Chapter 4 System Support
Compaq Deskpro EXS and Workstation 300 Personal Computers
Featuring the Intel Pentium 4 Processor
First Edition - December 2000
4-34
4.7.4 TEMPERATURE SENSING AND COOLING
These systems feature a fan integrated into the power supply assembly. A separate chassis fan is
also employed. Both fans are variable-speed type, and typically operate in tandem as long as the
power supply is active (producing 12 VDC). The fans are off in S3 (Suspend-to-RAM) and S5
(Soft-Off) states.
NOTE: These systems are designed to provide optimum cooling with the cover in place.
Operating a system with the cover removed may result in a thermal condition of system
board components, including the processor.
Figure 4-11 shows the fan control schematic.
NOTE: [1] Will be +12 VDC if chassis fan is connected and operating.
Figure 4-11. Fan Control Block Diagram
An ASIC monitors a thermal diode internal to the processor and provides a Fan CMD signal that
the Speed Control logic of the power supply uses to vary the speed o f the fans through the negative
power rail. The turning off of the fans as the result from the system being placed into a Sleep
condition is initiated by the ASIC asserting the Fan Off- signal, which results in the On/Off Control
logic shutting off the +12 volts to the fans.
Typical cooling conditions include the following:
1. Normal – Low fan speed.
2. Hot processor – ASIC directs Speed Control logic to increase speed of fan(s).
3. Hot power supply – Power supply increases speed of fan(s).
4. Sleep state – Fans turned off. Hot processor or power supply will result in starting fans.
Power Supply Assembly
P1
Speed
Control
Fan CMD
Fan Off- Fan
Off
PS Fan
CMD
Processor
Sensing
ASIC
On/Off
Control (+)
(-)
CH Fan CMD
Fan
Sense [1]
SMBus
ICH2
82801
ICH2
Therm-
On/Off
Control
8
12
24 PS Fan
(+)
(-)
2
3
4
Chassis Fan
Header P8
2
3
4(+)
(-)