Tony Duell ard at p850ug1.demon.co.uk
Sun Dec 18 17:03:15 CST 2005

> 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).


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