What's in a name?

by Jeff Payne,
AutomationDirect Product Manager
PLC, I/O and PC-Controls Group

"What's in a name? That which we call a rose by any other name would smell as sweet.”

PLC, DCS and PAC are a few acronyms used to describe what originally replaced the relays in the late 1960’s. So, what are the differences and why do we need to call them by so many different names?

The Farlex Dictionary defines these as follows:

Programmable Logic Controller
A programmable microprocessor-based device that is used in discrete manufacturing to control assembly lines and machinery on the shop floor as well as many other types of mechanical, electrical and electronic equipment in a plant. Typically RISC based and programmed in an IEC 61131programming language, a PLC is designed for realtime use in rugged, industrial environments. Connected to sensors and actuators, PLCs are categorized by the number and type of I/O ports they provide and by their I/O scan rate.

In the late 1960s, PLCs were first used to replace the hard-wired networks of relays and timers in automobile assembly lines, which were partially automated at that time. The programmability of the PLC enabled considerably faster changes.

Distributed Control System
A process control system that uses a network to interconnect sensors, controllers, operator terminals and actuators. Generally very large and costly systems, a DCS typically contains several computers for control and uses proprietary interconnections.

Programmable Automation Controller
A programmable microprocessor-based device that is used for discrete manufacturing, process control and remote monitoring applications. Designed for use in rugged, industrial environments, PACs combine the functions of a programmable logic controller (PLC) with the greater flexibility of a PC. They are also more easily configured for data collection and integration with the company's business applications than PLCs.

Although each PAC vendor uses their own development environment (IDE) and programming language, PAC networking is typically based on IP and Ethernet. Taking advantage of off-the-shelf microprocessors, PACs were developed in the 1990s to provide a single industrial controller that would provide the functions of a DCS (Distributed Control System) and PLC (Programmable Logic Controller).

These are fair assessments of three main types of control systems currently in use in today’s controller markets … but why so many and do we really need them?

Today’s PLCs have expanded well beyond their original design scope. They have continued to advance since their inception in the late 1960’s. However, there has been an obvious spike in the technological influence in the control world within the past 5 to 10 years.

As we’ve discussed in the past, I believe flexibility is becoming a higher priority for those who are specifying automation systems. Today users need multi-functional control, simple connectivity and easy access of process data, all wrapped up in a nice neat package.

Today’s PLCs have to be much more than the relay logic replacement which they were initially designed to be.

Insert the world of PCs
With PC technology becoming smaller, faster and less expensive, PLC manufacturers could not pass an opportunity to take advantage of these advancements in technology.

PACs are, for lack of a better term, today’s “high-end PLCs”. They still look like PLCs of the last decade, but introduce the intellectual properties of modern day electronics and computing into the industrial controllers we all know so well.

The advantage to the consumer is that you receive this technology in a platform you trust to perform in industrious environments while reaping the benefits of Commercial Off-The-Shelf (COTS) hardware.

Coincidentally, this new class of controller provides more memory capacity and processing power which allows for better data processing capabilities, and connectivity to enterprise business systems from the plant floor.

Additionally, we see the benefit of easy integration for multi-domain systems comprising Human Machine Interface (HMI), motion control and process control.

“So tell me again … what is a PAC?” It looks like a PLC with modular design for flexibility and reliability. It smells like a PLC, and programmability is basically very similar. (I can’t say I tried the taste, but I would be willing to bet it tastes like a PLC.)

Will it replace the PLC? No, but it will complement today’s control options to give you the best possible solution for your application.

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