Are you inserting cards while the power supply is in standby mode??? Some components such as the RAM are powered by the 5VSB line, I guess it also applies in some way for expansion cards.

Not sure what could be triggering the PSON signal in this particular case, but inserting cards while certain components on the board are powered on seems like a sure way to short something.

It's not in standby mode. It's definitely powered off. There is a green indicator light on the motherboard that just shows whether the power supply is on. You can even see me holding down the power button to turn the system off between tests in the video.

But even if it was in standby mode, I don't see how touching the PCI slot would cause the system to power on.

Could be asserting a power management signal (like PME#) which has some capabilities to wake the system

Kahenraz wrote on 2023-12-13, 00:25:

It's not in standby mode. It's definitely powered off. There is a green indicator light on the motherboard that just shows whether the power supply is on. You can even see me holding down the power button to turn the system off between tests in the video.

But even if it was in standby mode, I don't see how touching the PCI slot would cause the system to power on.

By "standby" I mean power switch on the back of the PSU in the "ON" position. Otherwise it's impossible for the PSU to turn on without someone flipping that switch manually (assuming no dodgy switch or anything of the sort). Based on some pinouts I saw online, there may be some pins that are used for power management events. Maybe temporarily shorting one of those to ground or VCC neighboring pins while the card is angled during insertion triggers the PSON signal... but it's mostly speculation as I'm not really familiar with PCI electrical details.

pancakepuppy wrote on 2023-12-13, 00:28:

Could be asserting a power management signal (like PME#) which has some capabilities to wake the system

EDIT: ☝️ Exactly, this is what I was thinking! One of those "Wake on xyz" events.

I never insert or remove components while the PSU is connected to the mains. Always thought I was just paranoid, but I guess this proves I'm just sensible 😀

debs3759 wrote on 2023-12-13, 00:39:

I never insert or remove components while the PSU is connected to the mains. Always thought I was just paranoid, but I guess this proves I'm just sensible 😀

As long as the mains switch is off there's no problem, it's as if the PSU was unplugged. But on ATX power supplies, flipping that switch turns on the 5V standby supply which powers certain components on the board. That's why many crappy ATX power supplies back in the day would end up killing the board/RAM due to the 5vSB line going rogue even when the computer was "OFF" (or more accurately "SOFT-OFF").

Many cheap OEM ATX PSU have no independent power switch (so less knowledgeable users wouldn't confuse it with the momentary power button on chassis, or complain "My computer is dead!" after accidentally switching it off).

It's better to avoid those PSU, but if you must use one (such as in this case), have a power strip with individual switches in between or just disconnect your PSU from mains completely whenever you add, remove, or adjust any component onboard.

Oh, I recall seeing that a few times during the p4 era. Freaked me out!

I have found it to be relatively "normal" for the machine to power itself up from standby when inserting cards like that, the machine should be off (unplugged or switched off at the PSU) for doing this as the mobo manuals warn you.

Maybe this motherboard has some kind of wake feature that I'm activating. I don't often power off the board on my test bench completely while swapping cards; having the system powered down has always been enough.

Kahenraz wrote on 2023-12-13, 03:19:

Maybe this motherboard has some kind of wake feature that I'm activating. I don't often power off the board on my test bench completely while swapping cards; having the system powered down has always been enough.

It's not safe. You could damage the motherboard and/or the component that you're inserting. Always completely turn off the power at the wall socket before adding or removing cards.

The easiest way to remember this is to connect your computer to a power strip with a glowing ON/OFF switch. After shutting the system down via Windows (or whatever), flip the power strip switch to the OFF position, check that it no longer glows, and you're good to go.