DCS... PLC... and DSP...

Hello ...
Saw this topic on the newsgroup quite sometime ago. Will like to share my views.
PLC are machines that perform control function via it's input and
output port. It can be a very complex system (example with SCADA, HMI etc). But I think it is common that PLC system is relatively small system and works independently.
DCS are large PLC system. It comes with HMI, and most likely SCADA. It provides more inputs and outputs as compare to PLC. It is normally DCS is a more integrated system. The word "Distributed" also indicted that most likely the system is kinda "decentralised" system, with a lot of CPUs working independently, but they are linked with information(data) passing around.
Sometime there is only a thin line between PLC and DCS. If a PLC system grows too big, it might be mistaken as a DCS.
DSP is Digital Signal Processing or sometime people will call it Signal Processing. I think Signal Processing is a better name as Digital Signal Processing is quite restricted in some sense.
Signal Processing mainly deal with signal. Analysis of incoming signal and processing the incoming signal. The DSP chip (embedded device) is a device used to analysis the signal using the programming (normally some math routines like Fast Fourier Transform, Discrete Cosine Transform, etc) it contains. It is part of Signal Processing, but not the main part.
Therefore, I think DCS and PLC can be consider as System, while Signal Processing is consider as Signal.
There is also only a thin line between Signal and System. Signal can be analysis using system method and system can be analysis using signal method.
Have a nice day.
Regards smallfriend
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Okay. I don't want to start another endless discussion but:

No, a DCS system is *not* a large PLC system. A DCS has features which are not available from a standard PLC/SCADA combination.
Although it is not too common, there are many DCS systems out there with relatively small numbers of I/O (compared to PLCs at the same site anyway) which were purchased for the features being offered and not only the price.
A you stated, a DCS is an "integrated system" - and a PLC/SCADA combination is not - ever. By "integrated" we mean that all the equipment is from the same manufacturer and each part of the DCS is designed specifically to work with all other parts of the system. And, yes, a DCS system is decentralised whilst a PLC system is traditionally not.

True, the line can be thin, but a quick look around the software at the features on offer reveals the differences.
For example: DCS software allows out-of-the-box full remote control of the remote nodes, whilst SCADA software does not.
Given enough money, some PLC systems (eg. Modicon, Siemens and AB offerings) can be made to "look-and-feel" like a DCS, but the end result will be *always* be more expensive, less flexible and more difficult to expand/upgrade than any comparable DCS.

All I'll say on this is that DSP is used by modern PLCs and DCS systems to do the processing for analog control - and outside of this, DSP is not part of the equation at all.
I hope this helps clarify things a bit...
Cameron:-)
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At one time there was a difference between PLCs and DCS. The I see todays is the DCS company, once you pick one you want, they want to sell a service and hold lots of meetings to discuss the project. Then provide the controls engineer to manage your system.
Cameron Dorrough wrote:

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It used to be that DCS vendors sold a product and provided service to support it. Now they sell 'solutions'. The difference between a product and a solution is that with the latter you have to pay extra to make the product work. It's kind of like an extended warrantee except that you have to pay for the free service whether you use it or not.
Walter.

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If your new to it, the basic SW package may go for <$500 to maybe $10K, but expect to spend >$15K to get all you need and the training or help YOU to get it up and running. Talking DSC's and multiple PLC SCADA's here, OK PLC's too ... lol
Some large installations spend tens of thousands a year on their systems for support and upgrades. Vendor support for large installations can run into millions, the software is almost insignificant in cost. An onsite engineer from vendor can run $ 1K a day and like $ 100K - $ 150K a year. I was a vendor application engineer, one large paint shop and tracking project had 3 of us for 2 years, plus there army of people that helped and took over.

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Original a DCS system is more reliable than a plc system (1) by redundancy and (2) distributed control. Normally you configure a DCS, instead of programming. And most of the time's the supplier guarantees that the system can "hang" with a maximum of once in 20 years. So, that's why they use it in critical (petrochemical) processes.
With a dcs I mean the real one's like Honeywell, Yokogawa, Foxborro... And not the modern new stuff like PCS-7 or SattLine. Simply because they are build with modern rubbish that can't stand the reliability proof of 20 years.

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No. Many PLC vendors claim that a big network of PLCs is "just as good as a DCS." Whenever a salesman claims his stuff is "just as good as" you know they have a weak case. It generally means he hasn't ever deal with a DCS.
Walter.

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Not all large control systems are for the refineries and similar continuous process industries you know! (DCS's are very good at those) I have seen several PLC/SCADA applications that are bigger and 'better' than DCS attempts. These are in the batch industries - food, brewing and some pharmaceuticals. In these types of application a DCS can be a straightjacket whereas a PLC/SCADA platform allows good automation engineers to design exactly what is required. And what seems to happen is that the DCS companies in their attempts to get into those application areas have made their systems more like PLC/SCADA, with the same problems. These problems are not because they are DCS or PLC products, they are because of the complexity of the applications. I do a lot of work in batch processes, with intricate sequential control (Complex batches, Clean-In-Place, Sterilising etc) and high levels of operator interaction. My experience of DCS's in these areas is that they require just as much design and integration work as PLC/SCADA, and that the DCS's are not really as integrated as the suppliers want us to believe. For example the Batch Managers products that are included in many DCS products do not really share the same 'global tag database' that the DCS has, they are bolted on via the same sort of integration that connects PLC and SCADA systems. In fact PLC/SCADA systems sometimes use the exact same batch manager products. Francis

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Do you work for a DCS company? I say a DCS is just as good as a PLC.

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Horses for courses. I'm talking about process control, not hub cap stamping. No, I don't work for a DCS company; they work for me. I buy them because my industry needs them. Sometimes a PLC is more appropriate. Then I get those.
Walter.

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