Intel BX80562QX6700 - Core 2 Extreme 2.66 GHz Processor Datasheet page 96

Boxed Processor Specifications
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
Section 1.2).
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