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I am trying to use a Raspberry Pi's GPIO pins in conjunction with a normal ATX motherboard's I/O pins (particularly the "power SW" pair) to trigger boot on the PC as though the Pi were a typical I/O panel NO (normally open) momentary switch.

The basic idea is that two pins on the Pi, say, pin A and pin B, will be connected to SW+ and SW- on the motherboard respectively. Neither pin A nor pin B are power or ground pins (maybe they should be?). Then, wiringpi or some other tool will be used to create a completed circuit which triggers boot for the PC. This may be less doable than I think it to be, but that's a different question.

According to this question's accepted answer, the I/O circuit for "power SW" belongs to the power supply and is a separate circuit "electrically isolated" from the rest of the PSU. The diagram is a bit over my head.

This detail, to me, means that connecting the Pi and the motherboard in this way is a bad idea, as it will be physically connecting otherwise isolated circuits with separate ground signals.

Am I right? Or is this a non-issue? Is it a bad idea for a different reason? Why?

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    use relay contacts – jsotola Feb 2 at 8:33
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If you want to connect the power switch pins using the pi, you could let the pi drive a relay or an optocoupler. Then the two circuits, the pi and the pc, would be isolated

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Question:

[WRT:] ...connecting the Pi and the motherboard in this way is a bad idea, as it will be physically connecting otherwise isolated circuits with separate ground signals.
Am I right? Or is this a non-issue? Is it a bad idea for a different reason? Why?

Answer:

The correct answer to your question as stated is:

Yes, you are right - THIS IS A BAD IDEA.

Why?

This is a bad idea because you don't know, and cannot tell us exactly what the interface specifications are for whatever it is you wish to connect to your RPi. In general, I would opine, it is always a bad idea to make uninformed electrical connections under any circumstances. If you don't know the specifications, it is your job to find them - phrases such as normal ATX motherboard typically don't help.

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It is unclear what you are proposing.

One thing is clear the Pi can not emulate a contract closure.

Attempting to connect to a 5V circuit will damage the Pi.

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If its pot-free/dry contact it shouldn't damage the pi, but if there is any kind of external power I would use an external relay as a switch. Use the pi to control external relays and you will be safe.

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It depends. I would beep both of MB pins to see if one is GND. Then other one can be either pulldown or pullup contact (i think it will be pulldown).
If there will be voltage on second pin (and first one is GND) there is high probability pulling this pin LOW will turn on MB. But you cannot put higher voltage of this pin.
If there is no voltage and you cannot beep both SW pins with ground, it is pullup contact and need some voltage to turn on. You can measure voltage between GND and both pins (One will have stable voltage), and then you need to put voltage of that level on second pin. If below 3.3V you can use voltage divider to turn on.

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The Power button and Reset buttons on a computer with an ATX motherboard are both momentary spst non latching switches.

The pins on the motherboard react when shorted and the motherboard responds accordingly based on what is set in BIOS (Power press, Power Long press (4sec), Reset).

To control this with a Raspberry Pi you need to create, or emulate a physical short of the same voltage.

I am not aware of any way to do this through code and GPIO pins, without an external device to create the short.

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