Re: PC Power Supply for IIgs

[mjmahon@aol.com (Michael J. Mahon)]

John E. Becker, Jr wrote:

>My IIgs has a new home within a surplus Compaq ATX case.  I'm using the
>Compaq power supply as well.  Just as anticipated, thermal problems are
>gone, and there are lower EMI and acoustical emissions.  However, when
>switching the power off, before completely de-energizing, the system
>yields about one second of chaos:  random screen characters and a big
>"zzzzzpppppppttt!" sound from the speaker.
>
>Besides being a slight annoyance, is there a possibility of damage to
>hardware (or an accidental hard drive write) resulting from this
>phenomenon?

Changing screen characters and noisy speaker mean a bunch of
random memory accesses, so I'd say, yes, there is a good chance
of something unintended occurring.

Sounds like the problem is that the +5, being very lightly loaded by
the IIgs, is dropping slowly on turnoff, causing digital chaos.

Although one could diagnose this by simply loading the +5 more
heavily, say with a 1 ohm, 25-watt resistor.  That should greatly
reduce or eliminate the problem if it is caused by a slowly falling
+5 supply.  Of course, this is not a satisfactory long-term solution!  ;-)

The best long-term solution would be to use the "power good" signal
from the power supply to release the Apple's reset line.  Then, when
you switch off the power supply, or a power failure occurs, the "power
good" signal will drop and hold the Apple in reset state while the
+5 supply is dropping.

Use a general purpose switching diode like a 1N914 to connect the
"power good" signal to the Apple /RESET line, with the cathode of
the diode connected to the "power good" signal.  This will isolate
the IIgs from the "power good" signal except when it drops, when
it will pull down the IIgs /RESET line.

-michael

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