On 17/07/2020 17:09, Dennis Lee Bieber wrote:
> 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

I know,but that is not what is at issue. Those chips are NOT TTL,
although they are guaranteed to work with ttl level signals, so they do
not *redefine* the TTL spec.

>
>  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.
>

You can, but not many people bother

>> 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.
>
>


--
If I had all the money I've spent on drink...
..I'd spend it on drink.

Sir Henry (at Rawlinson's End)

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