On Sat, 15 Aug 2009 09:10:26 +0100, Orchid XP v8 <voi### [at] devnull> wrote:

 
>> Good grief Andrew, 7400s, did you rob a museum?
>
>Maplin.
>

When I worked for Motorola way back in the mid 70's. Maplin and RS were sold the
ICs that failed all the tests except the functional ones. So I would not trust
the quality very much. 

>IIRC, they charge 21.22p per chip. (Presumably the 0.22p only makes a 
>difference if you buy hundreds of them...)
>

I suppose I was spoilt as I always worked for a company that would let me have
them free. FWIW It was considered an educational thing to allow engineers some
time and materials for home jobs.

>> Do you know the power consumption of TTL compared to CMOS?
>
>TTL has the advantage that it doesn't break if you touch it. ;-)
>

Fair point although I never had much problem with ESD and CMOS as long as you
earth yourself. 

>> Besides pin 14 being Vcc and pin 7 ground.
>
>OK. I was going from memory there.
>

I guessed as much.

>> You cannot assume that inputs will
>> float high you really should drive them high or low.
>
>...?

IIRC I think the specs said that TTL inputs would be at a logical 1 if there was
no connection. (Internal resistor to Vcc) 
This was not the case with CMOS, so I was wrong saying that TTL had to be driven
either high or low. But it is good practice to drive your inputs in case there
are things in your physical circuit that might affect what you think will
happen. Such as dry joints, solder bridges, RF pickup etc.
-- 

Regards
     Stephen

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