Comparing PLC and DCS.pdf

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rom the Automation List department...

Comparing PLC and DCSPosted by Anonymous on 6 May, 2004 - 6:34 pm

I am new in the field of automation. Could anybody tell me the difference between PLC and DCS other thanI/O handling capacity?

Posted by Randy on 7 May, 2004 - 7:15 pm

Now a days you cannot really tell the difference between a PLC or a DCS. Since the PLC was integratedwith Analog I/O it crosses the boundary of being just digital and crosses to the realm of DCS in handlingAnalogs, Bus Systems, Distributed I/O and etc. Also, since the DCS now handles logics of Digital I/O italso crossed the boundary to the realm of PLC.

As you know PLC as to its name Programmable Logic Controller. Its main purpose is to replace the relaylogic controls which is "On" or "Off". And DCS "Distributed Control Systems" its emphasis is Fast analoghandling because of communications through Bus systems, networking and etc.

Summarizing all these, PLC = DCS......

Hope that I shed light on you.

rtj

Posted by Anonymous on 7 May, 2004 - 7:17 pm

PLC=Programmable Logic ControllerDCS=Distributed Control System

A PLC can be a component of a DCS

A DCS can include Networked PLCs, PCs, or other control equipment sharing or distributing control of aprocess or processes. Key word being "distributed".

Posted by David Farris, Bristol Babcock Inc. on 7 May, 2004 - 10:40 pm

As I understand it, the two really don't compare as a PLC is Programmable Logic Controller and DCS isDistributed Control System. Generally, PLCs are stand alone and perform a particular task, where a DSC isa network of PLCs/RTUs that communicate in some fashion to accomplish a particular task. For example,in a water filtration plant, there might be a PLC that is used to perform a backwash of a particular filter, inthat same water plant a DCS may be communicating with 14 filter PLCs and starting the backwash routinewhen required.

Posted by Bob Peterson on 13 May, 2004 - 10:49 pm

While a trivial example, it serves as an illustration, although having programmed hundreds of such filters,I can't ever recall putting a seperate PLC oneach filter. Just not cost effective. the part about the DCS initiating backwash might be true in some casesbut in many cases it would be inititated on time, dp, or flow locally. In fact, most of the installations I

have seen of such things are indeed standalone and typically only report alarms and status to the DCS andrarely does the DCS actually do any control at all. But, YMMV.

Bob Peterson

Posted by Sutrisno on 7 May, 2004 - 10:52 pm

The differences between PLC and DCS:

1. PLC only handled sequential process than DCS can handled both Continue process and large loopcontrol.

2. If we see from security angle, PLC doesn't have dongle so peple can crack the software easy. DCS have adongle so it's only license to industry which have it.

Posted by Anonymous on 10 May, 2004 - 3:38 pm

Why this question, may we know?

Page 1 of 7Comparing PLC and DCS

7/23/2008http://www.control.com/thread/1026194428



Posted by Wieslaw Chodura on 10 May, 2004 - 5:12 pm

From my opinion DCS systems are more complex and include HMI. The realtime HMI database isgenerated when programming the PLC which is the part of DCS system. When you want communicatewith bare PLC the realtime database must be created "manually". In DCS systems the realtime database isalso distibuted so each operator station has its own RT database. There are also so called Hybrid Systemlike Honeywell PlantScape where RT database is created automatically during PLC programming but isstored at realtime server so it is not distributed.

Posted by Ron Beaufort on 11 May, 2004 - 11:44 pm

The following is basically a “cut and paste” from something I posted some time ago on another forum -specifically to answer a beginner’s questions about DCS; SCADA; PLC’s; HMI; MMI; etc. It seemed tohelp him so maybe there’s something in here which will help you too.

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

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

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 conveyormotors, and other assorted machinery - so let’s say we get to work and we build us an oven. Now thatwe’ve got the mechanical part of the oven built - we need some type of controller for it - something toaccurately control all of those different parts in order to turn raw material into a sellable final product. Sowhat type of control are we going to use? How about a PLC - a Programmable Logic Controller?

In very simple language a PLC is a type of computer. But the computers that most people are familiar withuse a keyboard as an input device and a screen for an output device. PLC’s don’t have a keyboard. So foran input device, we use an “input module” which is basically a little box with a row of screws on the frontof it. We wire up a bunch of pushbuttons, sensors, switches, etc. to the little screws ... and this will serve asthe input device for our PLC “computer”. We do something similar for an output device. Instead of using ascreen for an output device, we use an “output module” which is basically another little box with a row of

screws on the front of it. We wire up a bunch of solenoid valves, indicator lamps, motor starters, etc. to thelittle screws ... and this will serve as the output device for our PLC “computer”.

So for this first example, let’s say that we decide to go with a PLC system. We buy the PLC and install itby connecting wires between the oven and the PLC. Then we buy a copy of the programming softwarefrom 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 aboutready to properly control the system - except that we still need some way for the operator to set and tomonitor 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 justfine. 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 MachineInterface - but now-a-days we’ve got to be more politically correct.) So we buy us a nice desktop computerand some type of HMI software. We’ll need to program the HMI - and usually this is done by dragging anddropping 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 memorythrough a communication cable. And now we’re finally ready to go. Great so far - and we start makingsome money with our factory.

Later on, business is good and we decide that our factory could use two additional ovens. So we get themechanical parts built - and now we need to decide how we’re going to control these new ovens. Now theoriginal 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 newovens. But - here’s an idea: Suppose that we buy two new PLC’s - one for each new oven. Now that’scertainly 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 threecontrollers - 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 originalprogram into them - and we’re just about ready to go. But how about that operator control piece of thepuzzle? Since we’re already using an HMI for our operator’s control panel, all we have to do is make twocopies 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 nowwe’re up and running.

Next the boss hires a bean-counter - someone whose job involves maximizing our factory’s profits. Nowthis person requires data - he needs to know how much it costs to operate the ovens - and how muchproduct we run through them - and how much of that product is “off-spec” and wasted. The best way to getall 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 willbe called a SCADA system - for “Supervisory Control And Data Acquisition”. It will still have controlscreens with all of the virtual buttons and meters and other whatnots that the operator needs to control theovens - but it will also have some additional features beyond the HMI - features which will allow theSCADA system 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 littleheart’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 usedan 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 powerful software package- a SCADA (Supervisory Control And Data Acquisition) system. This new software still allowed ourhuman operator to Supervise and Control the system - and it also added some features for Data Acquisitionfor 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 rathersophisticated process - one which is going to require many interrelated steps - all of which must becarefully coordinated in order to produce a sellable final product. We’re talking about chemicals - orpharmaceuticals - or something along those lines. (The term “continuous process” is a familiar buzzwordfor something like this.)

Now yes, we COULD use PLC’s for this type of factory - and yes, we COULD use a SCADA system tosupervise and control the whole thing. But - many engineers would decide to go with a DCS for somethinglike 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 wouldwe control these? Instead of putting a PLC on each oven - we’ll use a separate DCS “controller” for eachoven. 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 outputsconnected to the front of them. So most DCS systems tend to look a lot like a PLC system. The bigdifference is that each of these DCS “controller/card” devices will be individually programmed. That’swhere the term “DISTRIBUTED” comes from - the control (or “brain-power” if you prefer) isDISTRIBUTED among many individual controllers. Specifically, in a typical PLC system we generallyhave only one “brain” (or processor) in each chassis - and then several I/O (input/output) modules in thechassis to handle the signal wires to-and-from the machinery. On the other hand, in a typical DCS systemwe’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 otherwhatnots that he (or she) requires in order to control the machinery. Second, it also has the features requiredto 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 thatreprogramming 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 individuallittle controllers. Our operator uses a DCS terminal (computer) to monitor and operate the machinery. ThisDCS 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 isgenerally provided by just one manufacturer. Some people think that’s a good thing - and other peoplethink that’s a bad thing.

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

Finally: Please remember that this was intended to be a general “beginner level” discussion - there areexceptions to all of these “rules” ... but hopefully this will give you a “starting point” from which to build.

Hope this helps ... best regards,

Ron

Posted by Matthew Hyatt on 27 May, 2004 - 4:22 pm

Ron,

In both cases the PLC or controller is sperately programmed and if programmed correctly can operatecompletely on its on and even share required data with other devices (PLCs, PCs, controllers...) and ineach case the controllres or PLCs or PCs could send data to a host computer which provides overalloperator interface, alarming, historical trending and such... you could even have local HMIs where youneed them.

In fact if you had twenty PLCs each programmed to perform a plant function and only send data to a HMIor SCADA computer would this not be a DCS system - the control operations are indeed distributed

among the various PLCs, the PLCs do indeed function on their own and are not dependent upon a hostcomputer to tell them wha to do or when to do it. Is this not the basis of a DCS scheme? Also, the PLCscould share data with the other PLCs so they could act upon the information obtained to adjust their givenfunction.

I don't know about the PLCs you use, but the one's I use can completely operate a 25+MGD water plantwith little or no operator interaction, except a little monitoring and house keeping via a host SCADAcomputer. From your two rather long explainations, I was not able to see a real big difference. Any morethe two are so intergal and integrated that it is hard to draw clear defining differences. Besides, I coulduse one PLC with plenty of I/O expansion capability to handle all of the filters in a water plant and eventhe rest of the plant's operations, vs buying seperate controllers to do the same thing - to the bean countersI know this is a real money saver when put into the context of operational cost over the life the plant vs

the cost of the equipment.

Though many will argue that there are significant differences and cost advantages, blah, blah, blah... Istand on the platform that supports both are very similar and each can perform the functions of the otherand can be integrated to provide solutions to a wide range of plant automation schemes.

MJH

Posted by Jonas Berge on 13 May, 2004 - 10:44 pm

PLC was developed as a replacement for many relays. DCS was developed as a replacement for many PIDcontrollers.

These days the difference between these two architectures is not very big. Both have a CPU card (controllermodule) and an I/O subsystem with I/O modules. In the past a PLC was purely logic and the DCS purelycontinuous controller. The PLC was programmed in ladder and the DCS in function blocks. Today bothhandle all kinds of I/O and can be programmed in multiple languages. In the past a DCS included servers





and workstations software whereas for the PLC the HMI software was purchased separately. I.e. with aDCS you got an integrated system whereas with PLC you did system integration. In the past a DCS usedonly proprietary networking whereas a PLC used open networking making it possible to connect third partyhardware. In the past only the DCS applications were proprietary whereas the PLC was an open system. I.e.with the DCS all applications were tailored for the native hardware minimizing configuration work butmaking impossible orunfeasible to add hardware and software from third parties. The PLC can freely use third party hardware

and software, required lots of configuration work, but at least it was possible. Today PLC use OPC to makedata available to software as a single integrated database with little of no duplicate work. At the same time,DCS also implement OPC as a gateway that makes access to some data possible although it is stillimpossible to choose theworkstation software and you still cannot connect third party devices to the DCS networking. These daysmost PLC manufactures have either bought or aligned themselves with HMI software companies supplyinga total solution. Other differences in that past included far better diagnostics and redundancy in the DCS,but this gap has been closed. Today, many PLCs are sold as and used in applications where in the past onlyDCS could be used. Historically a DCS was also far more expensive, but the competition from PLC andnew architectures have driven the initial price of DCS down although the long term cost may be highersince with a DCS you are pretty much locked to a single supplier.

Since a couple of years ago a technology called FOUNDATION(tm) Fieldbus introduced a new systemarchitecture based on standard networking providing a leap similar to that from DDC to DCS/PLC. Thenew system architecture is explained in chapter 1 of the book "Fieldbuses for Process Control: Engineering,Operation, and Maintenance" (buy online in hardcopy or downloadimmediately in softcopy):http://www.isa.org/fieldbuses

If you can't buy the book now, you can download chapter 1 (overview) for free in softcopy form. It's free,but you must register an account. If your email does not support this hyperlink feature correctly, pleasecopy the entire link and paste it into your Internet browser. Mind the line wrap, make sure to get thecomplete path all the way to the 4585:http://www.isa.org/Template.cfm?Section=Shop_ISA&template=/Ecommerce/ProductDisplay.cfm&ProductID=4585

Jonas BergeSMAR===========

[email protected] fieldbus at your own pace: www.isa.org/fieldbuses

Posted by FEMI ADULEYE on 14 July, 2005 - 12:49 pm

I have greatly benefited from this discussion.

However, I am of the opinion that there's no functional difference between a PLC and a DCS today. Both

can comprise intelligent devices networked over a data highway for control of sophisticated processes.

With PLC units manning process areas and then communicating with a supervisory controller(strictlysupervising, not controlling); one could without any controversy call that a DCS.

In essence, PLC = DCS, today!!!

Posted by Tom on 20 March, 2006 - 7:55 pm

Thanks for contribution of everybody. I think I should contribute, too.

I think PLCs are parts of a either DCS or SCADA system, so that the question should be DCS Vs.SCADA rather than DCS Vs. PLC.

As the previous writer said, DCS stresses on processing (PID) control variables, while SCADA stresseson supervisory (watching). Today, either system is capable of doing both jobs. However, due to limitedcapabilities of the CPU and budget availability, one have to choose which one (SCADA or DCS) is





more appropriate for a particular application, i.e 40% SCADA and 60% DCS or vice versa. Choosingthe ratio is implicited in choosing among several vendor/ sofware on the market.

I could be wrong, though.

Tom

Posted by p.k.kundu on 7 April, 2007 - 1:23 amToday both can serve the purpose of othersto some extent. But a DCS conceptual development isbasic/advance control and other higher level control/advance fn. Where as PLC is build basically for logiccontrol including safety logic upto SIL4 level but can accept analogue input preferred in 2oo3configuration.

Posted by Manoj Joshi on 19 May, 2007 - 3:04 pm

Main differences between TRUE DCS & PLC are:

1) Control2) Communication3) No. of I/Os that can be connected4) Scanning time

5) History6) MMI

Posted by Usman Alvi on 26 September, 2007 - 10:38 pm

Dear Mr. Joshi,

Please little illustrate the points of differences that you mentioned here. In this case it will be more clear.

Posted by EPK on 25 October, 2007 - 11:23 pm

The biggest difference between DCS and PLCs is that DCS systems provide:Level of intergration between the controller, HMI and historical database (Common database,Faceplates/Function blocks interlinked.

Control algorithms for advanced control strategies highly evolved and proven (Boiler Master,Distillation towers, Kiln control).Complete turnkey control solution from one vendor from P&ID development throught to startup.Huge number of I/O can be controlled 100K+ points.

In my over 25 years of experience in industrial control no expert in their right mind would everconsider using anything but a DCS system for control of a large plant that has a mixture of analog anddigital loops. DCS vendors have the experience and resources to make it happen. With PLC/HMI youneed to rely on systems integrators to make it all work. You get what you pay for.

Posted by STEVEN MATSEBA on 15 June, 2007 - 12:12 am

The difference between the PLC and the DCS is the database, i.e. when using the DCS the engineeringwork that you do is in one environment, for example mimics, programming, trends, reports, program

creation, etc. Whereas in a PLC environment you need two databases to do engineering, i.e. in a PLCenvironment you can do programming, I/O configuration, etc. To develop mimics you need SCADA whereyou can build your trends, alarm windows, etc. so you can see that you need 2 databases to develop yourengineering work on the PLC.

Posted by Jasir Sabri on 14 December, 2007 - 1:02 am

The discussion here was very useful. Thank you all.

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Comparing PLC and DCS.pdf