In article <4fvjak$>
  "Ole Christian Kollen Evensen" writes:

> I'm going to build a probe to sense logical "O", "1" or high-impedance state.
> I will have three LED's of different color and some logical gate's etc.
> My problem is how to sense the high-impedance state. Anyone have an idea ?
> Maybe some kind of 'current-detecting' or something. 
> Thanks in advance !
> --
> Ole Christian Evensen
> e-mail:

You can't just measure the node voltage as others have suggested 
because Hi-Z outputs are often connected to pull ups or pull downs.

                        1           L
      +-------------)           2   E
      |             )XOR>----)      D
o-----+          +--)        )XOR>--+
      |          |        +--)      L
      |          |        |         E
      +--/\/\/\--+----+---+         D  
                      |             +--/\/\/\--0V

I think this will work.  When both sides of the input resistor do the 
same, that is, probe connected to Hi-Z (or unconnected) then XOR 1 
output is low and XOR 2 output is clocking.  Both LEDs light.

When probe is connected to a Lo-Z logic high, XOR 1 output is clocking
out of phase with OSC (XOR 1 inverts the clock) so XOR 2 output is 
High and lower LED lights.

When probe is connected to Lo-Z logic low, XOR 1 output is clocking in
phase with OSC (XOR 1 passes the clock) so XOR 2 output is Low and 
the upper LED lights.

OSC may be made using spare XORs from a quad package and a couple of 
passives.  Don't forget the OV connection to the circuit under test.            
If the oscillator runs at around 10Hz, the lights will flash when Hi-Z
which will remind you to switch it off.

Beware that this probe will drive a Hi-Z node up and down and may 
thus have some effect on the following circuit.

Keith Wootten

Date: Wed, 21 Feb 96 22:55:19 GMT

Original Subject: Re: logic probe

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