PLC DCS Scan Time

Hi Folks,

When it comes to comparison of PLC and DCS we always have a point on Scan time..
But my question is what is it that makes PLC faster than DCS?Is it b'cause of software or hardware?
Please gve ur comments...

Cheers.
Duncan

I would guess in most cases it comes down to the amount of I/O and the type of application.

Many PLCs are used to controlled machines of some sort, these typically have relatively low I/O count, but speed of reaction, i.e. scan speed is of paramount importance.

PLCs are of course also used in Process Control which is also where the DCS is mainly at home. In my experience, PLCs have typically been used in relatively small installations having only a few hundred, or perhaps a thousand discrete I/Os and maybe 10 or 20 control loops. A typical DCS system can easily reach 10 - 20,000 discrete I/Os and several hundred loops. For typical Process Control applications, with the exception of a few exotic exceptions like exothermic reactions and distillation columns, a scan cycle time of 1 or 2 secs is more than adequate.

It's not really that one is "better" than the other, it's horses for courses.

Hey RMA,
I dont think number of I/Os is the only factor.Because today DCS are competing 4 small applications also.I have worked on a project with DEltaV DCS with around only 300 I/Os.Here in Deltav software u can't select scan time below 100ms.With Yokogawa I think we can go upto 50 msec.
So there should be some other fatcors which affects the scan time of DCS.

Duncan..

Number of I/O certainly isn't the only factor, although I must admit I haven't run into a DCS in the last 20 years with less than 1- 2000 discrete I/Os. I think it's more a case of the perceived market. DCS is usually aimed more at Process Control and apart from a few special cases, most of the time you simply don't need faster scan rates.

I can't see why anybody would want to use a DCS for a small system nowadays, anyway. The PLCs have developed to the point where they would wipe the floor with DCS systems on price/performance basis. The DCS systems are carrying around so much functionality (which has to be paid for), that unless there is some of this functionality that you absolutely have to have and which is not available in a PLC (Getting less and less all the time), then the cost is only justifiable for a larger system.

RMA said:

I can't see why anybody would want to use a DCS for a small system nowadays, anyway.

Click to expand...

People that make the decisions are unfamiliar with the differences and fall victim to a good sales pitch.

DeltaV must be good at it (the sales pitch), because I have seen a couple of small plants recently with just 2-300 I/O and a handfull of loops running their system. I am working on integrating a system into one and we are using ControlLogix on our end. They wanted us to use DeltaV, but when we showed them the price difference, they chose the PLC.

Marty

People that make the decisions are unfamiliar with the differences and fall victim to a good sales pitch.

Click to expand...

Must admit, I'd forgotten that aspect!

But my question is if we r using a DCS system with only 300 I/O points then why it is not possible to work with faster scan times in DCS...
waht is it that restrict DCS from fast scan times...

Regards.
Duncan..

waht is it that restrict DCS from fast scan times...

Click to expand...

Probably a conscious decision by the manufacturer not to provide that option!

Probably a conscious decision by the manufacturer not to provide that option!

Click to expand...

[sarcasm] But why would you want such an option. Look here see on page 32 of our catalog these nice comfy chairs (Foxboro P/N 32523590). Just think how nice these would feel when you're configuring this DCS. And the operators won't even want to get up out of them to bother you with minor changes like missing process actions because of a slow scantime.

Look at the oak desk as well. Note the fine craftmanship in this model (Foxboro P/N 32523577). You'll never have to buy another desk as this one will last a lifetime. [/sarcasm]

Now in Automobile Industry they use PLCs and not DCS.This is ofcourse due to scan time issues.Now why DCS manufacturers dont want 2 sell thier systems to automobile industries..I think there is something that not allow DCS to go for fast scan cycles....

Now in Automobile Industry they use PLCs and not DCS.This is ofcourse due to scan time issues.Now why DCS manufacturers dont want 2 sell thier systems to automobile industries..I think there is something that not allow DCS to go for fast scan cycles....June 24th, 2005 10:25 PM

Click to expand...

Many reasons really. Safety, security, scan time, reliability.

PLCs are generally designed to work in industrial environments. What are generally described as DCS system are not. PLCs are really DCS systems anyway (digital control systems). The areas a really grey these days.

PLCs have built in watchdog timers - so called DCS systems quite often do not.

PLCs check stored program integrity from scan to scan and, if the program has changed due to noise etc, the PLC systems shut down. So called DCS systems generally do not.

PLCs can, and are, used for what are traditionally so called DCS systems. Analogues are generally expensive in PLC systems compared to so called DCS systems and that is where they generally get priced out. But they are designed for industrial, noisy applications. So called DCS systems generally are not.

A Honeywell I/O unit is really a PLC. Tell the Honeywell guy that and he has been trained to the point where he will find any excuse to call it something else.

The software and expertise to program a PLC system is generally available. The software and compiled program for a so called DCS system is generally not available.

With respect to scan time, a PLC is designed to specifically scan and react to the inputs to create outputs in a timely manner for machine control etc. So called DCS systems are generally designed for control of air conditioning systems and the like. These syatems are generally very slow moving and do not require the speed response that a PLC can provide. The last time I worked with a so called DCS system it was so frustratingly slow with respect to scan and software monitoring time I thought I had gone back further than 20 year old PLCs with respect to scan time. Wow!! They are generally very slow. But they do not have to have a fast scan time to control the systems for which they have been designed.

Most of the confusion comes from the roots of both systems. PLC were first created for and by electrical engineers to replace electrical relay panels (I believe GM engineers start it all). On the other hand, DCS systems were developed to replace pneumatic controllers … initially with logic performed by integrated circuit in cards and later by microprocessors.



As a user of both systems for the pass 15 years, I have seen how technological advances have draw closer together both systems to the point that many DCS’s manufacturer are talking about hybrid systems (i.e. Honeywell Experion PKS: Allen-Bradley Contrologix hardware and Honeywell firmware) … Emerson Fisher-Rousemount Delta V is another example of a hybrid system. Hybrid systems target medium size applications and compete with medium to large PLC systems.


In my opinion the gap between the two technologies was reduced considerably by improvements in the communication and networking capabilities of PLC’s, along with the raising in popularity of field buses (which also imply the usage of better and faster processors).



PLC’s still are the preferred system for control applications in manufacturing environments where sequencing and interlocking are the main concern. DCS’s are and will be (for the years to come) the preferred system for control systems at the top end of the process industry (i.e. we commissioned and thermal power plant of 60,000 tags). The middle point between those two is where both systems fight for supremacy.



The main points to consider before making a decision on whether use one or the other are:



§ The system number of tags and I/O (system capabilities).

§ Control strategies and the right engineering tools (the right tool will make the engineer easier and faster).

§ Hardware and support cost.



Cheers,