Agronym for Supervisory Control And Data Acqusition. SCADAs are pure software running on industrial PCs to manage processes. As the software is very expensive, only major plants/companies/laboraties use it - CERN in Switzerland being one of many.
The main idea of the SCADA is to make it so simple that any fool can be a developer of a process control. And it works!
SCADA systems are not necessarily pure software, and quite often do not run on PCs either (though these days of cheap hardware and tight-wallet businesses, SCADA PCs are quickly becoming the norm). Nor, in fact, are they always for process control. All over the world, one of the biggest users of SCADA is the electric power industry.

In a nutshell, SCADA (in conjunction with a few other things) allows for remote and/or automated computer control of the power grid. Simple as that.

In more detail, it lives pretty much up to its name: Supervisory Control And Data Acquisition. It allows a supervisor (usually a person at a computer console back at the particular utility's control center) to operate devices on the power grid, and also relays to that supervisor/operator information from the grid. Now by "devices", this doesn't mean they control your toaster sitting in your kitchen. This is refering to the equipment most often housed in the little (or big) unmanned transformer plants you see sitting off beside the highway or in an empty field somewhere, which are commonly called substations. You usually see one of these for every few suburban blocks or housing projects. In these substations you've got circuit breakers, switches, fuses, transformers, and various other things. Also, you have to have power meters. These aren't the kinds of things you find in your fuse box at home. We're talking switches and breakers that handle any range from 13, 24, 68, 138, 345 thousand volts and sometimes several thousand amps, too.

What SCADA allows is: say you have a tree limb rip down a power line, and now you've got a live wire perhaps sitting in a busy public street. Using the Data Acquisition side, an operator can receive at his console information about a line fault and voltage/amperage drops from various meters and equipment designed to see and detect such things. With Supervisory Control, he can send a control signal to say a circuit breaker housed in a substation at one end of the line, telling that circuit breaker to trip, or open. It opens, cutting off power to that line. No more deadly live wire on the road, just a dead cable. Now sure, a repair crew could've done the same thing, but the trick here is that all of this occurred in minutes, three or four maybe. Having to send a repair crew out could've meant that line may have been live, and lethal, for much, much longer, especially if the crew can't immediately find where the actual downed line is. It can even be taken a step further, as the computer systems themselves can be programmed to react to situations like this, automatically opening the breaker when the line fault is detected, thus lowering the live-wire time to possibly a mere 30 seconds.

That's just one aspect of SCADA in the power industry, and the one I'm most familiar with. It's also used for yet another acronym, AGC, for Automatic Generation Control. This basically allows a computer system (with human supervision) to run the massive power generation stations far more efficiently than just people alone can. The computers can take meter readings from all over the grid to figure how much power (load) is being pulled, and generate for that amount. Thus, the power companies save huge sums of money because they're not generating more power than is necessary, so it's not so wasteful. It also protects you, the user on the end of the grid, from black outs (usually ;) due to under-generation. Both of these were very common back in the days of manual electric generation.

SCADA in the power industry has been around for decades, and its use on PCs is actually relatively new (even against the age of PCs themselves). My company still uses mainframes dated circa 1978 for their SCADA operations, and I still have to deal with them. These particular machines were built for the sole purpose of being SCADA systems, and they were far from alone. SCADA was and still is a huge part of the power industry. These days though, SCADA is indeed becoming less hardware and more truely just software, and most often found on PC-like machines. I've encountered AIX PowerPCs, DEC Alphas, and even Windows Server PCs running SCADA and power control systems.

My company recently bought ten Dell PowerEdge units with Windows 2000. Loaded with proprietary SCADA and EMS (Energy Management System) software, they're now a cluster intended to operate a portion of the Texas power grid covering at least a third of the state. God save us from the blue screen of death.

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