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Comparing PLC and DCS
PLCs and related questions. topic
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Posted 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 than I/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 integrated with Analog I/O it crosses the boundary of being just digital and crosses to the realm of DCS in handling Analogs, Bus Systems, Distributed I/O and etc. Also, since the DCS now handles logics of Digital I/O it also 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 relay logic controls which is "On" or "Off". And DCS "Distributed Control Systems" its emphasis is Fast analog handling because of communications through Bus systems, networking and etc.

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

Hope that I shed light on you.

rtj


Posted by William G. on 14 July, 2010 - 10:51 pm
If taken from a narrow point of view, maybe PLC and DCS are functionally the same. However, a lot of plant technicians and engineers are not aware of the other side of the debate, namely the engineering effort and the commercial ramifications. Of course, for huge plants with I/O's ranging from 10,000 to 30,000 I/O points, and beyond, it is silly to even think about PLC. From my experience, a DCS is not the same with PLC in the realm of huge systems such as oil and gas plants, as the following list will show:

1. A PLC is cost-effective up to a certain I/O count, and so is the DCS. But the difference is in their starting points: the PLC is cost-effective from 0 to a few thousand I/O points; the DCS becomes cost-effective starting from a few thousand points and beyond.

2. A PLC becomes a subsystem of the DCS in rare occasions when the situation calls for it, i.e., purchase of huge package systems with engineering schedules incompatible with the DCS schedule (I/O lists cannot be submitted on time before the DCS hardware freeze date). Note that this package system is a process system using continuous control, not discrete. Based on this, a PLC can never be larger than a DCS in terms of I/O count.

3. In large plants the DCS is king because most owners want a single source of hardware support and service, and this mentality naturally denies the PLC a foothold. Package vendors are no longer required to provide PLC for their system. Everything is connected to the DCS.

-WAG


Posted by Anonymous on 7 May, 2004 - 7:17 pm
PLC=Programmable Logic Controller
DCS=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 a process 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 is Distributed Control System. Generally, PLCs are stand alone and perform a particular task, where a DSC is a 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, in that same water plant a DCS may be communicating with 14 filter PLCs and starting the backwash routine when 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 on
each filter. Just not cost effective. the part about the DCS initiating backwash might be true in some cases but 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 and rarely 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 loop control.

2. If we see from security angle, PLC doesn't have dongle so peple can crack the software easy. DCS have a dongle 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?


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 is generated when programming the PLC which is the part of DCS system. When you want communicate with bare PLC the realtime database must be created "manually". In DCS systems the realtime database is also distibuted so each operator station has its own RT database. There are also so called Hybrid System like Honeywell PlantScape where RT database is created automatically during PLC programming but is stored at realtime server so it is not distributed.


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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 to help 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 all of these "rules".

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.

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? 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 with use a keyboard as an input device and a screen for an output device. PLC's don't have a keyboard. So for an input device, we use an "input module" which is basically a little box with a row of screws on the front of it. We wire up a bunch of pushbuttons, sensors, switches, etc. to the little screws ... and this will serve as the input device for our PLC "computer". We do something similar for an output device. Instead of using a screen 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 the little 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 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 beyond the HMI - features which will allow the SCADA 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 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 powerful 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 some 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 only 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: Please remember that this was intended to be a general "beginner level" discussion - there are exceptions 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 operate completely on its on and even share required data with other devices (PLCs, PCs, controllers...) and in each case the controllres or PLCs or PCs could send data to a host computer which provides overall operator interface, alarming, historical trending and such... you could even have local HMIs where you need them.

In fact if you had twenty PLCs each programmed to perform a plant function and only send data to a HMI or 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 host computer to tell them wha to do or when to do it. Is this not the basis of a DCS scheme? Also, the PLCs could share data with the other PLCs so they could act upon the information obtained to adjust their given function.

I don't know about the PLCs you use, but the one's I use can completely operate a 25+MGD water plant with little or no operator interaction, except a little monitoring and house keeping via a host SCADA computer. From your two rather long explainations, I was not able to see a real big difference. Any more the two are so intergal and integrated that it is hard to draw clear defining differences. Besides, I could use one PLC with plenty of I/O expansion capability to handle all of the filters in a water plant and even the rest of the plant's operations, vs buying seperate controllers to do the same thing - to the bean counters I 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... I stand on the platform that supports both are very similar and each can perform the functions of the other and can be integrated to provide solutions to a wide range of plant automation schemes.

MJH


Posted by Rhyan on 1 June, 2014 - 10:12 am
Ron,

thanks a ton for your response. What a breath of fresh air. I manage a dcs system but I have also been a system integrator for years. The differences are almost negligible and to be honest, my system costs so much that adding automation to the system is a major deal. As a side note, most of my processes are run by plc controllers in the field and then connected to the dcs to give operators access from the HMI. This is done through modbus to the dcs controllers ...

As for the explanation you were responding too. In both cases the gentlemen has explained either system. Having multiple controllers in one location to "distribute" the local control is a massive expense and complicates things. I have some drops with 3 redundant controllers (that's 6 total) doing the work that one controller could be doing. That's 25000 a controller for a total of 150000 in controllers with less io than I've seen on a controllogix processor run a whole plant. The specs on the processors are also less in almost all cases. Quite funny actually.

Anyway. The two have caught up to each other. And please everyone, look at the specs on PLC's PAC's and DCS's. Taking the stance that the dcs can handle more io is just ludicrous. Processors and memory dictate that, not the category you decide to put your product in. And as far as communications go, they all use very similar ways to talk and in most cases I've seen PAC's do it much better.

Great post Ron.


Posted by akash on 31 March, 2010 - 5:12 am
thanx sir ron..u are such a good professor..i have seen all ur bootcamp videos..waiting for some more lectures


Posted by Pete Ng on 1 April, 2010 - 4:49 pm
I love this discussion. There is not really wrong or right answere here as one could consider this like comparing oranges versus apples while others would say not.

To Ron: You must be one of the good ol' controls engineers who's at least been through the 80's, 90's, and the 2000's. Love your sense of humor.

Regards


Posted by jeff fraley on 3 August, 2012 - 1:06 am
In my quest years ago to obtain my Master Electricians license, I was fortunate enough to have taken a class under an instructor who could relate... or knew how to teach from the ground up, covering so many things in ways that most of us take for granted when teaching.

Ron B. is one of those people and the rest of us are fortunate for that. Thanks for all your posts and videos. I have relearned much of RsLogix 500... moving into Tags and 5000 now... Life is Learning

jeff f


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 PID controllers.

These days the difference between these two architectures is not very big. Both have a CPU card (controller module) and an I/O subsystem with I/O modules. In the past a PLC was purely logic and the DCS purely continuous controller. The PLC was programmed in ladder and the DCS in function blocks. Today both handle 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 a
DCS you got an integrated system whereas with PLC you did system integration. In the past a DCS used only proprietary networking whereas a PLC used open networking making it possible to connect third party hardware. 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 but making impossible or
unfeasible 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 make data 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 still impossible to choose the
workstation software and you still cannot connect third party devices to the DCS networking. These days most PLC manufactures have either bought or aligned themselves with HMI software companies supplying a 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 only DCS could be used. Historically a DCS was also far more expensive, but the competition from PLC and new architectures have driven the initial price of DCS down although the long term cost may be higher since 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 system architecture based on standard networking providing a leap similar to that from DDC to DCS/PLC. The new system architecture is explained in chapter 1 of the book "Fieldbuses for Process Control: Engineering, Operation, and Maintenance" (buy online in hardcopy or download
immediately 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, please copy the entire link and paste it into your Internet browser. Mind the line wrap, make sure to get the complete path all the way to the 4585:
http://www.isa.org/Template.cfm?Section=Shop_ISA&template=/Ecom merce/ProductDisplay.cfm&ProductID=4585

Jonas Berge
SMAR
===========
jberge@smar.com.sg
www.smar.com
Learn 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(strictly supervising, 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 stresses on supervisory (watching). Today, either system is capable of doing both jobs. However, due to limited capabilities 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. Choosing the ratio is implicited in choosing among several vendor/ sofware on the market.

I could be wrong, though.

Tom


Posted by Paresh on 17 June, 2014 - 3:06 am
The difference between PLC and DCS has narrowed but not completely wiped out. I mean we are using DCS of Toshiba, Yokogawa in plants in very harsh conditions, but hardly found the failure of cards. But we are also using PLC of Rockwell, GE, Siemens and found the failure of cards start after 3-4 years. So, the biggest difference between PLC and DCS is ruggedness and reliability. In DCS the biggest problem is the failure of the Process HMI, where the Hard disk is difficult to get. But, with use of PC as HMI this problem has solved. Another big difference in PLC and DCS is the ease of programming in DCS.


Posted by Rani on 19 July, 2014 - 4:00 am
Hey Paresh,

I am doing a research project on PLC & DCS comparison. I have few doubts. Can I get your contact number.


Posted by p.k.kundu on 7 April, 2007 - 1:23 am
Today both can serve the purpose of othersto some extent. But a DCS conceptual development is basic/advance control and other higher level control/advance fn. Where as PLC is build basically for logic control including safety logic upto SIL4 level but can accept analogue input preferred in 2oo3 configuration.


Posted by Manoj Joshi on 19 May, 2007 - 3:04 pm
Main differences between TRUE DCS & PLC are:

1) Control
2) Communication
3) No. of I/Os that can be connected
4) Scanning time
5) History
6) 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 ever consider using anything but a DCS system for control of a large plant that has a mixture of analog and digital loops. DCS vendors have the experience and resources to make it happen. With PLC/HMI you need 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 engineering work 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 PLC environment you can do programming, I/O configuration, etc. To develop mimics you need SCADA where you can build your trends, alarm windows, etc. so you can see that you need 2 databases to develop your engineering work on the PLC.


Posted by Jasir Sabri on 14 December, 2007 - 1:02 am
The discussion here was very useful. Thank you all.


Posted by cheers73 on 20 May, 2011 - 4:02 am
The basic difference is DCS and PLC lies in application. We generally use PLC for start up and shut down activities because of faster response of PLC and DCS for continuous control because the cost of implementing continuous control in PLC will be very high and secondly the response time of PLC will increase to great extent.

Evolution wise also, PLC is successor of hardwired relay logic and DCS is successor of SLPC's (Single Loop Process Controllers).

Operation wise HMI is an integral part of DCS, while for PLC we did no have any such HMI to start with. For data presentation we had to do serial communication between DCS and PLC to present all the tags of PLC in DCS. But with the advant of scada this difference does not hold any meaning any more.

Alarm and SOE wise, we can get the SOE with 1 msec resolution in PLC and in DCS resolution generally depends upon the scan time of DCS.

All said and done, we can implement logic in DCS also if the logic part is small and control part is bigger and also we can implement continuous control in PLC if Control part is small and DCS part is bigger.


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Posted by eng / karim on 13 July, 2011 - 10:51 am
the plc deals mainly the the digital i/p and o/p (1 or 0) for an ex. it gives a permission for a valve to totally open or totally close...but dcs deals with analog i/p and o/p it can gives the valve a permission to open 50% or 30% as the controller wants.


Posted by dinesh on 16 January, 2012 - 10:27 am
Dear friends i have a doubt please help me out in this "The system have at least 16 or more control loops" using PLC can i able to make out this or i need to go for DCS please give me in detail explanation.

Thanks & Regards,
Dinesh


Posted by bob peterson on 16 January, 2012 - 12:29 pm
PLCs can have as many loops as you need.

If one CPU cannot handle them all, you can always add another one.

--
Bob


Posted by Nasser on 8 February, 2012 - 4:48 am
Hi,

What is the difference between central and distributed control system applied to electrical network and industrial plants.

Also i need some notes on DCS and SCADA system, ptt slides will be fine or any website link.

Thanks
Nasser


Posted by AHK on 19 July, 2012 - 4:37 am
> Also i need some notes on DCS and SCADA system, ptt slides will be fine or any website link.

http://www.automation.siemens.com/mcms/process-control-systems/Site CollectionDocuments/efiles/pcs7/support/marktstudien/PLC_or_DCS.pdf

you will thank me for this


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Posted by Matthew Hyatt on 24 September, 2012 - 3:58 am
Hi,

If interviewers ask me what is difference between PLC and DCS? How I have to explain?

Thanks
asfani


Posted by Roy Matson on 24 September, 2012 - 2:32 pm
One thing they differ on is speed, the PLC will handle I/O lightning fast while DCSs used to be real slugs. I have seen as long as 1 second scan time. While 1 second is fast enough for most control loops it would be pretty useless for a fast moving machine for example a CNC lathe.

Now I know modern DCSs can probably handle high speed with the help of special routines.

Roy


Posted by Ramamoorthy on 21 February, 2013 - 8:16 pm
At present stage, there is no big difference between PLC and DCS. Both are same doing all jobs.

One and only major big difference between them is SCANNING TIME. PLC is much faster than DCS.

Thats the reason in many Oil & Gas Industry, Refinery, Chemical industries they are using PLC. Because less than seconds it will do Cause & Effects for Safety Systems and Process Systems.


Posted by Jared Bryan on 24 February, 2013 - 12:01 am
My impression (not having worked on what I think would be called a DCS) is that DCS is kind of a marking term used to differentiate from PLC based control without making direct reference to computer based control - which I think for many people has some negative connotation.

Also I think DCS is meant to imply large and integrated which it seems to me is more often the approach taken in certain types of process industries due to the nature of the systems involved.

While PLC's are often networked and interact in an integrated fashion, I think when someone says DCS they are implying computer based control which brings with it additional capabilities and complexity and other issues.

I don't think it is correct to imply that PLC's are not up to the task of analog control, but clearly a full computer will have greater processing power and resources in general.


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Posted by annapurna on 29 January, 2014 - 2:12 am
Plc requires an communication interface and DCS has inbuilt communication protocol.


Posted by James Ingraham on 29 January, 2014 - 10:53 pm
annapurna stated: "Plc [sic] requires an communication interface and DCS has inbuilt communication protocol."

No. Not even close. True, any DCS will have some native "inbuilt" communication protocol. But then so will any PLC, many of which these days are Ethernet based but even 20 years ago had SOMETHING, e.g. Modbus, DF1, etc., etc., etc. Meanwhile, DCS vendors will happily sell you expensive comm cards.

Why anyone is answering a 10 year old question that has plenty of good (and bad) answers, I'm not sure. Especially since that answer is WRONG.

-James Ingraham


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Posted by Iqbal Raza on 27 March, 2014 - 1:54 pm
People are still giving answers because many users still don't know the differences between a PLC and DCS.

In early stage PLC was used to replace relay base system and hence PLC deals only with digital IOs and DCS used for both digital and analog inputs and out puts special it eplace single loop controller. But with the passage of time plc modified to handle analogue inputs and outputs and hence now plc is considered a sister controlling system of DCS. But there is still you can find some differences in DCS & PLC.

1- Redundancy of Input cards are available only in DCS but not in PLC.

2- Termination unit card which used to condition the input signal.

3- Quality and reliability of all cards such as IOs card, CPU, CP and Power supply . specially input/output card. For example in Siemens S7-400 plc, if a two wire type analogue input wire get ground all input of this analogue card get badly effected and shows false maximum input values. There is no chance of this problem in any true DCS.

4- In DCS it is easy to install ignition barriers for those outputs which are going to use in Hazardous Area.

5- A DCS software has builtin HMI but PLC can be used with or without HMI software.

6- For less no of IOs PLC is more suitable price wise.


Posted by Cathy on 19 July, 2014 - 4:14 am
Dear Raza,

Can you provide some Input on cost comparison of PLC & DCS.

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