> 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.  The difference between "not connected to anything" (floating) and 
"logic low" is very important when working with data busses.  If you have 
several devices all with their outputs connected to a data bus, then only 
one device at a time must be driving the data lines to logic low/high, the 
rest must set their outputs to floating.

> 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.

> 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?  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.

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