Do you know analogue electronics? not just digital or programming?

*** KEY: Voltage sources have low impedance - so you can use this characteristic ***

Circuit
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Use two equal value resistors (say 10k) in series across the supply (VDD and GND). I assume VDD = 5v for TTL logic.

{The logic probe point is the mid point of the resistors}
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Logic States
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Hi-Z or not Connected
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When the probe is not connected to any thing or connected to Hi-Z the probe/mid point voltage (referenced to gnd) will be 1/2VDD.

Logic 0
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When the probe is connected to Logic 0, 0v (GND) the mid point voltage will shift to 0v becuase the GND impedance is much much lower than 10k.

Logic 1
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When the probe is connected to Logic 1, 5v (VDD) the mid point voltage will shift to 5v becuase VDD impedance is ALSO much much lower than 10k.

Measurement
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How to measure the mid point which may have analogue state?
You can use either a micro's ADC, or Op-Amp comparators as voltage threshold detectors.

The ADC voltage (proble voltage) will indicate the logic state of the probe point.
If probe voltage >0v and <0.6v then Logic 0
If probe voltage >2.4v and <5.5 then logic 1
If probe voltage >0.6v and <2.4v then Hi-Z
If probe voltage <0 then ERROR
IF probe voltage >5.5v then ERROR

Voltage comparator circuits
http://home.cogeco.ca/~rpaisley4/Comparators.html


Joe