DCS vs. PLC

One is a distributed control system. The other is a programmable logic controller.

Strictly a PLC is a programmable logico Controller, device able to make logicas and sequential functions, from programmings and configuiraciones according to the noncomplex required tasks, generally monitoreo and control of starting and unemployment of machinery. A DCS is a system made up of subsystems for complex applications of process control including advanced control and the own functions of a PLC. We can then deduce that for the process control the PLC and the DCS are the ends, the minimum for the PLC and the maximum one for the DCS. The tecnologic development has allowed to find an intermediate point between these limits, is to say devices that combine kindness of both technologies including those of PC´s in order to obtain a compact product, robust, with high availability, for universal applications and to low costs, same that denominates PA´s. Generally we used bad the terminos and we confused PLC´s with PA´s we could say that at the present time we have 3 significant technologies to the control of machinery and processes: PLC´s for noncomplex discreet applications; PA´s for everything aplicaion (noncomplex, medium and complex) and DCS For applications of medium to complex. I have had the opportunity to be present at expert connotados exhibitions of international using term PLC for everything which me paraece outside order and he favors to the confusion. I hope that my commentaries contribute and solicit to the people convinced with the exposed thing in spreading it to all the communities related to the automatization specialty in order to consensuar the terms since sometimes. greetings

PLC specific to cetain programme. DCL is not specific to any programme. otherwise can be more than one PLC is termed as DCS.

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That is the story that exists since the year 1979 aproximatively.

There are two main reasons to exist.

You have basically two kinds of signals:the digital and the analog signals.

Some constructors have choosen to built machines with high capacity to treat analog signals and a small capacity to treat digital signals.

The constructors that have choosen to built machines with capacity to treat digital signals, have only increased the analog signals , with the time passing.

The DCS were compact systems with terminal . The PLC were working blind because they where conected to electric logics builts on relays.

You just needed a terminal when programming.

For many years, for some applications requiring high speed execution ,like interlock because of dangerous operation like in boilers, the DCS had to be interconnected to a PLC.The PLC had always be able to work alone, when connected to computers with the supervisory system.

In 1979 , when you had only processors with 4 bits, it was a highly complicated operation to build systems with 16 bits.

As the 32 bits Motorola processor arrived, the situation was under control.

and it became possible to do what we still have today, PLC or AUTOMAX for 16000 digital I/0 resolution under milliseconds, at a high distance.

Today, the configurations are like twin brothers.

Some DCS are unique, because they are able to control the speed of numerous synchrone motors like in paper producing machine.

All systems are not able to be installed and set up easily

That is one of the point to differentiate one of the other.

What is new today, is that the instrument used used in the field to meter for exemple the pressure, are intelligent, it means they have the capability to execute locally, what was in the previous time executed centrally in the plc or the supervisory system, for example a PID, or a calculation of a density, something that has changed everything.

Another very important difference between the plc and the dcs was the language used for the programmation.

The people that manufactured DCS's were computer oriented , and the people that constructed plc were electricians.

The languages used for programing PLC were comparatively exactly the same that an electric diagram. I have never learnt to program PLC, that was a natural extension of what I knew. In 1987, working with the 40ae of Reliance, it was very easy to built any imaginable configuration because the machine was able to do anything in time., with powerfull block functions or with the ladder language. The DCS automax uses even a special "basic" program working with tics of a fraction of a millisecond.

Today the configuration are definitely remote systems based on small plc's talking together, with all interfaces on a fieldbus cable starting from the field and going up on internet if necessary for plant supervised far away.

Distributed Control System technique is found in Computers . Different components are located seperately and they are interfaced into one .

But in PLc what we see is all the components are interfaced into one unit and they are compact .

Rest of the things are same in both

In the beginning period of everything, the true thing was that american constructors had chosen the plc philosophy that had the advantage that the language used was based on ladder,the same than the language used by the electricians.It has been incremented the time passing with programming like instructions.

The DCS were too slow to resolve digital instructions, but they were customized for processes, thery had a scanning time of two , three seconds, and that is too long for digital instructions.

For many applications it was necessary to add a plc having a scanning time lesser than milliseconds.

The first plc had been constructed by modicon it seems in 1968, but the big business had started after 1978,and the plc's had ferrite memory. The RAM memory appeared just after that.

I know only few things about what was called DCS because I have worked in the industry manufacturing and selling PLC's and computarized supervisory systems.

Another kind of system had also existed instead of local classic I/O's, it was remote terminal units connected to computers, it was used in the energy distribution and permitted to poll information through telephone lines.

The brand DCS was used by very big manufacturers masters of the big markets, but some powerful PLC's are also called DCS, because they are so powerfull that there is a need to call them so, because the PLC has the reputation to be something inferior, and that is a wrong reason.

PLC 's today, at least some brands, are nodes of fieldbus and permit very flexible configurations and are so secure that they are approved for the nuclear industry and the defense.

The approach of every society is depending of what is known. at the given moment

The existing process to control are infinite, nobody in one life can have seen so many things that exist.

At any one of the place where people are living, people try to control their process the best way they can.

Before anything had been possible, everything was done with relays, simple contacts or timed contacts; or counters.etc. A temperatures, pressures etc had to be taken in account.Pneumatic was used for analog instrumentation in dangerous areas.Then tha analogic electronic had appeared, then the HARD protocol, then the Fieldbus.

Man intelligence has been used to provide solutions.

Before any processor had been disponible, people had to find solutions.and ask inventors to work.

In 1974, the programmable process were commanded with matrix and diodes.

In 1968 a device to be used by GM had been invented, I do not know wether it had analog I/O, but the thing had been growing in all directions. And the word programmable logic controller has appeared, because it was explaining what is was doing.

PLC's with 16000 I/O or even with 18000 I/O exist . You cannot tell that plc's is small and DCS is big.

The PLC's RELIANCE ELECTRIC had a CPU of very high capacity in the master rack.You could interconnect 32 racks , make hot-stand by. The system called automate had a CPU able to control 8000 I/O; the system called DCS automax was able to control 16000 I/O.with the same architecture, and some identic cards.

For other constructors, the concept of DCS was different.

The very important argument to consider was the price.

You could build a system with PLC's and computers , that was complete and making the job for just a part the money you had to put for a system called DCS with nice control desks , but not able to work alone, if you a had boiler.

Today, the solutions depend of what the customer prefers, because everything exists.

The only exigence is the reliability, in all situations

PLC - Programmable Logic Controller

This will control the simple process automation or a machine like CNC. PLC is a Sequential controller.IT's Control is based on IO's and its program and parameters can be edited. It can find the Failure and can't record and do furthur.

DCS - Distributed Control System

DCS is the system which controls the no. of PLC controllers that are connected through LAN and that can be monitored through DCS itself. It is process controller. This parameters can'n be edited by individual. DCS can record the event failures and it can give the soft as well as hard copy of the data.

Giving you an example that I/O card of Allen Bradley PLC-5 family can be replaced by the I/O cards of Honeywell C200 controllers and vise versa. Means at hardware level both are same even CPUs. The difference is Software. DCS is Software oriented and PLC is Hardware oriented. Automators