Paul B. wrote:
>   If the input follows the alternate states of the output, it is
> connected to a tri-state, if it doesn't it is fixed.  I suspect that is
> what was asked for.

There is the danger that you will inject a signal in the device under test
Speed will probably also not be great (though better than with an a/d
conversion)

the two resistor ladder with st inputs sounds like a better idea

another option , a 2 comparator window to two input pins

peter