Intel Core 2 Extreme QX6850 BX80569QX6850 User Manual

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If the new 4-pin active fan heat sink solution is connected to an older 3-pin 

baseboard processor fan header it will default back to a thermistor controlled mode, 

allowing compatibility with existing 3-pin baseboard designs. Under thermistor 

controlled mode, the fan RPM is automatically varied based on the Tinlet temperature 

measured by a thermistor located at the fan inlet.

For more details on specific motherboard requirements for 4-wire based fan speed 

control, refer to the appropriate Thermal and Mechanical Design Guidelines (see