>> 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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