standard protocols and components used throughout the IT industry.

Tag naming is another feature that PACs offer exclusive of their PLC brethren. This feature allows users to define data types during the course of programming, allowing for greater flexibility during system expansion and other scenarios.

Which Is the Right Choice?

PLCs are ideal for applications where simple controls are needed. But for complex applications where precision, flexibility and capacity are paramount, PACs are proving to be a better fit.

From a cost standpoint, PLCs are often seen as the initially better choice due to the lower purchasing costs and the lower expenses involved in programming and operation. However, applications that benefit from the flexibility and broader capability of PACs will not see an economic benefit from using PLCs in their stead.

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