PC crashes

[ppppenguin]

Gathering more evidence about method of switching off. I'm now convinced that using the power button to switch off but leaving mains applied means that there are no hangs. I tried this plus switching off mains for a minute and have not had a hang. I'll try it for (say) 10 minutes later.

I wonder what's still energised when mains is applied but PC is not switched on. And how this could make a difference.

[Nick]

There is a low-current 5V standby, termed +5VSB - this is ALWAYS on when the mains is powered - it's used to keep certain functions on the motherboard alive, e.g. "wake-on-LAN", even when the PC is nominally "off".

The generic term used for this signal is "Soft Power". On some (older) systems, the -12V supply is also kept active to keep serial ports powered for "wake-on-modem"...

The 5VSB line can provide up to 2.5A on some modern systems and is (should be) short-circuit proof - it can also be used to power USB ports even when the machine is nominally off...

See https://en.wikipedia.org/wiki/ATX#Power_supply for more information.

[ppppenguin]

As far as I remember I have "wake on LAN" etc disabled. I wonder what that +VSB is doing that seems to prevent the system hanging a few seconds after booting. Might be worth putting a DVM on +5VSB so I can see how fast is decays when mains is really off.

[ppppenguin]

I disabled the auto boot after powere returns. Then shut down and turned of the mains for an hour or so. PC hung a minute or so after booting.

So....

1: The auto boot after power down isn't reelvant.
2: At some point between a minute and an hour of power being removed the PC is vulnerable to hanging after a boot.

I'm guessing the actual time depends on how long it takes the standby 5V to decay below a threshold. Not sure it's worth trying to evaluate that time and voltage.

Ought to add I checked the CMOS battery and it was 3.05V. Even though Speccy reports it as about 1.5V.

[ppppenguin]

After many days of trials I can confirm that this PC works just fine when shut down at the power button. But if I subsequently switch off the mains it hangs soon after booting up. I haven't worked out how long it needs to be switched off to hang but it's more than a few seconds.

Conclusion: The standby output of the PSU is keeping something alive that prevents the hang. But what?

[Refugee]

Have you replaced the bios battery yet?

[ppppenguin]

No. Because it read 3V when I test it.

[Nick]

Do you have sleep or hibernate mode enabled? i.e. when you say you shut down your PC, that is does it actually shut down or sleep or hibernate?

[ppppenguin]

Definitely shut down. Not hibernate or sleep.

[Nick]

Interesting thought. If you shut down switch off at the mains and then boot into the BIOS screen and leave it,does it still crash?