DCS vs. SCADA in Modern Environments

There is considerable confusion today about the difference between DCS ("Distributed Control Systems") and SCADA ("Site Control And Data Acquisition") systems. As you can tell from expanded acronyms above, SCADA includes "Data Acquisiton" in addition to "Control". DCS, on the other hand, contains only "Control".

Understanding why this difference exists requires a 15-second history lesson. Historically, when computer networks either did not yet exist or had very low bandwidth, a SCADA system was the top-level controller for many lower-level intelligent agents. It was simply impractical to have a single system controlling every minute aspect of a system. In this technical environment, DCS devices did most of the detail work and simply reported to (and took high-level orders from) the SCADA system.

Today, computer networks have become so fast that there's no practical reason for SCADA and DCS to be separate. That's why they have blurred together into a single monitoring and control system. The choice of name - SCADA vs. DCS - largely depends on the region where you work. Some areas favor SCADA, others favor DCS. Ocassionally, some people who worked with the systems before they effectively merged or who have moved from another region will use a term different than their coworkers. This again leads to confusion when new employees must learn to manage SCADA/DCS.

Now that you understand the diminishing significance of the DCS vs. SCADA debate in modern times, it's worthwhile to understand some of the fading distinctions. First and foremost, SCADA is the preferred technology for monitoring processes and events that are spread out across a large geographic area. That's mostly due to the second key distinction: SCADA has distributed intelligence that allows monitoring and control to continue when communication to the central hub has been lost. Being heavily focused on local events and with no ability to compensate during communications losses, traditional DCS system would not be able to operate in a geodiverse scenario like this one.

Since SCADA has grown to encompass the traditional DCS role of distributed intelligent devices, it will help your understanding to look at a specific example now. Although there are many to choose from, I prefer to use an RTU (an intelligent remote SCADA device) with a good blend of common functions (and one that would actually be a good option in a variety of different SCADA projects).

With that in mind, let's take a look at the SCADA-Guardian RTU. As I explained above, half of the job of a SCADA system is to monitor remote events and report them back to you. For that task, the SCADA-Guardian has a few different technologies available. First (and the simplest technologically) are discrete alarm inputs. These are binary inputs that detect the presence or absence of a small current. They are wired into contact closure outputs from remote equipment to detect remote conditions that need your attention. The SCADA-Guardian has 8 of them, which makes it a small-to-medium RTU when measured based on discrete alarm inputs.

It's actually a larger RTU than this capacity implies, however, because it has devoted more of its inputs to other, more advanced technologies. The SCADA-Guardian also has 8 traditional industry-standard analog inputs. These measure either voltage or current input on a continuum. As you can see, these inputs allow for infinitely more granularity than the "yes/no" information that discrete inputs deliver. Instead of "above tolerance", you'll know that the temperature at a key location is "98 degrees Fahrenheit".

Since analog inputs are so powerful (especially in production, telecom, water treatment, and energy applications), this particular RTU model also has a second method of accomodating an additional 16 analog sensors. This set of 16 uses a technology that has been rapidly growing in the SCADA and broader remote monitoring industries: power and communication for remote sensors over a single wire. Known as "D-Wire" in DPS Telecom's incarnation, these sensors can be daisy-chained from one to the other. This is helpful when your SCADA system's sensors must be located a good distance from the central RTU. Daisy-chaining drastically reduces the amount of wire that's required, since each sensor is connected to the previous sensor in the chain rather than running all the way back to the RTU.

Finally, since monitoring temperature is such a common task in a variety of SCADA / DCS environments, well-built RTUs like the SCADA-Guardian will include an ambient temperature sensor built into the RTU itself. This is an easy feature to add that won't add any size outside the RTU chassis.

Other considerations when choosing an RTU for your SCADA/DCS system include looking for a strong, industrial-grade chassis. A powder-coated metal housing is always a good choice.

You also need to ensure that you RTU will be able to withstand any hot and cold temperatures within your operations. An RTU with an industrial temperature range is equipped to handle a span of sometimes hundreds of degrees Fahrenheit. Lesser RTU's must be run in temperature-controlled chambers (similar to a PC workstation's requirements), significantly limiting your flexibility during installation.

Make sure that the SCADA / DCS equipment vendor you choose puts its RTUs through rigorous in-house testing. Third-party testing is good, but it's possible for a company to pass through trial and error without really "knowing their stuff." In-house testing permits quick revision cycles and top-notch hardware. That way, if you need a minor hardware modification for a special project, your vendor will know how to make the change without disrupting the device's durability. They can also test the new design quickly without enduring an independent lab's multi-month wait times.

This in-house testing requirement applies to both temperature range and EMI (electromagnetic interference). To test temperature, just an industrial temperature chamber is required. These are about the size of a refrigerator and run both hot and cold tests. Testing EMI requires a larger anechoic chamber with carbon-construction cones. This is a vital test, however, because it ensures that your SCADA/DCS RTU will not output significant levels of disruptive interferance and that it will be able to tolerate reasonable interference from other devices in your operations.