Comparing PLC and DCS

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
 
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
 
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.
 
E
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.
 
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
 
B
PLCs can have as many loops as you need.

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

--
Bob
 
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
 
J
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
 
M

Matthew Hyatt

Hi,

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

Thanks
asfani
 
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
 
R
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.
 
J
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.
 
J

James Ingraham

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
 
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.
 
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.
 
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.
 
Hey Paresh,

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