In article <4fvjak$ut@beyla.ifi.uio.no>
olee@ifi.uio.no "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: olee@ifi.uio.no
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.
+--/\/\/\--5V
1 L
+-------------) 2 E
| )XOR>----) D
o-----+ +--) )XOR>--+
| | +--) L
| | | E
+--/\/\/\--+----+---+ D
| +--/\/\/\--0V
OSC
o---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