Re: gEDA-user: Spooky Action . . at a distance!

[Karel Kulhavy <clock@xxxxxxxxxxxxx>]

On Tue, Jun 07, 2005 at 01:08:32PM -0700, Samuel A. Falvo II wrote:
> OK, I just had the *weirdest* experience of my life.
> 
> Spooky.
> 
> I am working on wiring up the IPL circuit as documented at
> http://www.falvotech.com/projects/kestrel/repo/8k/prints/ipl-port_3.pdf
> .  I plugged power in, and I ran the software on my Linux box to
> program it (wrote it myself; sources coming soon).  I probed around
> with the o'scope, and everything was working.
> 
> Perfectly.
> 
> Shortly afterwards, I noticed that Vss of U301 was not hooked up to
> anything!!  Vdd was connected to +5V, as expected.  And yet, it worked
> fully.

I once debugged a circuit and noticed it somehow slightly damages the
signal going through. After long tinkering I finally found out that
the cause was that the circuit wasn't powered at all.

When powered, it started to work correctly. A popular wisdom says thast
things work better when plugged into wall.

Basically, HC family gates contain protection diodes which allow the
input signal to power the whole circuit. HC gates have also very low power
consumption.

Of course this is a great way to blow the protection diode because it's
only rated at 20mA DC absolute maximu.

> 
> Anyone else ever have something like this happen?  I know this is
> pretty much just a fluke, but I'm wondering *why* it worked?
> 
> Anyway, I hooked Vss back up, and the circuit operation didn't change
> at all.  :-)

This is a way instead of atomic power - design all circuits to work the
same regardless if power is applied or not. Then we save a lot of
electricity :)

CL<