22 Apr
1998
22 Apr
'98
7:35 p.m.
On Apr 22, 23:01, Tony Duell wrote:
A neat solution. Of course, anything you can do with minterms can also be done
with maxterms.
> I thought of that too. Then you might be able to
do it with an AOI
package,
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. 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