From: Pete Turnbull <>
Date: Wed Apr 22 18:35:45 1998

On Apr 22, 23:01, Tony Duell wrote:

> > I thought of that too. Then you might be able to do it with an AOI
> Oh, AOIs are fun, but not general enough for this...

You tend to need more than one small package to anything very useful

> > but I'd use a 156, which is a demultiplexer/decoder but with open-collector
> > outputs, which I'd wire-AND.
> Good guess. What you need is a fixed AND matrix to get all the possible
> product terms and then a programmable OR matrix to combine the right ones
> to form the desired output.
> That's _exactly_ what a PROM is, of course.
> It's also what a multiplexer is.

A neat solution. Of course, anything you can do with minterms can also be done
with maxterms.

> What worries me is that the above seems not the taught any more. And
> people don't seem to have grown up fiddling with TTL chips (or
> equivalent).

Here, 1st Year CompScis do a series of practical problem exercises with TTL,
one of which ends up building a multiplexer from basic gates. The next (or
maybe next but one) involves something that's complex to do with normal minterm
techniques, and often involves using a multiplexer as a building block.

Pete						Peter Turnbull
						Dept. of Computer Science
						University of York
Received on Wed Apr 22 1998 - 18:35:45 BST

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