AND-OR-Invert, was Re: Semi-OT: IDE & SATA to USB "dongles"
Roy J. Tellason
rtellason at blazenet.net
Sat Jun 3 06:50:12 CDT 2006
On Saturday 03 June 2006 03:31 am, William Maddox wrote:
> Roy J. Tellason wrote:
> > I've been wondering for a while now what the major use was of those
> > "AND-OR-Invert" gates, since the early TTL stuff I became familiar with
> > first had a bunch of them in the databook...
> > Anybody know?
> This particular configuration of gates comes up frequently in digital
> logic, and combining gates in this way provided more gates with fewer
> packages. The 7451, using 2-input AND and OR gates, was often used as
> a dual 2-to-1 MUX with unencoded select inputs. The inversion of the
> output was not so much a feature as consequence of the fact that an
> inverted output was easier to provide, e.g., NAND and NOR are more
> commonly used than AND and OR for this reason. NAND and NOR are
> also in some sense more universal -- you can compute any boolean
> function with sufficient numbers of either NAND or NOR gates alone, but
> not with AND and OR, which may require the use of inverters as well.
Now that you mention it, that early TTL book didn't have any AND or OR gates
in it, either...
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