Am 19.05.2010 15:29, schrieb scott:
>> 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.

... except of course in data buses that operate with one dominant and 
one recessive symbol (e.g. CAN), in which sending the recessive symbol 
is perfectly equivalent to not sending anything - which allows for some 
neat tricks to deal with collisions (i.e. the situation where two 
devices start sending simultaneously).

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