Some of you may have read my answer here to detect when two digital signals are either high or low, i.e. not both tri-state.
One method uses window comparators the other opto-couplers to detect when the line is actually driven. The first seems overly complex and I'm not overly keen on the LED currents in the second. Whatever the circuit is, it should not adversely affect the original signal.
I have been puzzling with this since then trying to come up with some simple discrete solution to this riddle, but I can't seem to come up with a discrete circuit that does not interfere with itself, that is, the low detector trips the high detector, or the high detector feeds current to the low one.
EDIT: To clarify, logic is one signal in, one output TRUE if input is floating.
Does anyone know how, or can figure out how, to do this with minimal components.