18 Dec
2005
18 Dec
'05
11:03 p.m.
I think the short answer is "second
sources"--you had at least 4 big
players, TI, National, Moto and Fairchild all producing 7400-series logic.
Some of the earlier TTL (Moto 400/500-series) had mid-line (pins 4 and10)
As do some 74xx parts (7490, for exmaple). And IIRC some of the later
high-speed (ALS, F, etc) parts went hack to middle power pins because it
gave shorted connections to the decoupling capacitor.
power supplies, which turned out to be not as
convienient for PCB layout.
And, although it's largely forgotten, 7400 TTL shares a fair number of
pinouts with the older DTL circuits.
By the time LSTTL was out, everyone had pretty much standardized on the
74xx line.
Juat watch out for some 74Hxx numbers (used in 1970's HP calculators,
maybe minicomputers). Those can have pinouts very different from the 74xx
device of the same number. There are even a few 74xx and 74LSxx parts
with different pinouts (the 7451 being the most obvious example).
-tony