The DH57DD product manual implies that PWM and non-PWM fans can be controlled by the motherboard.
That seems to be the case for the front fan. If I plug in a PWM (4 pin) fan it will supply a PWM signal and it puts 12 volts on the power pin (pin2). If I plug in a non-PWM (3 pin) fan it supplies no PWM signal and it puts 5.85 volts on the power pin. This is when my low power CPU is idling. I guess that voltage will increase if things get hot.
How does the motherboard know if there is a PWM or non-PWM fan plugged in?
However the rear fan header will only drive a non-PWM fan. It supplies no PWM signal, and it puts 5.85 volts on the power pin.
The CPU fan header always drives the fans in PWM mode. It puts a strong PWM signal on the PWM pin and 12 volts on the power pin. A non-PWM fan will always run at full speed.
Is there a BIOS or motherboard problem? I have the latest BIOS version 537. I'm particularly interested in knowing how the motherboard tells if the plugged in fan is PWM or not.
I figured out how the mobo knows if a PWM fan is plugged in. PWM fans have a pullup resistor to 5 volts (or 3.3 volts) on the PWM pin (pin4). Max current sourced when the pin is grounded is 5 ma. I use a 10k ohm resistor connected to 3.3 volts.
I still wonder why the mobo always drives the rear fan in non-PWM mode and always drives the CPU fan in PWM mode.