Strange grounding problem

Charles Dickman chd at
Wed Sep 27 20:14:34 CDT 2017

On Wed, Sep 27, 2017 at 8:23 PM, Adrian Graham via cctalk
<cctalk at> wrote:

> Schematic for the circuits is here - <> - top circuit is PWRUP and the bottom one is RESET that goes straight to the Z80.

CD4000 logic can't handle inputs pulled above the power pin or below
the ground pin without the possibility of malfunctioning or even
complete failure. Google for "CD4000 parasitic SCR" or "CMOS
latch-up". When power is cycled rapidly, the caps will still have
charge which could cause the latch-up. I did some industrial control
designs with CD4000 and used similar timing circuits, but always
included diodes to prevent inputs from being driven too high or too
low. I would have put diodes across both of the resistors.

Adding your probes may change the currents inside the chip that change
the latch-up behavior. Of course it also only makes sense if the power
is cycled quickly for some value of quickly. Others may have an
explanation, but that was my first thought from looking at the


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