Path: utzoo!utgpu!news-server.csri.toronto.edu!clyde.concordia.ca!uunet!comix!jeffl
From: jeffl@comix.UUCP (Jeff Liebermann)
Newsgroups: comp.unix.xenix
Subject: Re: Systems reboots after turning VGA monitor on/off/on
Summary: Monitor reboots computer
Message-ID: <56@comix.UUCP>
Date: 17 Jun 90 01:52:17 GMT
References: <4525@thebes.Thalatta.COM>
Organization: COmmittee to Maintain Independent Xenix
Lines: 41

In article <4525@thebes.Thalatta.COM>, campbell@Thalatta.COM (Bill Campbell) writes:
> If I turn the monitor off on my system, leave it off for a while,
> then turn it back on the computer reboots :-(

It's your power supply (methinks).  A VGA monitor draws
mucho amps (1.2A normal, 2.0 surge when the tube filament
warms up).  If you have a relatively high impedance power
line (long power cord, crummy connections, defective
surge protector, wacko UPS, dirty fuse connections) on
the 117vac, the power line voltage will drop momentarily
when your monitor gets turned on.  The reason you have
to wait a while with the monitor power OFF is to let the
filament cool so the inrush current is sky high when
you turn it on.  

You can create the same effect with a high wattage desk
lamp, laser printer, or terminal that sucks lots of power
when you turn it on.

If you want to monitor the effect, put a scope on P8-1 on
the power supply DC output connector.  The is the "power
good" output from the power supply.  If this line drops
low (0 vdc) from its normal hi (5 vdc) even momentarily
( < 20msec), the mother board will reset and the machine
will reboot.  Incidentally, the "power good" line was
necessary on the original IBM PC with 16k chips which used
-5vdc and -12vdc voltages as well as +5vdc.  If power
was applied in the wrong order, or one of these voltages
were missing, the chips would go POOF.  Today's dynamic
RAM are all run by +5vdc only and the "power good" line
could easily be dispensed with.  I usually cut this wire
from the power supply and pull the P8-1 connection on the
mother board high (thru 3.3kohms).  As a result, I've been
able to tolerate 300msec dropout without reboots or data
loss.

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