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SCADA and DCS: Differences and Similarities


December 23, 2021


If you work with an enterprise-level network for industrial automation, you probably heard about Supervisory Control and Data Acquisition (SCADA) Systems and Distributed Control Systems (DCS).

A SCADA system and a DCS system have many things in common as they both are a computer system for gathering and analyzing real-time data, allowing for remote monitoring and control of networks.

If you are evaluating which system is best for your monitoring project, it's important to know the similarities and differences between them. So, let's look at some aspects of SCADA and DCS systems and analyze both similarities and differences between them.

DCS and SCADA Systems in Modern Times

When computer networks either not yet existed or had very low traffic capacity, a SCADA system was essentially the main controller for many minor intelligent agents. At that time, it was not feasible to have only one system controlling every aspect of a network. So, DCS systems did the majority of the work and reported (as well as took orders from) a SCADA system.

Nowadays, computer networks have become so fast that there's no more reason for SCADA and DCS systems to being at different levels. They started as separate systems, but are now being combined into a single monitoring and control system.

Does your SCADA system have these 5 features?

Similarities Between SCADA and DCS

Taking a look at both SCADA and DCS acronyms - Supervisory Control and Data Acquisition System and Distributed Control System - you can tell that what they have in common is the word "control". This function is the main similarity between both systems.

Although SCADA and DCS might go about controlling operations differently, both systems can work with the same devices. These devices are:

PLCs and RTUs

Programmable Logic Controllers (PLC) devices work very well for the less variable process, where each step can be timed or expected to behave in a specific manner, like manufacturing and processing settings. PLCs are deployed at facilities to provide data and control equipment from a central master station. These devices can be used either for SCADA or DCS systems.

Another device used both monitoring platforms are remote telemetry units (RTUs). RTUs also provide data acquisition and control, but they are designed for use at locations where a variety of different equipment must be monitored. They allow technicians to take action as they notify technicians the instant a problem happens.

RTUs report not only on the status of the distributed controls, but also the communications network. They will also report on any external variables that can affect your systems, such as temperature and humidity.

SCADA Sensor Tutorial White Paper

How to Choose Between an RTU and a PLC

Both RTUs and PLCs can work well for if you have SCADA or DCS. But, to choose between these devices, you need to take into consideration your unique scenario.

PLCs are simple devices, but they are quite customizable. This means that you need to either need to know how to program and set them up or have an employee that can do this task. This customization aspect can be beneficial to you if you plant operations that vary widely.

On the other hand, we have RTUs - they are PLC-like devices equipped with multiple discrete and analog sensors. They provide a similar level of information and automation at remote sites, but they come programmed by the manufacturer. They're also specialized for networking, communications, and transportation processes.

RTUs and PLCs both have its pros and cons, but for whatever device you choose to implement in your monitoring system, you need to make sure your vendor puts its equipment through rigorous in-house testing.

Third-party testing is good, but if they are not the ones actually manufacturing your RTUs or PLCs, they can make mistakes easier simply because they are not the experts. In-house testing allows for quick revision cycle to make sure you get a competent hardware.

The in-house testing should apply for both temperature range and electromagnetic interference (EMI). To test temperature, just an industrial temperature chamber is required. Testing EMI requires a larger anechoic chamber with carbon-construction cones. It ensures that your SCADA/DCS equipment will not output disruptive interference and can tolerate interference from other devices.

Tests are important and they are extremely good indicators of quality and reliability. However, they will not completely replicate the conditions and stress your equipment will face in the real-world. Proven design is the ultimate test and the greatest proof of quality your devices can have. When selecting your vendor, make sure to ask for testimonials and success stories from companies similar to yours.

Find the perfect RTU for your spec, needs and budget

HMI Master Stations

Human Machine Interfaces (HMI) master stations can also be part of both SCADA and DCS systems.

An HMI can provide comprehensive network visibility because it brings in alarms for the entire monitoring system and provides an intuitive graphical interface for your SCADA or DCS systems. This way, if there's a problem your technicians will know what the issue is and what to do to fix it.

HMI is Important for SCADA and DCS Solutions

The HMI will receive the gathered information from PLCs or RTUs and then report it to your network operators in a visual form. When there is an alarm, the technicians are able to quickly make informed decisions about any required responses based on a comprehensive window.

This window should allow operators to see a thorough view of your entire SCADA or DCS system. It should also provide detailed information about any alarm when the network operator makes a request through the master station.

One of the most important functions of an HMI master station is to process all of the data gathered by your RTU or PLC. By processing this data, your master will generate report logs that summarize historical trends to indicate possible future problems within your remote network.

Intuitive HMIs

A master station installed within the SCADA or DCS systems often includes a drawing program that is used by network operators. The graphic representations provided by these drawing programs can be very simple or extremely complex. It will depend on your unique scenario and the level of detail that you want.

For example, your HMI drawing program give you an illustration of a traffic light that you're monitoring on your network. A good idea on this case is to have the light shown on screen as drivers see it on the road: switching between green, yellow, and red accordingly.

That is why efficient HMIs will usually be a web-based, graphical layout of the entire system. This layout will normally contain gauges, lights, field devices, and controls. All of which should be represented in the same place that they physically are at the remote sites.

Gauges normally show the normal range of operation. The lights might show if a determined machine is working or not, it can also indicate the presence of faults. The controls can send instructions to your RTUs or PLCs, which in turn control the machine and do the operator's command.

Master stations are critical devices within a network monitoring system. Without an HMI, operators would be unable to see or act upon the data gathered by RTUs, PLCs, and individual sensors. An HMI program allows operators to understand this data, as well as provide operators extensive reports related to network alarms.

Take a look at our T/Mon master station

Principal Differences Between SCADA and DCS

Now that we know some aspects that make SCADA and DCS similar, let's compare both systems. There are five main points that make them go about monitoring differently:

  1. Focus

    First of all, a SCADA system prioritizes acquisition of data and is event driven. DCS systems, on the other hand, focus on central control of processes and they rely on sensors and controllers to act as a master controller for the remote site.

  2. Areas of implementation

    Since it is scalable and flexible, SCADA technology works well for monitoring processes and devices that are disperse in a large geographic area. Conversely, DCS is an ideal system for controlling network operations in a single facility. And since it is centered on local events operations are usually controlled onsite instead of remotely.

    If you want to monitor only a single plant, then DCS will meet your capacity requirements. However, if you have a distributed network of industrial plants over miles of land, then SCADA would be the best case scenario for you.

  3. Ability to endure communication loss

    Also, SCADA has distributed intelligence. This means that monitoring and control functions even when the communication to the central Network Operations Center (NOC) has been lost. This is not possible with a DCS though, as it usually can't endure communication outages.

  4. Communication protocols

    The SCADA technology is able to support many different communication protocols, most of them being open protocols. While DCS has evolved throughout the years, it still works mainly with proprietary protocols. Remember that open protocols allow devices from different manufacturers to communicate with one another.

  5. Processing times

    Data processing times are slower in the DCS system than it is in the SCADA system. This is an especially important aspect if you are involved directly with network fault monitoring management.

Download our SCADA Tutorial

The Bottom Line

The main similarity between SCADA and DCS is that both systems control plant operations and the devices that they can deploy. On the other hand, the main difference is what they focus on - SCADA systems prioritize collecting and understanding data, while DCS systems is geared towards process control.

However, if you need real-time data and an open communication structure, then SCADA systems have an advantage since its processing time is faster and they allow for open protocol communications.

If you want to learn more about SCADA systems, our SCADA Tutorial white paper can help you. Simply download a free PDF copy and get essential, detailed information that will help you make informed decisions.