Orchid XP v8 schrieb:
>>> 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.

Maybe some other technology? While classic TTL gates (74xx) as well as 
Low Power Schottky variants (74LSxx) would internally pull up 
unconnected inputs, CMOS gates (74HCxx and 74HCTxx) would lack a pull-up 
resistor, causing unconnedted inputs to float erratically.

To work properly, all input pins of a CMOS IC /must/ therefore be 
actively pulled to either VCC or GND (a resistor is not required in that 
case, as CMOS ICs draw virtually no current on static input pins, so you 
can directly connect them to VCC or GND). It is even highly recommended 
to even connect all input pins of unused gates in an IC package.

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