Re: Motherboard Power Problem

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mcp6453 wrote:
Don Phillipson wrote:
"neumannu47" <neumannu47.3fkrfc@xxxxxxxxxxxxx> wrote in message
news:neumannu47.3fkrfc@xxxxxxxxxxxxxxxx

This problem is not directly related to XP, but this forum is the
closest one I could find for a hardware-only problem. (The reason I know
that the problem is hardware only is that it occurs even if I boot to a
DOS disk.)

When I press the OFF button, the computer does not turn off. Holding
the power button for 10 seconds or more does not cause the comptuer to
power down. Shutdown from within XP works as expected.

We need to know what type of motherboard and switching (case)
the OP uses. Since about 2000 MB and case builders have used
front panel switches with only one function, to turn on (or restart.)
This is not an on/off switch. Only the back panel power supply
switch has this function. Since Win98, Windows computers were
designed to switch off via keyboard / Start / Shut Down / Shut Down.


The motherboard is an ECS P4S5A. Plus, there is new information. The computer does not shut down even from Windows. It looks like it's powering off, but it's not. The power supply appears to have been replaced some time ago. The case does not have any identifying information that I can find.

The computer power looks like this.

A.C.---------Switch_On_Back----+------+5VSB_supply---- Standby_voltage
|
+------Main_supply----- +3.3,+5,+12,-12
^
| PS_ON# signal
| 0 volts = "Soft ON"
| 5 volts = "Soft OFF"
| (Main_20pin_cable disconnected = "Soft OFF")
| signal wire is green in color (black is ground)
|
|
Front_Panel_momentary_contact_switch--> Motherboard_logic_chip
Latches momentary contact front switch
Drives PS_ON# to ground, when "Soft ON" desired

If the computer won't "Soft OFF" the supply, then that means the green wire is
always at zero volts. Either the motherboard is faulty, or the power supply
is faulty. If you disconnect the main power cable from the supply, the
green wire should "float" at 5V (or the same voltage value as the
+5VSB signal), and that is the reason the power supply won't run if
it is just sitting on your test bench and plugged into the A.C.
and is switched on at the back. The power supply contains a pullup
resistor, that defines the "floating" voltage level. The logic
type is called "open collector" logic, where the driving circuit
(the motherboard), pulls the signal to ground or zero volt level,
to actuate it.

The power supply could be ignoring its own input logic signal, or
the motherboard may have failed and may always be driving the
signal. So fault isolation revolves around what the PS_ON# signal
is doing, both when the power supply is connected to the motherboard
and when it is not connected.

There is a reference here, on 20 pin ATX power supplies.

http://www.formfactors.org/developer/specs/atx/ATX12V_1_3dg.pdf

(On older power supplies, pin 18 carried -5 volts. The 1.3 version
of the specification, removed the -5V signal, and power supplies
no longer carry it. That is why pin 18 is marked as "reserved",
and on some supplies, there isn't even a pin present in the pin 18
position of the main cable. My guess is, that the 1.3 spec would
most closely match your power supply.)

A new supply may fix it, if the old supply is ignoring PS_ON#.
A new motherboard may fix it, if the driver to PS_ON# is damaged.

A multimeter connected to the green wire (you can probe the
main connector where the wires go into the nylon shell), will
tell you the voltage level on the PS_ON# signal. Anywhere between
2V and 5V is a logic 1 (and ideally nearer the 5V level), while
0V to 0.8V or so, is a logic 0. There is some room for voltage
variation there, so it doesn't have to be exactly 0.00V or 5.00V
when measured with a multimeter. Values between 0.8V and 2.0V,
will leave the power supply confused, and there is no way to
predict whether it will be on, half-on, or off. The above
voltage ranges, are as defined at the power supply input
receiving device.

HTH,
Paul
.

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