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from the Automation department...
DCS v/s PLC
Continuous process industries, DCS questions. topic
Posted by Sushant Chanana on 3 August, 2009 - 11:35 pm
Hi,

This is one of the questions which keep on popping up in many circles of automation. Many have a view that, nowadays, the line between PLC & DCS is almost blurred. Whether is I/O count or networking features, almost PLC has all the features.

However, what is left after that almost??? PLC has fast response time compared to DCS. DCS can support much more I/O than PLC but PLC's of rockwell have comparable IO to any DCS.

Many Views but at last i'm not sure what exactly the difference is!!!
If no difference at all then why two names??

And why a new breed of PAC's (Process automation controllers) emerged??

How its different??

Thanks & Regards
Sushant Chanana


Posted by Dick Caro on 4 August, 2009 - 9:50 am
Excellent question. Chanana's statement that most PLCs have the same set of features offered by the DCSs is true. The difference between them is that the PLC is sold as a component of a system, while the DCS is sold as a system pre-engineered and tested with all of the components required for continuous process control. If your application is not continuous process control, then the DCS becomes a less optimal choice.

There is also a difference in the techniques required to configure control loops. A DCS offers a complete integrated loop-builder designed explicitly for the construction of complex control schemes. The programming software for PLCs also provides this capability in their function block diagramming support, but in a much more complex software package that also includes other forms of programming for the other parts of IEC 61131-3 programming: sequential function charts, structured text, instruction list, ladder logic, and often flow-chart programming. It is these other programming methods that make the PLC better suited for batch process control and discrete parts manufacturing automation.

DCSs have also added many of the other IEC 61131-3 programming capabilities, but always in a form that retains their inherent function block orientation.

Dick Caro
===========================================
Richard H. Caro, Certified Automation Professional, CEO, CMC Associates,
2 Beth Circle, Acton, MA 01720 USA
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Posted by Sushant Chanana on 5 August, 2009 - 1:05 am
Hi Mr Caro,

"The difference between them is that the PLC is sold as a component of a system, while the DCS is sold as a system pre-engineered and tested with all of the components required for continuous process control."

So,is it means that there is lobby of DCS players which want to retain there monopoly by maintaining a hype!! As nowadays PLC's are engineered enough and ease of selection for a particular application is not a rocket science in case of PLC.

"If your application is not continuous process control, then the DCS becomes a less optimal choice."

We go back to 25 years when PLC are meant to replace ladder logic and DCS for process control. Because PLC have fast scan rates in 10-50 ms while DCS has 250 ms. But if we look at the PLC of Nowdays,they have same speed and other features. Both PLC & DCS support IEC 61131 complaint languages.

"A DCS offers a complete integrated loop-builder designed explicitly for the construction of complex control schemes."

Again we can find the PLC's having the same features.you draw PID in embedded controller program ,tag them and automatically a faceplate will be created in SCADA with automatic mapping of TAGs

Also,again we forgot the new breed of PAC's.(having OS like windows CE and embedded XP)!!!

So,still confused why three names PLC,DCS & PAC!!!!!!!!!!

Regards
Sushant Chanana
sushant.chanana [at] sedl.in


Posted by Dick Caro on 5 August, 2009 - 5:10 pm
Sorry that this industry has such confusing names.

PAC is a name originated by ARC Advisory Group (for whom I used to work) and adopted by Rockwell and Siemens. It stands for Process Automation Controller. It's just a name where they were implying that a PAC is more than the old PLC. A PLC by any other name is still a PLC.

The difference between PLC/PAC suppliers and the DCS suppliers is mostly a business model and involves the marketing presentation of their products and responsibility for the entire system integration.

DCS is a complete system: I/O and/or Foundation Fieldbus Interface, multifunction controller, HMI for operators, and data highway/local network/communications. All this is engineered to work together and is tested.

Any other connections are custom at extra cost.

PLC is a component of a system: multifunction controller, I/O and/or communications network interface. If you want an HMI, you may use theirs or some other HMI. If you want to use their own I/O that is OK, but you may use 3rd party I/O if it works via a supported interface. In other words, YOU get to do the integration and testing, or pay some system integrator to do it.

I don't see this as a monopoly. The DCS is engineered for petrochem and heavy chemical manufacturing. These companies are willing to pay for a single vendor to be responsible for the DCS to work.

Of the PLC suppliers, only Siemens is willing to accept system responsibility for process control applications.

You can get system integration from a number of (usually) small companies, but large companies prefer to have a large company to be responsible for large systems.

Dick Caro
===========================================
Richard H. Caro, Certified Automation Professional, CEO, CMC Associates,
2 Beth Circle, Acton, MA 01720 USA
E-mail: RCaro@CMC.us <mailto:RCaro [at] CMC.us>
Subscribe to the new CMC Wireless Report <http://www.CMC.us>
Web: http://www.CMC.us
Buy my books:
http://www.isa.org/books
Automation Network Selection
Wireless Networks for Industrial Automation
http://www.spitzerandboyes.com/Product/fbus.htm
The Consumer's Guide to Fieldbus Network Equipment for Process Control
Buy this book and save 50% or more on your next control system!!!


Posted by Sushant Chanana on 6 August, 2009 - 12:54 am
Hi Dick,

Thanks for this wonderful explanation. If there is any technical difference among three!! or it has to do with service and support only apart from being a system whilst other is the component of the system (old days of college books,its written over there)

Any take of the control forum editor on this matter !!

Regards
Sushant Chanana


Posted by wboyes on 6 August, 2009 - 11:41 am
With all due respect, Dick, when ARC wrote that report it was more true than not that you were correct. But that has not been true for well over 10 years. In fact, it is backward. A PAC by any other name is a PLC.

Now there're significant differences between PACs and old-style PLCs. Some PLCs are still old fashioned proprietary real-time o/s ladder-programmed digital i/o devices. Some PLCs are actually PACs-- capable of running higher order programming languages like C++ and function block, structured text, and so forth...and some PACs run the real-time (don't argue, that's what M/S says, not me) version of Windows and can do many things that an embedded computer can do.

Within five years, all programmable controllers will really be what we're calling PACs...and it won't matter any more.

Best,
Walt

Walt Boyes
Editor in Chief
Control and Controlglobal.com
www.controlglobal.com
Mailto:wboyes [at] putman.net
Read my blog SoundOFF!! At www.controlglobal.com/soundoff


Posted by anynomous on 6 August, 2009 - 1:59 am
1] Why you want to know the difference between PAC/PLC/DCS, do you have application or Capital investment to be done?

2] It may be hype from PLC vendors to claim close functionality of DCS, which may not be the case?

3] It is only when people have started using DCS for batch application, this whole concept have started and possible vice versa?


Posted by Francis on 6 August, 2009 - 7:03 am
Moderators note: This message went out earlier, but because of the denial of service attack on Twitter, some people got it multiple times in email and it did not appear on the website. For those of you who received it multiple times, sorry.

I have a recently made a couple of blog comments on this topic.
http://s88control.blogspot.com/search/label/DCS%20versus%20PLC


Posted by Vahid on 13 August, 2009 - 1:56 am
I have read all your replies, but refer to the today industrial PLCs (such as Mr. Dick stated like Siemens) are going to support all the industrial requirements, may be limited to the number of I/Os, so I think we could not note to a specified difference between DCS and PLC, Apart from this I think because of improvement in industrial networks, DCS is replacing with new technologies.

I would be thankful if someone corrects me.

Thank you,
Vahid


Posted by P Khurana on 21 August, 2009 - 10:44 am
I seriously think that there is not much difference left between the SCADA and DCS now a days. But the DCS vendors are just mis-guiding the customers against the SCADA systems (Since DCS has been considered to be better than SCADA in the past and can do a lot which SCADA couldn't do couple of years back) to grab the market.

Also the DCS vendors have reduced the price of the new system so much that the customers dont think of purchasing SCADA system (Even when they can easily fulfill there requirements with it) but they forget that the maintenance cost of DCS is much much higher than a SCADA system and with DCS system you have a monopoly!!!


Posted by Dick Caro on 21 August, 2009 - 11:26 am
Clearly written by someone who does not understand petrochem process control. SCADA is great for oil/gas pipelines and electric power distribution networks where control is local. It could be OK for Foundation Fieldbus networks where ALL control is in the field device, but I don't known of ANY of those.

The DCS is a vehicle for distributed process control. It includes methods and processes for distributed process control including a full high-level control package in each distributed controller, fully integrated with control in field devices including full cascade loops.

Dick Caro
===========================================
Richard H. Caro, Certified Automation Professional, CEO, CMC Associates,
2 Beth Circle, Acton, MA 01720 USA
E-mail: RCaro@CMC.us <mailto:RCaro@CMC.us>
Subscribe to the new CMC Wireless Report <http://www.CMC.us>
Web: http://www.CMC.us


Posted by anynomous on 24 August, 2009 - 2:02 am
Taking this discussion in slightly lighter mood the difference between DCS & PLC is , the same difference between "George Washington & George Bush Jr"

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