On Sat, 15 Aug 2009 12:37:30 +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. 
>
>I'm told 10% resistors show a curios bimodel resistance distribution 
>too. ;-)
>

It would not surprise me but I could not say for sure,

>BTW... What tests are there *other* than functional ones?!
>

Well it was quite a while ago and I assume that they will have changed.
IIRC: Rise and fall times of the output, propagation delay of the gates, input
and output voltage range, operational temperature and humidly. There was a
centrifuge test up to 200 g for mil spec and a test where the ICs were dipped in
liquid helium and tested to see if there were any leaks on the encapsulation.


>> FWIW It was considered an educational thing to allow engineers some
>> time and materials for home jobs.
>
>Indeed. I'm told Cadbury let their employees eat as much chocolate as 
>they want. Not so much for educational purposes, but because the amount 
>of chocolate that one employee can eat without being horrifyingly sick 
>is insignificant compared to the amount of chocolate they make per day...
>

I believe that too.

>>>> 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. 
>
>What voltage does CMOS use anyway? IIRC, TTL can be driven with a 
>triplet of 1.5 V batteries.
>

Why don't you look it up as I can't remember,

>>> OK. I was going from memory there.
>> 
>> I guessed as much.
>
>It's been a while since I did all this.
>
>> IIRC I think the specs said that TTL inputs would be at a logical 1 if there was
>> no connection. (Internal resistor to Vcc) 
>
>Oh.
>
>Well that would be a problem then. I was assuming it would be logic 0. o_O
>

:-)

>> 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.
>
>So connecting to the negative rail should give me 0, and connecting to 
>the positive rail should give 1?

Yes, by George you've got it!

:)
-- 

Regards
     Stephen

Post a reply to this message