PLC Circuit

Ok, now that I have a semi good understanding of how PLC's work, my next question is where might I get a general circuit of exactly what one looks like. i.e. A circuit that I could build and simulate in PSpice. Also, thank to everyone who repsonded to my previous post.

>Hi

Try www.thelearningpit.com I have tried their software and its quite good and very cheap..

paul

Give up now. You won't find the circuit diagram for a PLC. Andi it wouldn't help you if you did.

1) PLCs circuit diagrams are proprietary to the company that manufactures them. They are also patented, so I suppose you could find the diagram at the patent office.

2) The circuit diagram is only half the show. PLCs also have firmware flashed onto their chips that allow them to work their magic (I'm sure that Terry will ream me for referring to what PLCs do as "magic").

Not to diminish your thirst for knowledge, but you don't need to know the circuit diagram for an Intel chip to use, or program, a PC. By the same token, you don't need to understand how the magic works to make the magic do your bidding.

But simulating the circuit working in PSpice isn't going to teach you what you need to know. At this point, you need to get one and USE it. ABUSE it. Push it to it's limits, and beyond. Find things that it can't do, then figure out how to do them anyway. That's where the true learning lies.

Some PLC manufactureres show an equivalent circuit in their manuals, that would probably suffice for a PSpice model, but they only go as far as the signal converter, not inot the backplane.

Why do you want to model this anyway? The discrete I/O is simple - voltage or no voltage, polarity right, and for DC sometimnes sink or source (which is really a polarity question anyway). Ditto on analog - right signal, right polarity, you are done.

I can't figure out what useful info you would get from the circuit model!

Build (and simulate) a PLC in PSpice???

There's one little problem with trying to do that. It's the letter "P," which means "Programmable." PSpice won't simulate everything a PLC can do, wire-for-wire, without a ton of help from you. That's because the inputs and outputs on a PLC don't follow a set pattern (logic table), until you tell them to.

You may be able to simulate a **SIMPLE** PLC program in PSpice. The key is the letter "L" for "Logic." If you're using discrete I/O (switches and lamps), and not too many timers or math functions it is possible to create a circuit using AND/OR/NOT components that will simulate a known PLC program. This would be similar to simulating a PLD. But, you won't have down-to-the-microsecond timing. So don't pay much attention to the time axis of your system response graphs. When I was building logic circuits as a student, I actually did something similar. For me it was faster to develop ladder logic and replace it with gates later.

If you're trying to simulate analog I/O, good luck. You'll have to become comfortable with the gain, offset, addition, multiplication, integral, and other functions in PSpice.

AK