PLC Question

Hi, Can anyone help me, I've just started studying PLCs at college and need to know how PLCs are classified in relation to memory capacity and small, medium and large scale PLCs. Any help would be greatly appreciated!

What is a PLC?

These HNC course questions drive us nuts. Because the terminology that's used is archaic, and ultimately meaningless.

Before you can answer your questions you have to ask "What is a PLC?"

Is a Programmable Limit Switch (along the Seimens LOGO / AB PICO lines and programmed with ladder) a small PLC?

Is a DCS just a large PLC?

Is a desktop PC running Steeplechase a PLC (and if so, what size)?

Once you know what constitutes a PLC, then you might be able to find look up the specs of twenty or so, and then you might see a pattern emerge that you can classify into small, medium, and large.

But even then, it's all relative. The AB SLC is generally thought of as a small-to-medium PLC. But a 64 K SLC 5/05 has more memory than a medium-to-large AB PLC 5/40 (at 50K).


I know that you'll come back with a sob story of how your tutor isn't available and that how you've been handed this assignment with no lecture notes, and that your text doesn't have it. But TRY. Go to the tutor and DEMAND that he show you how to answer these questions. Show him that the questions are derived from a textbook that is no longer in use, and ask him which one will answer the questions. Ask HOW this meaningless terminology is going to help you undertand ANYTHING about PLCs.

You have hired this guy to perform a service for you (i.e., teach). You must insist that he fulfill his end of the bargain. Copying 10-year-old questions is not teaching.

Allen's right

I used this question when I wrote my PLC course, 10 years ago.

Now, with device-level networks, and memory expansions, and
distributed computing, is seems like a meaningless exercise.

The vendors don't help, since they are all trying to make
the latest <fill in size here>-sized PLC obsolete by introducing
their latest slightly-better-than-the-last-model.

Good Luck.

Of course, remember that we're talking academia here, so,
ultimately, the right answer is whatever the professor decides
it is, since there are no absolutes inside the walls of a
college campus.

SET FLAME-GUARD==1

Are you doing a college course on PLC's? or just electronics or something. I would be interested in details if you have any!