On Sat, 15 Aug 2009 07:21:00 +0100, Orchid XP v8 <voi### [at] devnull> wrote:

>>> And then I got stumped by the fact that the gates don't appear to do 
>>> what the diagrams suggest they should...
>> 
>> In what way?
>
>The 7400 is a quad 2-input NAND gate. In other words, it's a box with 
>four NAND gates in it.
>
>What's supposed to happen is that you connect pins 1 and 14 to a power 
>source, and the remaining pins are grouped into 3s. Each group is the 
>connections for a single NAND gate. When the inputs aren't connected, 
>the output pin is at logic high. When you connect both of the input pins 
>to the (+) rail, the output should go low.
>
>...except this doesn't seem to happen. I swear to God I built circuits 
>like this when I was a kid, and it worked. But when I tried it as a 
>teenager, it wouldn't work for toffee. No idea why.


Good grief Andrew, 7400s, did you rob a museum? Do you know the power
consumption of TTL compared to CMOS? 
Besides pin 14 being Vcc and pin 7 ground. You cannot assume that inputs will
float high you really should drive them high or low.

-- 

Regards
     Stephen

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