X6800 Datasheet, PDF (120/122 Pages) Intel Corporation – Core2 Extreme Processor

Balanced Technology Extended (BTX) Boxed Processor Specifications
the motherboard that sends out a PWM control signal to the 4^th pin of the connector
labeled as CONTROL. The fan speed is based on a combination of actual processor
temperature and thermistor temperature.
If the 4-wire PWM controlled fan in the TMA solution is connected to a 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 T_inlet 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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Datasheet