difference between

Hi, could someone explain the exact difference between plcs,d.c.s and scada.i am very much confusd in this regard.
thanks in advance.
raju.

raju, this topic has been on the site before so a search could have lead you to more extensive information.

Your question is one that many programmers asks. Here is a very short list.

1. First of all, one can see less differences in those systems today. In the old days there where manyer differences but every days now they come closer to being variations of the same thing.

I will start with the oldest one, the DCS.

It stand for Distributed Control System. They where fully "closed" propretairy systems. The communication protocols where not for the open market. Therer architectures where "distributed". This means that there was a "central" command system and many distributed sub-systems, often able to operate on there own. A Sub-system could receive its "Set-points" from the central system and manage its own PID loops but the central system could do it also. They where VERY expensives and one would almost NEVER see them in a small enterprise. The central system would handle logic and data gathering.

The PLC stand for ...

They where first introduce to replace relays. So they where central with "home run" wires going to the rest of the plants systems. Imagine a BIG relay panel becoming a SMALL panel where the wire logic could be modifyed without touching a single wire..

The SCADA.

This was the answer to fight the monopoly of the major DCS vendors. These systems would use the PLCs for there main source of I/Os and since they where running inside a PC, they could manage the DATAs. This really hurt the DCS industries. One could setup something closely related to a BIG DCS but in a smaller scale and a much smaller price. Wonderware was the one that popularized the SCADA in the industrie just like Lotus did the PC in the offices.

Today, one can find DCS with drivers to connect to other brands and SCADAs having realtime kernels (SoftPLC) and PLC with inbeded PC modules running Web Servers... Its almost look-alikes.

Tomorrow, they will be the same, in fact they already almost are.

My answer is far from complete and many will argues some facts and add to it but basicaly this is it.

DCS= Expensive, Close systems and protocols
SCADA= Cheap, Open systems and many drivers to link to others
PLC= Relay logic + functions.