SCADA Systems vs. DCS

For the last part of your question, googling "ABB DCS 800xA" gets you to this is 2 clicks.

thanx john but what about the first part ??

SCADA vs DCS

thanx for your time

That SCADA vs DCS discussion was probably true in 1997--13 years ago--but little of that is true today. Hardware and software have come a long, long way in those 13 years.

thanx for your comment, but i think you concentrated on the financial value of both systems and the economic facet of the project. Academicly speaking, i want to know exactly the differences between the two architectures.

The company i work with is Using the ABB's DCS 800xA system. and it's successful in such a huge cement plant. but i want to know just why we used it and not the SCADA system, what are the functions that the SCADA system can't take in charge?? thanx a lot for making this point clearier, and the discussion more enjoyable. :)

Sorry, but the difference between DCS and SCADA is primarily economics. A DCS from the Big Boys provides all the hardware from a single vendor--from the field instruments to the HMI terminals and everything in between, including signal conditioners, I/O racks, fieldbus/Ethernet communications, wireless, cabinets, controller modules, and networks of various kinds. All of this is packaged in a nice, neat system at a very high price.

A SCADA system uses the exact same KINDS of hardware in the same architecture (except for fieldbus), but the parts come from different vendors, and at a much more reasonable price. Architecturally, they perform the same functions (acquire signals from sensors, transmit them to the control room, and carry control commands back out to the field).

As for DCS vs SCADA software, they are also functionally the same, and often are the exact SAME software packages, such as an historian from PI, HMI software from Wonderware, or ERP from SAP. You'd be amazed at how much software--regardless of the brand name--comes from the same few HMI/SCADA and enterprise software vendors.

So there is no architecture difference between DCS and HMI/SCADA systems. There was, once, but that was 13 years ago.

OK, I will give you one architecture difference: Fieldbus. Only the Big Boys with DCSes have fieldbus. Nobody else can afford the development costs. But when the PAC boys (see discussion elsewhere) invest the money needed to get their own fieldbus systems, then everything will be architecturally equal.

Of course each system is unique, but there is nothing about the term DCS that means that the field instruments come from the same vendor, they could but they don't have to and in most cases they would not.

One big difference I am very familiar with, I managed a Wonderware System on top of a network of PLC5's and a Honeywell TDC 3000 system at the same facility. If I made a change to the Operator Interface, on what you're calling the SCADA System, I had to walk around to each Wonderware PC and update that computer with the changes. BTW, I even used the wonderware system as an inexpensive interface to the DCS, this saved having dedicated Op Stations from Honeywell.

On the Honeywell DCS I just made the change in my office, compiled it and it was automatically updated on all of the screens.

In general a DCS will provide you with a complete system that is easier to build and maintain, but as has been said, at a cost.

The SCADA/PLC can do the same thing but it is more work to build it and maintain it but it cost less initially.

I was always a PLC/Wonderware guy because that's how I made a living but once I started using the Honeywell DCS I saw the advantages, especially when it was me doing all of the updating and maintenance work.

Ah yes, but, most of the "DCS" vendors also have instrument manufacturing capabilities and give you a good discount on the instrumentation you will need for your plant if you buy their DCS. (i.e.Emerson Delta V and Rosemount instruments - never see one without the other in a plant) So now it will only cost you in the hundreds of thousands more than SCADA, not millions.

In the good old days, instrument vendors enjoyed "company plants," where everything in the plant--from control valves to the control system--came from the same vendor. Then, in the 1980s and 1990s, many "instrument companies" divested themselves of valves, transmitters, single loop controllers and lots of other hardware, and just concentrated on DCSes. Standards such as 4-20mA, HART and now fieldbus make it possible to buy a DCS from Honeywell, valves from Fisher, transmitters from E+H, and so on, and they will all (allegedly) work together. Now that you can assemble a DCS using equipment from multiple vendors, competition made prices drop a little from millions to hundreds of thousands. Still expensive, though.

A SCADA system is just what these letter means, Supervisory Control and Data Acquisition. It does not include the logic functions necessary to control a cement plant, additional hardware is required, typically a plc (Programmable Logic Controller).

A DCS (Distributed Control System) is typically a complete system of hardware and software including the Operator Interfaces. DCS's cost more and usually have a steep learning curve but once you learn how to use them they are easier to configure and manage.

I speak from experience, I started out 30 years ago using PLC's with lights and push button interfaces, moved to using PC's, with SCADA Software, to replace the lights and push buttons. Tried some PC Controls solutions on non critical applications and eventually using full blown DCS's.

A function that is much easier to configure in a DCS is a control Loop, Typically it is defined, given a name and the IO is assigned, then it is available in the operator interface as a single entity. This might take literally a few minutes in the DCS but in PLC/SCADA System there are many more steps starting with the PLC and then creating the tags in the SCADA System, each and every one will have to be defined in the SCADA System, including all the tuning parameters, this can take much longer and there are many places where errors can be made. These are 2 separate systems usually from 2 separate vendors, there might be export and import utilities but usually these are not dynamic, they are run as a batch during project startup. It is very easy for something to go wrong and the communication to be interupted.

This creates job security and whoever created the system has a tremendous advantage in troubleshooting things that go wrong.

I still use all three types of systems, there are applications for PC Control, usually non-critical applications where there is no danger of someone getting hurt or of an environmental issue.

PLC's are great for discreet manufacturing requiring high speed decision making but they have also improved their process control functions so they work well for continuous control as well, but it is a little more work to configure, tune and maintain than a true DCS from one vendor.

For a cement plant with with a 2000 Degree Kiln I think a DCS would typically be used but a PLC based system like ControlLogix from Rockwell would also be a good choice.

Another consideration might be a system like Delta V from Fisher/Emerson, it is almost a small low cost PLC System with DCS like software. But the more I think about it Delta V is basically what we would categorize as a DCS.

Thanks for pointing out the PLC aspect of all this. Guess I forgot about the PACs (programmable automation controllers) from Rockwell, Beckhoff, Advantech, Opto 22, and a host of other companies. PACs are PC-based PLCs that offer both PC and PLC functions, and have HMI/SCADA software.

Rockwell, in particular, is making a big deal these days about how ControlLogix systems compete head to head with DCSes. You could consider a PAC a mini-DCS, I suppose. Rockwell is promoting the fact that they have the same capabilities as the Big Boys (Emerson, Honeywell, etc)--that is, complete turnkey capabilities for a control system and software all the way up to enterprise systems--even though their "DCS" is based on a PAC.

My point is, the distinction between HMI/SCADA, DCS, PACs, PLCs, and all the other control system types has disappeared--they all do the same functions with slightly different hardware. Still, the term "DCS" will always be associated with the Big Boys, who supply the entire package, from field instruments to fieldbus to HMIs to enterprise sofware and have an army of engineers and experts on hand to put it all in for you. If you can afford it, that is.

One way to select between a "DCS" and another type of control system is to do a little Googling to see what other cement companies have done recently. A billion years ago, I did a project for TopTools in France, where they were using IBM PCs and PLCs to control cement plants. Wonder what they are using now?

I am kind of partial to the Schneider (was Barber-Colman, Invensys, TAC) systems. One stop shop for all devices and control logic. Tridium or BACnet based, but then again that is what we design and install.

Now we're crossing another line, commercial vs industrial.

Cement Plant sounds like an industrial application to me.

I'm surprised that no one has brought up how DCS hardware architecture has changed. In 1975, when Honeywell turned the process control world on its ear with the TDC2000 DCS, it really wasn't a distributed system. Honeywell developed a microprocessor-based multiloop controller that sat between the minicomputer and the field instrumentation, but the controllers weren't "distributed"--they were still in the central control room.

Today, with the concept of "control in the field," we have finally gotten to the original concept of true distributed control. Today, with smart transmitters, valve actuators, fieldbus and HART, it is now possible to truly distribute control functions to the field, so that if anything fails--computers, DCSes, networks, etc.--the loops keep running and the plant doesn't have to shut down.

I bet somebody on this forum can probably explain how we got there architecturally.

I thought about bringing that subject up but the term DCS seems to be more of a marketing term than an architecture description.

The TDC 3000 system I worked on is also a centralized system, there is remote IO in the MCC for motor control but everything else is in the raised floor computer room.

The first system I helped build used Single Loop Controllers at the reactors and a PLC with a communication loop tieing them to the HMI and Historian. This was truly distributed but what we call DCS's typically don't have physical distribution of the control hardware.

DCS seems to be more of a marketing term than an architecture description

We call our plant control system "PICS" (for Plant I&C System). But everyone has to constantly explain to outsiders that we really mean DCS (by contemporary standards). This may have to do with the fact when the plant was built that 15 to 20 years ago, the control system architecture was in transition.

Unfortunately, we cannot change the acronym from PICS to DCS today that easily. I don't know of many other plants that use this reference.

It got pretty weird for a while there, about ten years ago, when nobody wanted to call their DCS a DCS--especially Foxboro (oops, Invensys). We went through a whole bunch of name changes, including "process automation systems." Sam Herb, who teaches a DCS class for ISA, had to change the name to "Distributive Control Systems" to get people to attend. The vendors can call a DCS what they want, but everybody still knows them as DCSes.

especially Foxboro

Yep, many of the guys here came over from Foxboro (plant is located in MA). Probably does explain the bias against term "DCS" back then.

Hallo coquita:

Have you googled SCADA Systems vs. DCS,see this,http://www.google.com

and also for specifications of the DCS 800xA from ABB see this, http://www.google.com.

Also you can search at the wikipedia,http://en.wikipedia.org/w/index.php?title=Special%3ASearch&redirs=1&search=PLC+%26+DCS&fulltext=Search&ns0=1