Thread Starter

Shyam Adsul

hi all,
in almost all books it is given as the PLC is better for discrete (ON/OFF) control, and interlocking operation. Although PID loops can also be implemented. similarly it is said that DCS is better for PID control.

my question is why there are restrictions on PLC and DCS to the tasks of each other? example if one PID loop can implement on PLC, why cant multiple? and why DCS is not good for discrete operations?


Donald Pittendrigh

Hi All

This is one of life's ugly little generalisations, there are definitely many
PLC's capable of great numbers of PID controlers and there are certainly enough DCS's around with the ability to handle high speed digital interlocking. The area between DCS and PLC has greyed out considerably since these books were written, in fact most texts in this area which are older than 6 - 12 months probably aren't written about the same generation of equipment on the market today, anything written today will be out of date in less than a year, and by then there will probably be no definable boundary between DCS and PLC ayway, that is presuming there still is today.

Donald Pittendrigh

William F. Hullsiek

The best explanation is ... market forces.

DCS - are oriented toward large scale plant / continuous process.
PLC - are oriented toward a cell or machine.

Both are good solutions - but you select the solution based on the project economics, i.e., cost/benefit / limits on capital.

Look at the $ per point - and the $ per monitor point and $ per alarm. Put the point count into a scale going from 10-100, 100-300, 300-750, 750 - 1500, 1500 - 2500, and 2500 to 5000. Then look at the control loops in a similar fashion.

You wlll get a good economic picture of where the price/performance curve shifts from one technology to another.

Then you factor in the fear, uncertainty and doubt - this is known as the FUD mark-up.
FUD can add 50-100% to the cost of a system.

Lawyers, lawsuits, liability add the LLL mark-up to the economic equation.
In some markets - this actually can be exponential.

Have fun !
Mostly - because the books say so.

It's not really a restriction as such, it's just that products which concentrate on discrete control get called PLCs and products which concentrate on PIDs are called DCSs.

In reality, these days, most products can do both and sometimes it's more tradition than reality whether they get called PLCs or DCSs. Still, they're likely to be somewhat better in their traditional strengths.

Jiri Baum <[email protected]>
MAT LinuxPLC project --- --- Machine Automation Tools
Invention of PLC was the next logical step to implementation of ladder logic consisting of relays. Hence, codes for all PLCs were written around discrete logic and controls. Subsequently, PLCs were designed to take care of analog controls for applications where minimum number of analog loops are involved.

But, PLCs are best suited for discrete controls in the sense that it 'acts fast', i.e the execution time for each loop is typically in milliseconds (Of course, a few DCS systems in the market are also competing in this regard). As far as analog controls are concerned, DCS is superior as it contains lot more function blocks, better interconnectivity, dedicated HMI etc.

Having said that, however, selection between the two involves good engineering judgement.


Bob Peterson

The reason the books say this is because they are far behind the times. This was true 15 years ago, but with the advent of processors such as AB's PLC5, this is no longer true. There are still differences, and there are reasons to use a DCS over a PLC but simple PID loops are not it. OTOH-PLCs handle typical discrete control tasks in a much simpler and more efficient fashion.

I have done PLC systems with 50 or more PID loops, and it worked quite well.

The main reason DCS systems do not deal well with discrete control tasks is because the vast majority of such tasks do not fit well into the preconfigured control blocks they give you.

I think most DCS systems can now give you ladder logic, but why bother when you can have a perfectly good little PLC that actually does this stuff for a living?

Bob Peterson

Jimmy Saldivias

Also Genetics. They were born from different mothers and later discovered they were related.
DCS was born in process industries controlling loops. i.e. you need a lot of processing power to solve 1 equation i.e. think of analog signals: how many bits you need for them?
PLC was born in factory controlling sequential activities. i.e. you need low processing power to solve "first do this, then do that" i.e. think of discrete signals: how many bits you need for them?
But then again, this is ancient history.
Silicon went down in price and up in sofistication. Today is very cheap to
buy a chip, with everything on it.
And you can place that chip in an electronic equipment and call it: PLC or
DCS or Hybrid system.
And you have a very complicated set of names and control philosophies which go to the same end.
As William said, this a place where you can have a lot of fun!
MBA Ing. Jimmy Saldivias
Phone: 591-4-4523438
Fax: 591-4-4523413
DCS systems are costly for Digital systems. DCS systems have predefined control blocks related to PID loops like CASCADE/THREE element control, Feedforward, etc.
PLC have simple PID control implemetation and the PLC cost is less and powerful in Digital operations.
In old days the PLCs were not having hi end microprocessesors, so they were not able to do mathematical calculations and as the name suggests PLC (Programmable Logic Controller), it was used to control the logic and not mathematical calculations and even only in open loop control so they were low cost.

On the other side DCS were used for close loop controls where there was need of computation power and so hi-end processors. They were not doing any Logic \ discrete control.

But with the course of time the PLC started building the features that were required for close loop control. And today you will find there can not be a clear line in terms of applications those can only be implemented using DCS or PLC.

In some PLCs today you can run multiple PID or even some complex algorithms and on other side DCS can do discrete control.


In the past this was because:

PLC = simple little devices, manufacturing oriented - not enough software and hardware horsepower any serious process control. DCS for the mostly costly digital control you'll ever install.

Here is where a good server based system can technically slam-dunk both PLC and DCS solutions. Because IT servers are used 100's even 1000's of PID loops, no matter how complex are essentially little or no-cost. At the same time, access to Profibus, Device Net or Ethernet/TCP mean hoards of cheap digital are a couple of terminals away.

Software based systems can do both, in large or small applications with no penalty to the user. Old rules need not apply, in fact we are happy to chuck them out the control room, or plant floor window!

Paul Jager, CEO
[email protected]