Have PLC's become DCS's?

The hardware and software capabilities of the PLC have increased dramatically in the past 10 years. Have PLC's advanced so far that term PLC and DCS are now synonymous?

No.

I think that target is 5-10 years away.

Ken Moore said:

No.

I think that target is 5-10 years away.

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I've not worked with a DCS system in over 15 years, but haven't they evolved as PLC's have evolved also ?

I would imagine that DCS will always have a market of their own, in the same way that PLC's will always have their own.

Just thinking out loud.

I do not see why PLC's should ever "evolve" into DCS's. The issue is whether the two types of system can perform the same tasks. Some top end PLC's (CLX, S7, Mitsubishi Q series) can certainly do that but the architectures and methodology are different.

Andybr

I think that there is not much difference between "high end" PLC's and DCS's. There is not much you can do out there now that can't be done with either/or arrangement. The siemens PCS7 platform using S7-400 controller seems to me "like" a DCS as do the ControlLogix PLC platform in I/o arrangement, controller placement etc. . It seems to me that the PLC manufactures are taking the best attributes from DCS systems and making it there own. But that's just my opinion.

Perhaps there's room for someone to start a "DCS.net"?

I think some folks are missing the point here.

A DCS encompasses not just the 'controller' part of the system but the entire architecture all the way from enterprise-level down to remote IO and serial interface options. HMI connectivity is built in, comms are redundant and secure. Scan time is guranteed and there are a ton of diagnostics.

I have yet to find an HMI/Controller combination offered by a PLC vendor that even come close to the functionality and stability of a DCS.

I work on DCS systems from the 1970s (TDC2000) to right now (CS3000 R3.08) and they have come a long, long way, as have PLCs.

A DCS is aimed at a market where 10,000 IO and 40+ operator stations is normal. A PLC and its associated comm structure would struggle to accommodate such an undertaking, coupled with its limited redundancy options I doubt it would ever seriously take the place of dedicated DCS systems from Honeywell, Emerson or Yokogawa.

I'm not trying to flame anyone in the slightest.

I have come across some installations where PLCs have been used and a DCS should have been.

Every one was a train wreck.

Now PLCs are wayyy more common because they are perfectly suitable in 90% of industrial applications and generally very cost-effective.
Its the other 10% thats the territory of a DCS and a DCS alone.

Of course a DCS costs about 10 times as much too

my opinion

There will always be a market for both, so both will stay and not only for the financial side of it.

To simplify ...

Continous process (cement, chemical, ...) = DCS
Stand alone (press, palletiser, ...) = PLC

However it would be nice for future if everything could sit on the same 'platform'.
Let me explain myself. In the past ethernet was quit expensive, but these days it is very cheap, even cellphones have it. Why can't it be integrated into the eyes and ears (actors/sensors)of the machine/plant. Is it not time to get rid of combinations of different networks like profibus dp/pa, modbus, interbus, asi, ... and standardise to one platform like ethernet. Each actor/sensor or decentralised node gets his own mac-adress, and there is only one network for the whole plant, accesable from plc up to mes, even over the internet.
Imagine what possibilities this would create ...

To simplify ...

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That really was a simplification!
Continuous versus stand-alone? So if I have a single PLC controlling something that's stand-alone, right? And if it's controlling a valve that's continuous, right? And if I have 12 PLCs networked together on a printing press, they're not standalone, and because the machine only runs sometimes and not others, it's not continuous either, right? So sometimes I have PLCs which fit in both categories and sometimes fit in neither.

How about treating DCS and PLC a bit like phones and cameras. Years ago they were entirely separate technologies solving entirely separate problems. Talking and taking pictures - no common ground at all. Nowadays phones and cameras themselves have changed over many generations, and hey what do you know, some of the phones have even got cameras in them! But are these cameras really any good other than for trivial tasks. And I've never seen a camera you can use as a phone. Does that mean cameras have become phones or have phones become cameras? And where will the change lead in future? And why do specialist photographers still stick to specialist cameras? Oh, we could go on debating phameras and cones for ever, and where would that get us. Probably the same place as PLSs and DCCs

Ken

Hit the nail on the head there if you ask me.


This debate has been raging on as long as I can remember.
The only solid statement is DCS cost = PLC costx10.

Thats all alot of clients worry about!

Systems are getting so varied and complex now they dont even bother trying to unravel this mess themselves but hire an outside integrator instead.

Keeps us in business.

My 2 cents

I came out of the DCS world into PLCs. I look at a DCS as a completely integrated solution from a single vendor; engineering station, control processors, I/O blocks, historian, batch manager. I am very familier with the Foxboro I/A series DCS. Everything in this system is well intgrated and works well together; displays with standard faceplates, historian, trending, batch sequencing, etc.

However, I believe that the controllogix PLC or GE PAC is every bit as capable as any DCS control processor. It has all of the engineering tools and is well capable of implementing complex control schemes using function blocks, SFCs, and structured text. In addition, Rockwell has integrated batch management quite well as long as Bizware batch is used.

However it is a large engineering task to integrate all of the other systems such as a SCADA, historian, enterprise system integration as well as it is accomplished in a DCS system. In addition, I do not know if all SCADAs are capable of handling 40+ operator stations.

With large processes whether batch or continuous, DCS can compete on a cost basis. Small standalone systems are much simpler to inegrate and lend themselves well to the PLC/SCADA solution.

I see here doubts as to whether SCADA can handle 40 plus stations, DCS doing 10,000 I/O etc etc.

Might I suggest that the "doubters" have a look at a project here in Ozz. It is Olympic Dam. Citect SCADA system is now over 1 million tags to the best of my knowledge. All PLC control (AB in this case - way before Control Logix too - no PAC controllers on this site). PLCs dial in faults from remote stations to SCADA. SCADA dials out to remote stations to collect data etc etc. Scada has historian, recipe control etc etc. Overall scan time of the SCADA system is under 2 seconds for 1 million tags.

Pretty damned impressive. Can DCS do that I wonder?
Historian, recipe management etc etc. The great thing is the PLCs are all industrially hardened and reliable, the SCADA has 10 redundant servers (may have changed since I looked at this site).

Not tied to a DCS manufacturer that wants to rip you off every time there is to be a small change made to a timer either. Just about all changes to the PLC system/programs and SCADA are performed by technicians on site employed by the company and not a DCS or integration company. The system is fully transparent, reliable and open.

I agree that the high end PLC cpus have comparable horsepower and functionality to most DCS controllers.

Where they fall down (as mentioned by infineum) is the total package. With a DCS you can do pretty much anything. They have full-scale historian packages, advanced control packages. They have a proper, integrated alarm and operator 'area of responsibility' structure that I have yet to see in ANY scada system. Lets not get confused between an RTU SCADA system and a DCS. We work on Cygnet systems that pull in thousands of points but the scan time isnt that critical and neither is the redundancy. There is not alot of heavy-duty real time complex control happening.

The fact that every time you want to add a PLC you have to start with an 'empty box' also makes engineering costs (compared to a DCS) much higher. It had imorived somewhat with CLX in that you dont have to map out all the memory anymore but never the less, the 'canned functions' in a DCS cant be beat.

In answer to the previous post:
To be fair I havnt messed with Citect, but every other scada I have used (RS View 32/SE iFix, WW, Cygnet) doesnt come even close on the HMI side to what a DCS is capable of.

Siemens PCS7 is definitely based on the standard S7 PLC (either S7-400 or WinAC). How it stands up against other DCS packages I dont know. But for sure it has the integration, scale and redundancy that is mentioned by others.

PCS7 is the oddball out there in that Siemens really put some thought in to the software side of things, making a pretty neat package. It is classed as a 'hybrid' I guess in the fact that they were never a legacy DCS vendor but have the know-how and funding to try and push into that market.

Still in my experience most plants DCS are either Honeywell, Emerson or Yokogawa.