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>> I always thought of it as logic high meaning "connected to the +V
>> rail" and logic low meaning "not connected to anything".
>
> Logic "high" and "low" are just names given to a certain agreed range of
> voltages.
Indeed. You can use any two states you want, so long as it's easy to
distinguish them, and all the components are using the same rules.
> The difference between "not connected to anything" (floating)
> and "logic low" is very important when working with data busses.
There is that...
>> From the looks of things, it seems that actually logic low means
>> "connected to the -V rail". So it's like a bipolar setup...
>
> But don't assume a "logic low" output will be exactly -V, it is only
> guaranteed to be within the range of voltages given for "logic low" in
> the data sheet.
Heh. It gets weirder and weirder...
>> Yes, that's the other fun thing. Depending on which type of gates
>> you're using, they supply about 20 mA. But some of the LEDs I'm
>> looking it draw 40 mA...
>
> What you mean is, they draw *up to* 40 mA?
No idea. The spec sheet just says "forward current".
> How much they actually draw
> depends on what value series resistor you put in there. Using Ohm's law
> and the V-I curve of the LED from the datasheet you can work out exactly
> what resistor size you need for a given current.
Heh. And here I was thinking it's just a case of "if the voltage is less
than X, you don't need a resistor"...
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