Compaq 4200 Family - Manual (Page 36)

Enclosure Power and Cooling 5–3
The fan mounting assembly is the element for mounting the second fan. This element has
no power circuitry. It does contain a circuit board for processing signals to and from the
fan.
Temperature Sensing
The power supply temperature sensor provides a temperature range signal to the EMU.
The EMU can use this signal to set the fan speed.
Fan Interface
The fan mounts on the rear of the power supply. A power supply connector is the interface
between the fan and the enclosure. The interface signals include:
1. Fan speed control from the EMU
2. Fan speed to the EMU through the power supply
3. Power supply high-speed enable
4. Fan operating voltage
Fans
The power supply-mounted fans cool the enclosure by circulating air through the
enclosure and elements. The rate at which air moves, the airflow, determines the amount
of cooling. This airflow is a function of fan speed (rpm). These fans, under the control of
the EMU or the associated power supply, can operate at multiple speeds. This ensures that
when the enclosure temperature changes, the fans can automatically adjust the airflow.
Should a fan fail, EMU and power supply circuitry automatically increase the speed of the
operational fan to high speed. Simultaneously, the error condition is reported to the user in
several ways.
NOTE: The failure of the power supply 12.5 VDC circuit disables the associated fan.