scott wrote:
>> 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.

OK, so if I'm understanding this right... The 74126 (go look it up) 
contains 4 "buffers" - gates who's output is logically equal to their 
input. But each gate also has an "enable" pin. When the enable pin is 
high, the gate works like normal. When the enable pin goes low, it's 
like the output pin isn't connected to anything any more.

And it seems that this allows you to connect several outputs together, 
forming a kind of wired-OR configuration, provided that at all times 
only one gate is "enabled".

Is that right?

-- 
http://blog.orphi.me.uk/
http://www.zazzle.com/MathematicalOrchid*

Post a reply to this message