3 Jun
2006
3 Jun
'06
12:31 a.m.
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