I ordered up some 74LS245 chips from a "trusted" ebay seller and received 74LS645 chips instead. Reacting with knee-jerk I sent him a message saying he sent me the wrong part but he gets back to me and says they are the same. So I go and look it up. They are the same. Looking at the spec sheet they are nearly identical except the '645 has slightly longer maximum PD (15ns vs 12ns).

Further research shows that the '245 vs the '645 is not the only example of this. There are many identical parts in the series with exactly the same function and specs. Anyone have any insight into why this is? Could it really be because TI wants to sell 74LS245s that fall a tiny bit outside the PD specification?

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Bill

Ah -- the '645! An oddity now, but a miracle at the time. I recall my excitement procuring some of these! The vendor was proud to stock them, describing them as "the number one hard-to-get part right now." The 74LS244 was already a thing, but a single chip that could serve as an octal transceiver was a non-trivial innovation!

So, yes, the demand was definitely there! Therefore it seems plausible to me that TI would rush the slightly half-baked '645 to market before the '245 was ready for prime time.

But I seem to recall that the 645's deficiency had to do with the spec for the input levels, resulting in slightly less noise immunity. Now it seems I may've mis-remembered. Or maybe it was that and tPD as well.

-- Jeff

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In 1988 my 65C02 got six new registers and 44 new full-speed instructions!
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No, there was that too. Vil for the '645 is 0.6V, Vil for the '245 is o.8V. The both have a Vih of 7V.

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Bill