DSP vs. PLCs for control equipment

From what I gather, it seems that the two means of electronically controlling something are DSP and PLCs.
What exactly are the merits of one over the other? In what types of
applications are PLCs better, and in what types of applications is digital signal processing superior? What drawbacks does DSP contain that have prevented it from replacing PLCs?
Jason Hsu, AG4DG usenet@@@jasonhsu.com
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Jason Hsu wrote:

That's an exceedingly limited view.

Most PLCs -- I assume the common meaning, Programmable Logic Controller -- measure their world and change their control signals at zero crossings of the power line: 100 or 120 times per second. They are designed to work in a factory environment, taking into account the electrical noise and the skills of the workers there. (Most are programmed with Ladder diagrams, originally developed for relays.) PLCs contain general-purpose processors. I could build one with a DSP instead, but most of the DSP's special facilities would be wasted and some of the general-purpose processor's would need to be provided with additional hardware. One doesn't normally use a team of thoroughbreds to pull beer wagons.

Jerry
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Engineering is the art of making what you want from things you can get.

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Mike Rosing wrote:

You're plain wrong if I'm right. A thoroughbred is a particular breed of horse, as is a Clydesdale, a trotter, or a quarter horse. They're all purebreds, but that's not the same thing. [time passes]
I just checked; your sense is listed. The Century Dictionary lists the sense I used after yours. I don't follow horse races, but I believe that my sense is universal among those who do.
Jerry
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Oops! I forgot to ask about DCS!
What are the advantages and disadvantages of DCS as compared to DSP and PLCs?
Jason Hsu, AG4DG usenet@@@jasonhsu.com
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Jerry,
A DCS (Distributed Control System) is an integrated system including ALL functionality from power supply to operator chairs and everything in between. They are big, expensive and are used on big projects such as oil refineries, etc. They originated in the world of analog control but now can do just about anything so long as high speed is not a requirement. (Typically 1/4 second top scan rate.) A typical system costs about $2,000,000 US. (How are DCSs like the Titanic? They are big, expensive and everyone wants them to go faster.)
A PLC is a collection of I/O modules and a logic processor. They inherited the world of relay control but today have much of the analog functionality of a DCS as well. Everything from power supplies to cabinets to operator interface must be bought separately. Buying a DCS is like buying a bus. Buying a PLC is like buying an engine. There is no typical PLC price. They range form $200 to $1,000,000.
I have no idea what a DSP is. I've been in process control for 25 years and I've never heard the term. Web search tells me it means digital signal processing. If that is what you mean then your question is like asking, "What are the advantages and disadvantages of batteries as compared to lawn mowers?"
But keep prodding this thread. It will go on long time since the DCS vs. PLC split is once that commands strong loyalties. With luck you will even learn something through all the snow.
Walter.
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Jerry,
OK, I get it. Then clearly DSPs are embedded OEM (original equipment manufacturer) components. There may be lots of them inside a PLC or DCS for all I know. So to extend my original analogy, if a DCS is like a bus, and a PLC is like an engine, then a DSP is like a cam shaft.
Walter.

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