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There's one important thing that everyone forgets: more modern logic families might have the same logic levels, but they can switch much faster than old logic families. If your board is not designed to cope with such fast switching, it will either completely stop working, or will work erratically, once you start swapping out old chips for newer, faster ...


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I'll assume you're talking about 'glue logic' and you're not trying to build a CPU out of logic chips. I think you're better off using popular types of HC CMOS chips since they'll likely still be available in some years. 4000-series CMOS is too slow when operated from low voltage, and LS TTL is becoming rare, has lower fan-in and requires that you use HCT ...


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A few arguments: with HC and HCT, the fan-out (at lower frequencies) is essentially unlimited, for LS it s IIRC 10 (or was it 20)? the HC and HCT families are newer than LS, so my bet would be that they will be around longer than LS HC and HCT use much less current than LS (except maybe for a few gates that switch at a very high frequency), which eases the ...


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Capacitors are used as test loads in CMOS logic because the actual load when connected to other logic (the gates of additional MOSFET transistors) is primarily capacitive.



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