OT: pc power on

[Erik Trulsson]

On Sat, Jul 28, 2007 at 01:56:13PM -0400, fbsd2 wrote:
> After a power outage my FBSD server does not restart automatically.
> Someone has to push the PC power on button on the front of the case.
> I tried to jumper the motherboard pins the wires from the power on button 
> go to but that did not work. It starts for 3 seconds then goes off.

Of course, since the power button in front of the PC is not just a 'power
on' button, but also a 'power off' button.

The way it works is that when the machine is in a 'soft off' state then the
PSU (Power Supply Unit) constantly delivers a small amount of stand-by power
to the motherboard. When the power button on the front is pressed the
motherboard will detect this and send a signal to the PSU to start running
for real, after which the machine starts.

If you then press the power button when the machine is running  the motherboard
will usually detect this and send a signal to the PSU to power down.
(Although if the machine is fully up and running this action can sometimes
be overridden by software.)

> 
> How do I make the pc boot automatically after the power comes back on????

There is usually an option in the BIOS governing how the machine should
react when power is returned.  There are usually three choices labeled 
'Soft off', 'Last state' and 'Power on' or similar.
Not all BIOSes have such an option, but many do.


Another option if you are not afraid of messing around with electrical stuff
is to simulate the signal the motherboard sends to the PSU telling it to
start.  This can done by connecting pin 14 (PS_ON) and pin 15 (GND) of the
20-pin cable going from the PSU to the motherboard.
(For a 24-pin connector the lines are numbered 16 and 17 instead.)
Do be careful if you try this route though, and don't blame me if
you damage something.



-- 
<Insert your favourite quote here.>
Erik Trulsson
ertr1013 at student.uu.se