>>> 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. ;-)

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

> 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...

>>> 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.

>> 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?

-- 
http://blog.orphi.me.uk/
http://www.zazzle.com/MathematicalOrchid*

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