AND-OR-Invert, was Re: Semi-OT: IDE & SATA to USB "dongles"

[William Maddox]

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.

--Bill