DCS vs. SCADA

Dear Control Engineering: What’s the difference between a DCS and a SCADA system?

That’s an interesting question because the two terms seem to overlap somewhat, and I have seen them used interchangeably. Let’s start by defining what they mean specifically.

DCS stands for distributed control system. (Which means you shouldn’t say, “DCS system.”) This term came into currency during the 1970s and attempted to describe control systems that could be broken into smaller sub-units and distributed around a plant. This followed “monolithic” control systems, where all the computing power was in one centralized system. With the coming of microprocessors, more of the control functionality could be spread around the plant communicating digitally with the central command, now functioning in more of a coordinating role. The coming of personal computers in the 1990s made control even more localized.

SCADA stands for supervisory control and data acquisition. (Generally pronounced SCAY-duh.) In many respects this is a good description of most control platforms, so it has been applied widely and therefore has become a lot less specific.

Consequently, the difference between the two varies a great deal. Some see DCS as a subset of SCADA.

Probably the best differentiation I’ve heard, and the one we tend to use when doing articles for Control Engineering, is that the system changes at the fence. (I can’t remember who told me this, so if anybody wants to claim credit, go ahead. In any case, I wasn’t me.) Processes that go on in the plant (inside the fence) are run by a DCS. Things that go on outside the fence are run by a SCADA system. The best example is an electric utility. The generating plant with its boilers and turbines, is run by a DCS. But once you get outside the plant to the distribution grid, it is governed by a SCADA system. This implication is that SCADA controls more remote and spread out infrastructure. As another example, an oil refinery is run by a DCS, but the pipelines use SCADA.

This is not a universally accepted idea, so you may still have to make some evaluations based on context. But if you see the terms here, you’ll have a better idea of what we mean.

–Peter Welander, process industries editor