Google returned following link as #1 hit for >tristate logic probe<
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If you know how it works you will notice that uses same logic like
the previous link - it creates "tristate" signal if input is neither
zero nor one.
Using comparators as Nigel suggested would give you chance to
easily define "tristate window". The smaller the window, the higher
probe sensitivity. It's because even smaller current leaks would
disturbe the balance of the tip.
You are more than welcome to build it but I am not too convinced
that results would be as reliable as you might expect. More over,
you would have to look for special parts or sacrifice performance.
Maybe it's me but logic probe that cannot handle at least 20MHz
is not worth the trouble (unless you want to use it on something
as slow as LPT port). I didn't use comparators in a long time but
if I remember correctly, most of them used to be usable up to
one or max 5MHz. (on the other hand, things do progress fast so
who knows, your local Radio Shack might have inexpencive parts
that can handle 100MHz).
Here are some simple probes using OPs as comparators:
http://www.swansontec.com/sprobe.htm
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