I would say the next step would be to take out one of the fan units, and see if one can properly control and measure its RPM on a breadboard circuit. Other than the 32 kHz PWM frequency, they should behave exactly as Intel-spec PWM fans. Collecting a few data points on the relationship between input duty cycle and RPM (output tach frequency) would be useful, too.
Thank for the informative post once again.
I am looking forward to hooking this up on a breadboard and playing, that's a great idea. It will help me become more acquainted with my scope again, it's been a while since I dug into any project.
Still not sure why the system was shutting down with the test fan piggybacked. If it is sensing current, then this may be more complicated yet again. I have reached out to Cisco as a hail mary, we'll see if I get anything back.
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