Intel Xeon 3500 Series Datasheet page 106

Boxed Processor Specifications
If the boxed processor fan heatsink 4-pin connector is connected to a 4-pin
motherboard header and the motherboard is designed with a fan speed controller with
PWM output (CONTROL see Table 8-1) and remote thermal diode measurement
capability the boxed processor will operate as follows:
As processor power has increased the required thermal solutions have generated
increasingly more noise. Intel has added an option to the boxed processor that allows
system integrators to have a quieter system in the most common usage.
The 4th wire PWM solution provides better control over chassis acoustics. This is
achieved by more accurate measurement of processor die temperature through the
processor's Digital Thermal Sensors (DTS) and PECI. Fan RPM is modulated through the
use of an ASIC located on the motherboard that sends out a PWM control signal to the
4th pin of the connector labeled as CONTROL. The fan speed is based on actual
processor temperature instead of internal ambient chassis temperatures.
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. 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, see the appropriate processor Thermal and Mechanical Design Guidelines (see
Section 1.2).
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106 Intel® Xeon® Processor 3500 Series Datasheet Volume 1