Difference between PLC & DCS

The manufacturers of distributed control systems have developped prepared packages for your biggest customers.It was like a car, you just had to choose the color.

PLC manufacturers had a different approach.

The DCS of today can be PLC s associated with computers.

That will do anything you want for the better price.

PLC is a control instrument may be very sophisticated now a days.

DCS is a scheme to achieve control. DCS may utilise PLCs.

Sure a distributed control system can be built with plc's.

plc's are distributed systems in the field and can control everything that what is required from them and send results or receive orders from a system that is higher in the control logic and this system can be a DCS;

But if you connect that PLC's to a net of computer you can have what DCS does but not being called DCS.

One thing is true,PLC's and DCS are different things, but for a customer who has a process to built and control it, the name given may not be the most relevant thing. the DCS may be a turnkey sold system because all libraries are known by the manufacturer.A system built with PLC can have the advantage that you write a new book, freely, and have not to copy paste and modify existant things, even if that is always possible for both solutions, but it will depend of the level where it will be done.

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I'm not too well versed on either but where I work, both PLCs and a DCS are in use.

The difference as I see it is the PLC is sort of a basic building block for larger control systems or it can be used by itself for smaller control applications.

The DCS we have is a very large control system which consists of digital input termination cabinets, analog input termination cabinets, digital output termination cabinets, analog output termination cabinets, power supply cabinets, PLC cabinets and output relay cabinets. It occupies a small temperature controlled room by itself.

Snave is absolutely right. If I may add, If PLCs are 10 seater aeroplane , then DCS will be a sophisticated Boeing 777.

Oops! I hv a simple answer. PLC works as a PC and a DCS is a server. Depends on the size of inputs one wants to workon and where to use these inpurs as outputs. Mostly larger organizations with total automation preferes DCS where in smaller organizations with semi automatic controls at user level adjustable features incorporated in PLC.

Regardinf inside electronics better google yourself......

Plc is a super pc, specialized for processing digital and analog data

A pc , even the pc of today are not able to be competitive with a powerfull programmable logic controller when the task is to control physical input outputs.

A PC , if you compare with medecine is a generalist; plc is a specialist.it has been built to execute some chosen function and it does it well.

A dcs , distributed control system can be composed of plc's and it is true to say that big organisations prefer dcs.

I know both systems; I actually work in a refinery having DCS;

What I observe is that a dcs structure can be more easier to revamp because it uses a lot of space, while plc use to be compact, and if you install new equipments in a running plant, you may work with more security when you have a DCS because is everything big, the panels are big, with few things inside, while plc's racks use to be mounted in small panels.

I was using very big plc's of Brand AUTOMATE 40AE, RELIANCE ELECTRIC, with hot stand by, coaxial cable; speed of 800.000 kbytes in the year 1987, very fast, with a watchdog at 100 milliseconds for thousands of I/O distributed in a 2 km radius area.

I was loving that devices, because they were able to execute anything.

Sistema Automation , a brazilian company had bought the license to built them in Brazil and had migrated the programmer into a pc compatible.

The programming software was provided with commentaries. accessible in monitoring mode or on_line running.

In that time DCS were already wanted by some customers because of their characteristics, but some customers , in the oil industry prefered PLC with supervisory systems in PC;

Dear Sir, Thanks for the correction to my clarification. Sharing helps increasing knowledge base...thanks to globalspec for making it a platform on Engineering...

Regards,

JK

Jayakrishna,

Most of the people have expressed their views about PLC & DCS systems. Let me present my view.

Now a days, PLC & DCS systems have can handle all functions?? - The answer is slightly NO.

Even today, PLCs are more preferred in Machine control applications, Motion Control applications where scan time should be very low and a very fast response is needed.

DCS is a control system which is used for Continuous / Batch Industries automation where there is a need for complex control loops implementation, advanced process control algorithms, sequence of events etc. DCS is also used where high level of redundancy / security / fault diagnostic features are needed.

Now a days, DCS systems are sometimes assisted by PLCs to take care of small part of the process where a large amount of DI/DOs are needed to be controlled.

Consider a refinery process. In refinery / Oil & Gas, you have highly critical & complex loops. These can be better controlled by DCS systems. However, in refineries, Safety is a major concern and hence safety shutdown systems are better handled by TMR (Triple Modular Redundancy) PLC s from Hima / Triconex systems.

I hope everyone would agree with my view.

Thanks

Sastry MRKS

I believe a significant difference between PLCs and a DCS is that a DCS can incorporate analog inputs and outputs whereas PLCs are essentially digital.

I have worked mostly in plants using PLCs. I think that nowadays the difference between PLC based automation and DCS are becoming just an idea of automation structure. True, there are DCS systems built for special industries that have been proven to work great and no need to re-invent the wheel. But what I am saying is, a DCS is a distributed amount of little brains with I/O. I have built control systems where I used PLC in the same fashion (but we did not call it DCS system). We have many huge I/O cabinets, also called remote I/O, and depending on the area there is a PLC to control the process involved. It is not correct when you say PLCs are just digital. I have worked with all kinds of analog signals, like thermocouples, flow meters, etc. Also worked with high pulse counters (rpm readers), and others. I think PLCs nowadays can be as sophisticated and maybe even a lot more broad than before. In conclusion, DCS is simply the idea of how you layout your system. You have only one PLC as the main brain of the system, then it is not a DCS, but if you have many plcs controling I/Os then you could say it is a DCS system, in escence. Hope this helps. Thanks!

Not anymore. Nowdays, PLCs are used in analogic control too.

Dear Sir,

Thanks for you detailed explaination and is more useful to broaden my knowledge on the subject.

Best Regards,

JK