On Fri, 17 Jul 2020 14:40:35 +0100, The Natural Philosopher
 declaimed the following:

>FWIW there is somewhere the original TTL spec where 0 is less than some
>voltage and 1 is more than some voltage with in between being 'here be
>dragons'
>
>"A TTL input signal is defined as "low" when between 0 V and 0.8 V with
>respect to the ground terminal, and "high" when between 2 V and VCC (5 V)"
>

 Which is true for genuine Transistor-Transistor Logic (74xxx series)
chips... CMOS chips nominally (I believe there is a 74xxx series of CMOS
that may actually use TTL drive levels) use

Low 0V .. 33% VCC
High 66% VCC .. VCC

 Note that, in going from CMOS /to/ TTL, a 3.3V CMOS High still falls
into the region of a 5V TTL High. You'd need a pretty long cable run to
have a 3.3V High degrade to below the TTL High threshold (and a lot of
leakage to bring a 0V driven Low above the TTL Low threshold). Can't speak
for the other direction -- having something that drops 5V High to 3.3V CMOS
might result in dropping 3.3V High below the CMOS threshold. If it is a
proportional drop, maybe... 3.3V is 66% of 5V, and 66% of 3.3V is the
threshold for CMOS High...

 I don't believe one can even buy true TTL chips anymore. They are
replaced with 74HCxxx series CMOS as I recall.

>Whether or not what you are discussing conforms to that, I do not know.
>Just my 2 pennoth..

Coated zinc, I presume -- as copper is too valuable for those coins.


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 Wulfraed                 Dennis Lee Bieber         AF6VN
 wlfraed@ix.netcom.com    http://wlfraed.microdiversity.freeddns.org/

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