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Old December 29th, 2002, 05:39 PM   #1
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Dcs, Scada

I am having trouble learning about DCS, and SCADA. So far the only thing that i have come up with on the internet is for SCADA. From What i can gather a SCADA system is basically a PLC but with processors spread very far apart. It takes sampling of data from different places and then sends commands to these different Places. If this is correct please tell me, also if you guys have found any great websites that could help me learn more about this then it would be great. As for DCS I really dont have a clue I was thinking it must be similar to a PLC/SCADA System. Correct? any good websites on DCS would help. Also, what kind of programming are these systems (DCS, SCADA) programmed in ladder, SFC, word text, etc?
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Old December 29th, 2002, 06:05 PM   #2
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SCADA PRIMER
http://members.iinet.net.au/~ianw/primer.html

DCS-Distributed Control System is similar to a plc, it is a microcontroller based system that is designed to control various components.
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Old December 30th, 2002, 06:02 AM   #3
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Forget about what you call something and select the right technology for the application.

To some people SCADA and DCS mean the same thing. Others will insist that they don't mean the same thing and are totally different.

SCADA can be just a single computer collecting high speed data (like a test stand). SCADA can be multiple PLCs and computers. SCADA often mixes and matches different hardware and software from different vendors.

DCS usually is a single, large computer that provides all functionality (hardware, software, etc) from the same vendor. Typically there is not a lot of mixing of different vendors. Typically they cost 10 times what SCADA systems do.

Is this a test question (in which case the only right answer is what the instructor thinks) or do you have a real-world application?
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Old December 30th, 2002, 07:59 AM   #4
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A DCS (Distributed Control System) is usually used in the process industries, and historically consisted of a communications network linking proprietary PID controllers to a SCADA operator interface on a PC or workstation. The programming dor the DCS was usually limited to alarms and loop tuning, but other functions could be programmed into the controllers, usually with proprietary languages.

SCADA (Supervisery Control and Data Acquisition) is simply a graphic front end on a PC or workstation that allows display and logging of data and tuning of a PLC network or DCS. The control logic is not usually performed by the SCADA.

This topic has come up many times. Do a search on this site for DCS.
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Old January 1st, 2003, 08:48 PM   #5
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just wanted to learn

I am not a student, I am just trying to learn more about it since it seems that there are a large number of jobs that require you to know how to program and configure a dcs or scada system. Also if anyone knows of any good text books on the subject then it would be really apprecitated
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Old January 1st, 2003, 09:15 PM   #6
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language???

I have searched the back threads but there doesnt seem to be any information on what type of lanuguage that these two (dcs, scada) use.
Is it ladder logic, SFC, C, basic etc? and which is most commonly used?
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Old January 2nd, 2003, 09:18 AM   #7
Tom Jenkins
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It depends on what programming you are talking about.

The SCADA system software itself is a PC appliction, and can be programmed in whatever language the SCADA supplier chooses. (Blast from the past: One of the early ones I used was done in Pascal.) However, that generally doesn't have any bearing on the application.

The SCADA software will have tools to configure graphic displays, read data from the DCS or PLC network, log alarms, and display trends. No true programming is required - it is more like configuring a data chart in Excell or creating a layout drawing in CAD.

The PLCs can be programmed using their own software, which is usually going to include ladder logic but may also have other lnaguages available for the application.

A DCS is usually programmed in some kind of proprietary script language, almost like creating macros. In some cases this may be done from the DCS hardware itself, not from a separate PC. The functions available, the syntax, etc. are all platform dependent. In some brands the programming is fairly obvious, in others it is more llike programming in assembly or machine language. Some higher cost and newer DCS equipment may also have ladder logic - I think this is true of the new Bristol stuff. The individual DCS manuals need to be consulted for details.
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Old January 14th, 2003, 03:14 AM   #8
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The confusion is caused by people using terms loosely and manufacturers wanting to reach a bigger market thus calling their product anything that will help in the selling.

DCS eveolved from process plants loop controllers and traditionally tends to be :everything from one vendor. Even the workstations hardware are made by them, network protocols are specially designed etc etc.

PLC eveloved from relays and switch panels. So tend to be smaller and focused on the discrete side of the factory. But of course technology has moved since and most DCS can do 70% of what a PLC does. And most established brands PLCs can do 70% of what any DCS does....70% is just my take do not shoot me for that. There is now a bloody battle btw/among DCS and large PLCs in the process industries esp for processes that are considered mixed (food, pharm)

Scada evolved from telemetry and simple remote contol units used in pipelines and water works. PLCs have quite effectively moved into that arena and displaced some Sacda mkt share by improving its speed, data archiving capability, comm flexibility etc etc.
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Old January 14th, 2003, 06:57 AM   #9
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Tradionally, DCS was prefered to PLCs for process control because of its ability to have PID control, fully redundent ( it had two processors per node so if one fails the second automatically takes over with no downtime ), and being able to have its I/O distributed over a wide area due to each area having its own node and each node being connected in a ring network with reliable but very slow communications.

DCS is programmed, usually called "configured" by a special engineering console which is permanently wired into a node. Its interface looks very basic.

SCADA systems for DCS are usually a extra viewing package setup up on the Windows based PC that reads data off the DCS nodes. The SCADA system is used for operator viewing and control.

The disadvantages of a DCS is that it takes up an awful lot of real estate. A node cabinet is the size of a fridge, and any I/O modules that are wired in a different cabinet take up more space.
PLCs are also much faster with processing speed.

With the advent of Foundation Fieldbus for control loops ( smart instruments in a loop that control their own PID ) the DCS system appears like it could be on the way out, but being the rock sold reliable system that they are logic would suggest they will be here for a long time yet.

This link
http://www.rs3.com/product/arch.asp
takes you to a page with RS3 Fisher Rosemount DCS site. It has a pic to show you the basic layout of a DCS system.
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Old January 14th, 2003, 01:04 PM   #10
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maybe this will help

Let’s see how simple we can make it - by first building a SCADA system - and then by building a DCS system - each from the ground up.

Disclaimer to one and all! What follows is a general discussion - there are exceptions to all of these “rules”.

Suppose that we’re building a brand new factory - and suppose that our first piece of equipment is something like a big industrial oven. This thing will be made up of heaters, and valves, and conveyor motors, and other assorted machinery - so let’s say we get to work and we build us an oven. Now that we’ve got the mechanical part of the oven built - we need some type of controller for it - something to accurately control all of those different parts in order to turn raw material into a sellable final product. So what type of control are we going to use?

For this first example, let’s say that we decide to go with a PLC system. We buy the PLC and install it by connecting wires between the oven and the PLC. Then we buy a copy of the programming software from the PLC manufacturer - and then we write a program for the PLC - we’ll probably use “ladder logic” programming, since that’s what most PLC’s use as their native language. And now the PLC is just about ready to properly control the system - except that we still need some way for the operator to set and to monitor the temperatures - and to start and stop the conveyors and so forth.

Now for this small system, some meters and pushbuttons and some thumbwheel switches might do just fine. We could wire those up and build us an operator’s control panel for our oven. But another (better?) way would be to use an HMI - a Human Machine Interface. (This used to be called an MMI - Man Machine Interface - but now-a-days we’ve got to be more politically correct.) So we buy us a nice desktop computer and some type of HMI software. We’ll need to program the HMI - and usually this is done by dragging and dropping pictures of meters and knobs and buttons onto our computer screen. In other words, we build a “virtual” control panel for our operator to use. We link these on-screen controls to the PLC’s memory through a communication cable. And now we’re finally ready to go. Great so far - and we start making some money with our factory.

Later on, business is good and we decide that our factory could use two additional ovens. So we get the mechanical parts built - and now we need to decide how we’re going to control these new ovens. Now the original PLC that we used for oven number one is quite capable of controlling the two additional ovens. We just might need to add a few additional I/O modules to the chassis - and we’ll certainly need to run some more wires - but basically the same old PLC “brain” has plenty of extra horsepower to handle the new ovens. But - here’s an idea: Suppose that we buy two new PLC’s - one for each new oven. Now that’s certainly going to cost us more money, but at least this way each oven could operate - or be shut down - completely separately from the other two systems. That’s going to make scheduling maintenance a lot simpler - and generally give us a lot more flexibility in all of our operations. Plus - by having three controllers - we’re not putting “all of our eggs in one basket” as the old saying goes. We talk the boss into it - and we buy the new PLC’s and install them - and download copies of the original program into them - and we’re just about ready to go. But how about that operator control piece of the puzzle? Since we’re already using an HMI for our operator’s control panel, all we have to do is make two copies of the screens from our original oven - and set these new copies up on the operator’s HMI computer. Finally, we extend the communication cable from the HMI station over to the two new PLC’s - and now we’re up and running.

Next the boss hires a bean-counter - someone whose job involves maximizing our factory’s profits. Now this person requires data - he needs to know how much it costs to operate the ovens - and how much product we run through them - and how much of that product is “off-spec” and wasted. The best way to get all of this production data is to ask the PLC’s - after all, they’re the “brains” that are controlling the system. So let’s upgrade the old HMI that the operator has been using - to something with more features. This will be called a SCADA system - for Supervisory Control And Data Acquisition. It will still have control screens with all of the virtual buttons and meters and other whatnots that the operator needs to control the ovens - but it will also have some additional features - features which will allow it to suck the production data right out of the PLC’s - and to store that data in some type of computer database. Later, the bean-counter can retrieve that production data and analyze it to his little heart’s content. All is well.

Quick review so far: The machinery in our factory is being controlled by PLC’s. For a little while we used an HMI (Human/Machine Interface) software package - so that the Human operator could Interface (that is, monitor and operate) the Machine. Later we moved from the HMI up to a more power software package - a SCADA (Supervisory Control And Data Acquisition) system. This new software still allowed our human operator to Supervise and Control the system - and it also added features for Data Acquisition for the bean-counter’s benefit.

Now let’s start over with a new factory - and this time we’ll use a DCS (Distributed Control System).

Suppose that this time we know in advance that the factory we’re about to build is going to involve a rather sophisticated process - one which is going to require many interrelated steps - all of which must be carefully coordinated in order to produce a sellable final product. We’re talking about chemicals - or pharmaceuticals - or something along those lines. (The term “continuous process” is a familiar buzzword for something like this.)

Now yes, we COULD use PLC’s for this type of factory - and yes, we COULD use a SCADA system to supervise and control the whole thing. But - many engineers would decide to go with a DCS for something like this. And that’s what we’re going to do.

Now suppose that our new factory still needs something along the lines of our previous ovens - how would we control these? Instead of putting a PLC on each oven - we’ll use a separate DCS “controller” for each oven. Now at first glance, these controllers will each look a lot like an individual “I/O module” or “I/O card” in a PLC system. They usually slide right into a chassis - and have wires for inputs and outputs connected to the front of them. So most DCS systems tend to look a lot like a PLC system. The big difference is that each of these DCS “controller/card” devices will be individually programmed. That’s where the term “DISTRIBUTED” comes from - the control (or “brain-power” if you prefer) is DISTRIBUTED among many individual controllers. Specifically, in a typical PLC system we generally have one “brain” (or processor) in each chassis - and then several I/O (input/output) modules in the chassis to handle the signal wires to-and-from the machinery. On the other hand, in a typical DCS system we’ll have several “brains” (or controllers) in a chassis - and the I/O wiring associated with each particular “brain’s” machinery will be connected directly to the front of that individual controller.

Now what about the operator control function? Well, one integral part of a DCS system is a large computer (usually a quite powerful one) which looks a lot like a SCADA terminal. And it does exactly the same job. First, it gives the operator a series of control screens with all of the virtual buttons and meters and other whatnots that he (or she) requires in order to control the machinery. Second, it also has the features required to suck the production data right out of the individual controllers - and to store that data in some type of computer database. And in most DCS systems, there is a third function of the DCS terminal: The programming software for the individual controllers is also usually available on this terminal - so that reprogramming the controllers is possible right over the existing data communication cables.

Quick review of the DCS approach: The machinery in our factory is being controlled by many individual little controllers. Our operator uses a DCS terminal (computer) to monitor and operate the machinery. This DCS terminal also has features to acquire production data and store it in a database for later analysis. Additionally, the DCS terminal usually has the programming software required for the individual controllers available. And all of the hardware and all of the software required for our DCS system is generally provided by just one manufacturer. Some people think that’s a good thing - and other people think that’s a bad thing.

So which is the better approach - PLC or DCS? This is usually decided by the engineers who initially design the factory. And in practice, there are a lot of factories out there who use combinations of the two approaches.

Finally, if you want a painless, fun, and free introduction to SCADA, go to:

http://www.theautomationbookstore.com/public.html

Click the "Go!" button next to "View Current Order" and then order item number 9398-VW32STCD. This is a FREE CD with a (more-or-less) fully-functional copy of AB/Rockwell's RSView32 software. It has a decent tutorial and online reference manuals to get you started and lots of interesting samples to play with. (Once you've played with these for awhile, start dissecting them to learn how they work. This is an EXCELLENT way to learn marketable job skills). The catch? The demo only works for two hours at a time ... but you can always save your work and just restart it over and over.
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Old January 14th, 2003, 09:51 PM   #11
rogerhollingsworth
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Ron Beaufort

Ron, I think that was the best comparison I have ever read.

Good Job, thanks.
Roger

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Old January 15th, 2003, 12:13 AM   #12
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PLC IS THE HEART OF THE DCS SYSTEM OR THE DCS SYSTEM
IS USED 1NPUT/OUTPUT CARD AND SCADA IS LIKE COLLECTIVE
SYSTEM OR GATHERING REAL TIME DATA AND ALSO HISTORICAL
DATA THAT THE CUSTOMER CAN USE IT IF VENDER HAVE SOMETHING WRONG
LIKE EVEDANCE
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Old January 15th, 2003, 01:58 AM   #13
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PLC/DCS

I might have mentioned this before but where I used to work the hard-core DCS group would wrench if DCS and PLC were mentioned in the same breath and they walked around with their noses held slightly above your head if you were a PLC man.
From my experience the DCS that I worked with had vast amounts of memory capable of doing true log & trending and storing historical data, which is not a PLC’s long suit.
But with the ability to share the controls by networking several PLC’s and expanding the PLC systems to include PC’s and other sophisticated HMI devices the line between the two is fuzzy.


Be gentle
Roger
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Old January 15th, 2003, 07:51 AM   #14
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Like you Roger the plant I worked at was hybrid system of DCS and PLC. The DCS did the plant controls and the PLCs had control of various OEM equipment but they were remote controlled by the DCS. The DCS trending was super, you could trend from a DCS engineeing terminal, but also had a dedicated PI server on the LAN that stored a trend for any IO that went back further than I ever needed to look at. It was awesome when operations would complain at a weekly meeting about a level transmitter or something had 'played' up the previous week and cost production, usually you could trend back and found they had shut off a valve, pump etc and blow their cover story out of the water.
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Old April 11th, 2005, 07:11 AM   #15
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Scada Dcs

Ron That Was Something Really Good To Readout
Could You Guide Or Focus On Communication That Has To Be Established For Scada And Dcs
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