Difference Between DCS And PLC

The strengths and weaknesses between DCS and PLC systems was once well understood. Nowadays, this is simply not the case. With advancements in modern microprocessor technologies the difference between DCS and PLC systems is no longer apparent.

The power that was once exclusive to DCS systems alone can now be found in modern PLCs just the same. This means hardware alone is not the real differentiator here. Rather, implementation determines whether your installation models a traditional DCS or a machine level PLC system.

In this article we are going to dive a little deeper into these two constructs, as there are still some key differences. Keep reading if you’re interesting in finding out what these difference are…

Before we dive into the difference between DCS and PLC, let’s get the acronyms out of the way:

DCS stands for Distributed Control System
PLC stands for Programmable Logic Controller

Traditional Uses Of A PLC

The function or intent of traditional PLCs was to provide a stand-alone control system that was economical and very efficient at processing various I/O types. This included primarily digital I/O (On/Off) and analog I/O (continuous measurement).

PLCs were originally designed to replace relay control systems and their low relevant cost made them ideally suited for production automation and machinery. Typically, their interfaced I/O points would range from a few dozen to a few hundred at most.

Of course, with the advent of modern PLCs or more appropriately PACs (Programmable Automation Controllers) all of this is out the window. Modern PACs have the capability to control literally hundred’s if not thousands of digital, analog, and motion I/O points without batting an eye.

One could argue that a modern PAC = DCS from a processing power and remote I/O expansion perspective. In fact, many modern PAC’s are at the central processing point of modern day DCS.

If you’d like to review our article on how to become a PLC programmer, give it a read after this one!

Traditional Uses Of A DCS

The fundamental difference between DCS and PLC is that distributed control systems were designed and architected for larger and more complex applications.

True, a DCS can do everything a modern day PLC can do, however, it is designed to do it faster and on a much larger scale. As discussed above, perhaps a PLC and DCS are functionally the same in principle, however, from an implementation perspective very different.

Rockwell Automation is leveraging their Logix Platform in combination with its PlantPAx software to deliver what they are calling a “Modern DCS”. You can check out their short Youtube clip here.

You most likely wouldn’t consider a traditional PLC architecture if you were spanning a plant that had 10,000 to 30,000 I/O points such as what you would find in an an oil and gas refinery. This is no exaggeration, when your plant or processes are large in scale and scope, a DCS is the only way to go.

Fundamental Difference Between DCS and PLC

To explain the difference between DCS and PLC let’s take a look at a typical architecture of each.

A PLC is a specialized and dedicated processing system with the necessary hardware and software to solve a specific automation task. A typical cell level design could like something like this.

The actuators and sensors reside in Level #0 and are being processed and updated by the PLC in Level #1. There is some human interaction or control via the HMI (Human Machine Interface) Level #2, and the cell will connect into a larger plant network.

In a DCS the entire logic and hardware is distributed into different microprocessors or Functional Groups (FGs) to provide controlled segregation, redundancy and ease of implementation.

These separate Functional groups (FGs) are linked to the same HMI/SCADA/interface. The entire control of the plant is broken into smaller groups, where each group makes up a functional unit. For example, the HVAC (Heating Ventilation and Air Conditioning) system can be logically separated from the production part as it is not related functionally.

If separate FGs need to communicate with one another it is typically done through hardwired interlocks, or networking protocols such as OPC, Ethernet/IP, Modbus, Profibus and the like.

Safety critical systems are typically split into separate FGs as they typically have lower I/O requirements which results in lower computational loads. This allows the FG to focus on safety critical tasks such as redundancy and processing speed.

Difference Between DCS and PLC – Pros and Cons

PLC Pros:

In most small to medium size applications a single processor (though this is changing now too) has immediate access to all data and I/O and processes it as required.
PLCs have a simpler architecture, as is evident from the diagrams above. This makes it easier to implement and maintain.
The PLC is cost effective from 0 to a few thousand I/O points.
The reduced hardware and software requirements reduces cost.
A PLC can become a “sub-system” of a DCS when the application calls for it.

PLC Cons:

Since all computations are done by a single processor (again, this is changing) it increases the loading on the controller. For large systems the processing power required may be too high.
Single points of failure will lead to complete shutdown of your process. As of late we have seen redundant “safety PLCs” come to market, however they are typically residing in the same chassis and there intent is to “fail safe” not necessarily maximize machine up-time.

DCS Pros:

DCS is designed and architected for large, complex batch control.
DCS supports advanced analog loop control which will include – ratio, cascade, model-predictive, and feed-forward control, for maximum speed and accuracy.
DCS become cost effective when I/O points range in the order of a few thousand to 10’s of thousands.
DCS are architected for redundancy so they are single fault tolerant.

DCS Cons:

DCS systems are expensive and more complex.
The added complexity of various hardware and software makes it more difficult to install and maintain.
DCS’s priority is high reliability so they don’t lend themselves to customization.

Where Does SCADA Fit Into All This?

In the section describing the difference between DCS and PLC, I threw a new term at you that you may or may not have heard of before…SCADA.

The term SCADA refers to Supervisory Control And Data Acquisition. SCADA is nothing more than software used to gather and aggregate data from instruments and controllers in the field to present a graphical snapshot of your plant or process.

This means that a SCADA system will leverage the data and information being served up by your PLC or DCS and organize it in a graphical user interface. In simple terms, it is the “big brother” to your more traditional machine level HMIs.

SCADA systems have the processing power to pull data from multiple data sources or large geographical areas – typically 10’s, 100’s or even 1000’s of different sources spread out across vast distances. Typically, SCADA software will run on larger servers or PC’s and will provide all the key process metrics for your entire plant, process or system at large.

Typical SCADA packages include: iFix, Wonderware, RS View, FactoryTalk SE, WinCC, to name a few. These systems are typically found in large control rooms such as those found in Oil and Gas Refineries, Steel Factories and Power Generation Plants to name a few.

Conclusion

Admittedly, as an engineer working in the controls field, I too find it confusing when people use the term PLC and DCS interchangeably. Since the difference between DCS and PLC from a hardware perspective can be somewhat blurred, from an implementation, execution and architecture perspective it certainly is not.

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