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naeemmsc May 18th, 2009 01:25 AM

What the Main difference between PLC and DCS
I want to know what is the main difference between PLC and DCS.if PLC Perform every task why we use DCS.Plesae can some one tell me about DCS5000 literature web sites.

sumanthb4u May 18th, 2009 02:31 AM


Originally Posted by naeemmsc (Post 323606)
I want to know what is the main difference between PLC and DCS.if PLC Perform every task why we use DCS.Plesae can some one tell me about DCS5000 literature web sites.

DCS contain both SCADA & logic programming in it.

tomalbright May 18th, 2009 06:17 AM

A PLC system CAN work by itself. However, if it relies on, or supplies signals to other components to operate, then it MAY be called a DCS. It's the idea of "distribution"; many components, or many components working together? The term of "DCS" is pretty vague, to be honest.

Steve Bailey May 18th, 2009 06:25 AM

In manufacturing you make either 'things' or 'stuff'. Traditionally, PLCs have been mostly involved in the manufacture of things and DCS have been mostly involved in the maufacture of stuff.

The dividing line between the two gets less distinct with each new generation of controllers.

danw May 18th, 2009 08:37 AM

DCS is always tag based and server based.

PLC's are memory based.


FrancisL May 18th, 2009 10:38 AM

The size of the suppliers expense accounts
DCS's are bigger

shooter May 18th, 2009 11:37 AM

PLC means programmable logic circuit
in general it means a sophistacted relay box that can be programmed with a ladder program. So simple on/off in a line and some timers/counters.
DCS means digital control system Almost same , however programming is done in basic, C, pascal. The in and outputs are not standard

FrancisL May 18th, 2009 11:49 AM

PLC means Programmable Logic CONTROLLER
DCS -Distributed Control System
And we do not use Pascal or C in Control

Tom Jenkins May 18th, 2009 12:41 PM

Historically DCS developed in the process industries and were the result of adding capabilities and communications to single loop PID controllers and using mini-computers for HMI functions. Many DCS systems use proprietary communications protocols and have proprietary programming languages, although most modern systems are more open and flexible.

PLCs grew out of component and product manufacuring as relay replacement systems. Most program in Ladder Logic but modern units have a wede variety of programming languages, analog I/O capabilities, math functins, and communications.

PLCMentor May 18th, 2009 12:56 PM

You have hit upon a discussion that has been raging in the industry for 20-30 years. Over the years, the two have come together to the point that there is not a whole lot of difference. The biggest difference is probably in their origins.

PLC's originated with discrete control. Designed to replace large amounts of relays. This also is the reason for PLC's dominant programming language being ladder logic. The target audience was electrical engineers and electricians. Ladder logic is well suited to the core training of both disciplines so EE's and electricians can understand ladder programs due to their similarity to electrical ladder diagrams. Over the years the functionality required by advanced ladder programs has somewhat negated this advantage. Many of the commands available are in no way comparable to electrical components. PLC's have also expanded to include structured text programming, function block programming and step programming. Analog I/O and capabilities have been developed to cover a full spectrum of possibilities. SCADA systems have developed to be paired with PLC's to allow full visualization and data manipulation.

DCS's originated to fill in with analog control. Developed for process industries, they were designed around process engineers - usually chemical engineers. They were designed for large systems and generally were programmed with some form of high level language or function block programming. They now have ladder and discrete functionality and even tend to look more like the PLC's. DCS's tend to integrate the visualization and data manipulation better than their PLC counterparts as their configuration is usually a single system designed to work as a whole; however, the PLC manufacturers have made strides into duplicating this capability. The costs of DCS systems are generally higher than comparable PLC systems.

I really dont see a lot of difference today between the two platforms. EE's tend to gravitate toward the PLC's still and Chem E's tend to still gravitate towards the DCS's. The PLC manufacturers are constantly trying to break the image of small system and/or discrete machine control even after all these years. Many of them have gone to calling their upper end controllers PAC's or programmable automation controllers. Change the name lose the stigma.

plc noob May 18th, 2009 05:01 PM

And thus we have the PAC

Higher level models of contrologix plc's are considered a PAC an example is here

Add a client / server hmi system and maybe dta collection such as a historian and you have what in the olden days would be termed a DCS or SCADA

Yuryas May 19th, 2009 06:34 AM

An Example with Siemens:

Simatic PCS 7 is DCS
As a rule it is based on Simatic 414 416 PLCs.

FrancisL May 19th, 2009 06:45 AM

A more focused question is to ask What are the Main differences between PLC's and DCS controllers?
By which I mean let's not look at the networking, HMI etc, but just at the controllers themselves.
And here there are still differences that go back to the Discrete origins of PLC's and the continuous origins of DCS's
With a PLC you can code close to the metal, easily setting and responding to individual bits, bytes or words, or even tables of them with confidence that the program will use little memory and execute fast.
This is not the case with the controllers in most DCS's.

Andybr May 19th, 2009 08:17 AM

The best definition I ever heard was that if the salesman tries to sell you a chair its a DCS.
My view is that the historical definitions are no longer valid. A Control Logix processor has never been a PLC in the original sense and the PC based modern DCS's are a million miles away from a Rosemount System 3. Selection needs to be based on engineering criteria rather than outdated terminology.

naeemmsc May 20th, 2009 04:06 AM

Thanks for all those response my question.But one thing is still not cleared that if any task can perform PLC so why we use coslty DCS system Is there any limitations Of PLC that we have to use DCS>Thanks

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