Quote Originally Posted by DanceswithUnix View Post
I would hope not. I was told early on at university that anything I designed on the course would be expected that you can drop a spanner across any connections without fear of the consequences, or that design was a fail. IME that's a pretty normal mindset, only broken by the cheapest of illegal Chinese imports.
I was merely a jobbing wireman promoted to test engineer. Nobody I worked for designed PCBs to be safe when powered up with the case removed.

Poking at a PC fan header with multi-meter probes while the board is live presents an obvious risk of shorting the tacho on Pin3 to the 12V supply on the adjacent Pin2. In the document I posted earlier Noctua are very clear that shorting the 12V line to the tacho or PWM line can break the fan circuit instantly. ITE super i/o data-sheets are not so clear but the logic pin current limits and "not 5V tolerant" warnings suggest a 12V short may end badly. Generally speaking I wouldn't trust a 3.3V logic pin to survive connection to a 12V supply rail.

The "motherboard schematic" earlier was for PWM only header with no voltage control.
The fan header is backwards compatible and suitable for brushed and brushless DC fans with fixed, thermostatic relay, voltage regulated, PWM and MCU speed control. The schematic contains nothing that would prevent one technology working with any other. You can't really call it a standard but everyone follows Intel.

But the more I think about this, the more I think such things don't matter.
You're right, it doesn't matter. Either side of the interface manufacturers can implement as they like provided voltage, current and timing parameters remain compatible at the header. The common header and pin out allows PC fans and motherboards to be widely mixed and matched.

This is not a simple DC motor plugged into a 12V rail. It is a brushless motor taking inputs into its control system, plugged into a PWM header with its own control system and the two are interacting in a way that is unstable.
You are so over-thinking it. Brushless PC fans have a BLDC control chip on the fan head. Apart from needing a correctly polarised supply, brushless fans with a BLDC chip behave remarkably similar to the brushed fans they replace.

There is no need to guess how it might work. Manufacturers have documented just about every fan control arrangement known to man. https://www.nxp.com/docs/en/white-paper/QD4DCFAN.pdf

I would still want some protection on that to avoid customer returns if they ham fisted plug the wrong thing into a header, or backwards or something (we all have our moments, not judging customers)
You have to be pretty ham fisted to force a Molex-KK on backwards. We did have a gamer bring a PC in with the connector wired backwards, after his mate thought it would reverse the direction of the fan. The tacho was toast.