mynick wrote:
On Feb 1, 12:47 pm, Paul wrote:
mynick wrote:

thanks for great explanation
so in modern mobos the on switch grounds a pulled up line, straight to
southbridge
(Possibly there is a Schmitt trigger/inverter in between the two)
+5VSB +5VSB
| |
Pullup Pullup
Resistor Resistor
| PS_ON# |
PWR X----+---- Motherboard logic ---- Open -------------------+- ...
Collector (to
GND X----+ Driver ATX +
| supply) |
(Front GMD GND
Panel
Switch)

Using an ohmmeter, between PWR and PS_ON#, doesn't tell you anything.
There is a silicon chip in the way.

Paul

Thanks
checked no shorts and it turned out empty bios batt prevented power
on,
however now there is no display with onboard or addon graphics
although rest seems ok(fans spinning,cpu warm,keyboard flashes)

Then you'd simplify the setup a bit, and retest.

At this point, are you hearing any BIOS beep pattern or not ?

Possible answers a

1) No beeps - implies processor can't run very much BIOS code.
RAM is bad in the low memory area. Processor 12V is unplugged.
BIOS code got erased by user accident.
2) One beep - implies processor has completed POST and is booting
the hard drive. Check for hard drive light flashing.
If the flashing "looks like Windows is booting normally",
it could be that only the video card output signal is
defective.
3) Multiple beeps - now we know the BIOS has detected some kind of
subsystem failure. Either video or RAM. If you
have two video devices, remove one and retest.
If you have two sticks of RAM, retest one stick
in place at a time.

When it comes to the display, your computer has multiple video connectors.
The output signal could be on a different connector, than the one
you're currently connected to. Alternately, the system is using the
built-in video, and wasn't able to detect the add-in video card.
Moving the monitor cable from connector to connector,
may be sufficient to sort that out.

You can remove the video card, and connect the monitor to the motherboard video.

If still no luck, power off the system, and use the Clear CMOS jumper.
This is on the theory, that the default BIOS settings include
enabling the built-in video. The system must be powered off, because
some motherboards get damaged, if you "Clear CMOS" with any power present.
I always unplug the computer, when doing this. After Clear CMOS,
if you can see a video screen, you'll need to re-enter the BIOS,
select the boot device and so on.

The fact that your keyboard flashes, implies you're not in (1) above.
So now it's a question of how many beeps you hear, is the hard
drive LED flashing a normal boot sequence, and so on. If could be
just a video issue of some sort.

Paul