On Jan 31, 10:46*am, Paul wrote:
mynick wrote:
On Jan 31, 5:20 pm, Adrian C wrote:
On 31/01/2011 15:16, mynick wrote:

What you should get between those pins of *a good PC mobo when you
test with ohm-meter
Normally, something other than a dead short.

--
Adrian C

are those directly connected to 'green and black wire' on atx power
connector on motherboard

Not exactly.

First, look at the power on button on your PC.
It is a normally open, momentary contact switch.

When you push the button, it creates a pulse.
The logic input on the motherboard, has a pullup to +5VSB,
and when you push the button, the logic signal is shorted
to ground. A current of a milliamp or less may flow
through the switch. (So the front panel switch can be
a flimsy piece of crap, and still work. The front panel
switch doesn't need a multi-amp current rating or anything.)

The signal enters one of the motherboard ICs, and is
conditioned. It is eventually converted into an active low signal
called PS_ON#, driven by an open collector driver. The
motherboard IC means there is no direct path, from front
panel power on switch, to the PS_ON# signal. The motherboard
IC doing the conditioning, is running off +5VSB at the time
you push the button. If +5VSB is not available, then the
signal from the switch can't be conditioned, and can't be
acted upon.

(Note - in the following, I'm illustrating the principle
of cause and effect, not the timing. I didn't verify that
the timing looks exactly like this. But it gets the idea
across.)

PWR * * *-----+ * +------ *Momentary pulse, active low
(Mobo * * * * | * |
Header) * * * +---+

PS_ON# * -----+ * * * * * OFF
(main * * * * |
20 pin) * * * +---------- ON

The decoupling is more apparent, if you attempt to turn off
the PC, after the PC is booted. They have an option to check
a timer, which validates the state change on the PWR switch. You
have to press the front panel switch for at least 4 seconds,
to get the PC to power off. And the switch can be set in the BIOS,
to either do a controlled shutdown of the PC, or do a power off
instead. In the following diagram, I'm showing the "immediate"
power down option in action (it's how my PC is set up right now
in the BIOS). So after the four seconds is up, the power just
goes OFF, without warning the OS. This gives a "dirty" shutdown,
and potentially needs a CHKDSK later, to fix the file system.

PWR * * *-----+ * * * * * * * +------
(Mobo * * * * | * * * * * * * |
Header) * * * +---------------+

PS_ON# * * * *- 4 sec - |--------- OFF
(main * * * * * * * * * * |
20 pin) *-----------------+ * * * * *ON

For a sample motherboard schematic, you can take a look at
this old 440BX design.

http://www.intel.com/design/chipsets...x/BXDPDG10.PDF

On page 32, B_SUSC drives pin 14 on the ATX power connector. That
is the green wire (PS_ON# signal) in the ATX standard. B_SUSC
stands for "buffered SUSC signal".

On page 18, you can see the creation of the B_SUSC (PS_ON#) signal.
A 74F07 open collector driver is used. That is a beefy OC driver,
with lots of current sink capability to ground. Modern PCs
are probably using something a bit weaker than that. To operate
PS_ON#, probably requires sinking a milliamp or two (I don't
know the exact figure right off hand). It shouldn't need a lot
of beef, but the beauty of the 74F07, is it is more likely to
survive all insults. Occasionally, on modern motherboards, the
equivalent to the 74F07 function, fails to sink properly to
ground (logic 0).

The SUSC# signal is coming from the Southbridge. So that is where the
"conditioned" control signal, comes out of the motherboard chipset in
this case.

Now, still on page 18, you can see in the Power Management section
of the Southbridge IC, they have a "PWRBT#" (Power Button) signal,
which is active low. That is the signal the Southbridge is going
to be looking for a pulse on. The power button circuit is back on page 32..

And on page 32, they kinda ruined my explanation. They chose to use
a momentary high pulse from the switch (switch pulls to 3VSB), plus
a CMOS Schmitt trigger/inverter to clean up the edge of the signal. The
74LVC14 turns that signal upside-down again, so as the PWRBT#
signal leaves page 32, it is an active low pulse. But as far as
I know, modern motherboards don't have that additional step.
The switch would be set up to pulse low, so the 74LVC14 would
not be present.

Using an ohmmeter, on the PWR/GND pair on the motherboard panel
header, should have little to do with the PS_ON# signal on the
main 20 or 24 pin cable, as they're separated by the logic in
at least one chip. In the Intel schematic, that was the Southbridge.
So ohming from PWR to PS_ON# wouldn't be telling you anything.

What you want to do, is check the voltage level on PS_ON# (green wire),
while you're fiddling with the front panel power button. If the
motherboard open collector driver, pulls the PS_ON# signal towards
ground (zero volts), then you should be seeing the power supply
fan come on and the main rails pop up.

On the input side, you'd monitor the voltage between PWR/GND
pair, when you push the front panel button. PWR should
drop to zero volts, for as long as the front power button
is pushed. Alternately, you can connect the front panel PWR
switch to your ohmmeter, and see if it reads zero ohms, when
the button is pushed. Sometimes, the flimsy button breaks,
and when you push the button, it no longer makes a proper
momentary contact.

HTH,
* * * Paul

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)