How does PC power-on work ?

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Apparently PCs often have problems if left off for weeks?
Understandably the battery/cell becomes discharged.

But I think 'dry joints' also develop in some of the many connections.
Once my PC failed to run after it had been out of use for 6 weeks, and
eventually I determined that the 'Start switch' had become a dry-joint.

Now I've got the opposite problem on another PC.
After the previous owner had not used it for 20 months it failed to power up.
IIRC the CPU fan didn't start.
After I left it on-power for 24 hrs, while I was getting some 'sevisol'
[aerosol for dry-joints] it recovered.

I've been using it for 4 months: 24 hrs a day for 4 days a week.
But when I came back to it after 3 days off, the CPU-fam started immediately the
PC got power, and before the 'Start switch' was activated.
Apparently the CPU isn't running, but the IDE connector voltages are OK
[5 and 11.8] and the CDrom/music runs/sounds independently of the CPU.

What is a likely fault cause?

In principle I'd like to know how the 'start' key/circuit/system works.

== TIA.

Re: How does PC power-on work ?

On Dec 15, 1:31=A0pm, wrote:
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Reseat memory, cpu and any add on cards...corroded connections are
usually fixed by reseating....
Check other connections and reseat (power etc) to remove any corrosion
caused connection issues.

Corrosion isn't so much rust as it is tarnishing of the card edge and
plug connectors.....reseating will "scratch" off the corrosion
restoring connection.

My system has pretty much been on for over 4 years now and I haven't
had issues.

Re: How does PC power-on work ? wrote:
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I had a laptop that wouldn't boot.
I checked the CMOS battery and it was low.
I charged the CMOS battery.
Still wouldn't boot.
The clock chip seemed to be counting.
I reset the cmos help.
I removed the cmos battery for a few minutes
then put it back in.  The computer booted just fine
and has worked ever since.

The battery was soldered in on the bottom of the mother
board necessitating complete disassembly of the system
and hooking the parts back up so it would run disassembled.

My hypothesis is that the battery discharged far enough
for the internal state of the cmos chip (or peripherally connected
devices powered by the cmos battery) to be corrupted
in a manner that wouldn't be fixed by resetting the chip.
I don't have any idea why this could happen, but removing
battery definitely fixed it.

I've fixed similar problems in a Magellan GPS by shorting
the backup battery momentarily, but I DO NOT RECOMMEND
the process.

Re: How does PC power-on work ?

["Followup-To:" header set to comp.os.linux.hardware.]
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In desktop computers, a BIOS setting determines whether the computer
starts when power is applied, or you have to press the power button.
The BIOS may also have been set to wake on LAN.

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Another year someone won for letting the spellchecker do the work and
running an ad for a "Chrysler Lesbian". --ronniecat

Re: How does PC power-on work ?

On Wed, 15 Dec 2010 18:31:00 +0000, rearranged some electrons
to say:

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My machine did this same thing yesterday.  Reseating the power connector
header on the motherboard fixed the problem.

Re: How does PC power-on work ?

On ATX machines there is a standby circuit on the M/B, live whenever the
mains are connected, that has a latch circuit contolling the full power
application from the P/S.  If the power supply has problems supplying
the standby circuit the machine will not power up. Sometimes you can see
the indicator led flashing when the standby power is flaky.
Once the standby circuit power is stable pressing the power button on
the front of the machine sets the latch and the power supply brings the
rest of the rails up.
The front power button effectively shorts 2 pins in the I/O control
connector set.  Corrision anywhere in the front power button path can
cause the normal power up process to fail.  Test method is to pull the
connectors off the 2 pins and short them with a screwdriver blade or
some other conductor.
I had a DELL GX260 that took about an hour to activate the standby
every time I connected mains.  This went on for 3 or 4 years until the
supply finally failed.

Re: How does PC power-on work ? wrote:
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The power control is implemented partially in the SuperI/O and
partially in the Southbridge. The reason for that path and those
chips, is to include "wake events", so certain waking events
can turn on the soft power (PS_ON#) control.

As Bill says, one way of getting immediate power on, is a setting
in the BIOS that is there for that purpose ("Restore On
AC Power Loss"). But in your case, you may not have changed
the BIOS setting. And if the BIOS settings were randoonly
corrupted, there is a checksum that the BIOS can use, to
detect a percentage of those corrupt bit patterns.

The other way of causing immediate power up, is stressing
the Southbridge. The Southbridge is a multi-rail chip. For
example, the RTC and CMOS RAM, sit in the "CMOS Well", a
chunk of circuitry powered by the CMOS battery (or by
3VSB when the ATX supply makes +5VSB available). The well
is isolated with transmission gates, to prevent backfeed
current flows. That is there, to prevent the CMOS battery,
from being used to power the entire computer (wearing
out the battery in short order).

The CMOS Well may also have some of the logic gates used for
turning on the power.

Now, I've had the following happen to me, on a 440BX. One
day, I was inside the computer, and half unseated the IDE
cable. Half the pins were still making contact, and the
other half were not. Upon switching on the power at the
back, the computer started immediately. After firmly seating
the cable again, with the power off, it operated normally

It could be, that something that plugs into the Southbridge,
is shorting or putting an abnormal load on some logic
signals of the Southbridge.

The fact the processor can't start, could be related. If
the Southbridge is "under attack", then it isn't likely
that a probe to the BIOS interface is going to work.
Then, the processor may be ready to run, but "bus faults"
as soon as it tries to read the BIOS chip. And then it
can't start.

Double check your cabling. Reseat cables. Do it with the
power off. Then, try again.


Re (2): How does PC power-on work ?


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Thanks for the in-depth info, which I've filed.
I didn't want to expose all the complications initially:-
 I'm operating in an african/3rd-world environment.
 I move the PSU & 2 IDEs between 2 PC [locations] every week;
 I forgot to remove the mains, before I unplugged the PSU from PC2,
 and when PC1 'got' the PSU it behaved strangely - as described.
 PC1's CMOS battery is 'down', but apart from having to set the
 RTC and boot-sequence every week, I don't know the meaning of
 most of the BIOS settings.
 When I came back to PC2's location [where I have inet access]
 I was able to confirm that the IDEs [which are most important]
 were still OK, but PC2 now has strange, apparently unrelated faults:
 hda,b are undetected; so I had to 'set' the BIOS and /etc/fstab
 to continue via hdc.
 I'm taking a scrap but tested MOBO and PSU to location PC1,
 so that I can process my inet-fetched material on the 'movable'
 IDEs, and some servisol [engineer in a spray-can] to check PC1.
 BTW, I had already tried removing the CMOS battery/cell.
 I guess the BIOS just goes to workable defaults when there no
 CMOS-power, since this worked OK for months.
 == TIA.

Re: Re (2): How does PC power-on work ? wrote:
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It could be, that something got damaged, when it was unplugged with
the power still applied.

As for the CMOS battery, yes, in some cases, the defaults reloaded
by the BIOS, while running off +5VSB instead, are useful for booting.
But I have several computers here, that when the battery is flat, and
I power them for the first time, it means a few minutes work turning
stuff back on in the BIOS screen. Generally, the "dumber" the PC
(like a Dell), the better the odds the BIOS default values will work.


Re: Re (2): How does PC power-on work ?

In wrote:

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This may be the problem. The PSU connectors are rated for
a relatively low number of insertion cycles, typically 50.
It gets worse with heat/dust/humidity and when you insert
manually, i.e. not perfectly aligned. The faults in your
drives may just be bad power connections that intermittedly
cut power to the drives while they are writing.

Arno Wagner, Dr. sc. techn., Dipl. Inform., CISSP -- Email:
GnuPG:  ID: 1E25338F  FP: 0C30 5782 9D93 F785 E79C  0296 797F 6B50 1E25 338F
Cuddly UI's are the manifestation of wishful thinking. -- Dylan Evans

Re: How does PC power-on work ?

On Wed, 15 Dec 2010 18:31:00 +0000 (UTC), wrote:

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YOu left on for 24 hours while the CPU fan wasn't spinning, while you
got a lubricant?  I think you should have turned it off until you got
the lubricant.  Isn't that obvious?  You're lucky you didn't burn it

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