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?