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Enhanced Monitor Fan Control
Software Control of
Fan Speed
Chapter 2
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With the front panel fan switch in the VAR position, the Enhanced monitor
controls the fan speed based on slot temperature limits you specify. With the
fan switch in the FULL position, the fan operates at full speed.
Essentially, the Enhanced Monitor's fan control has two contrasting
functions: 1) keep the VXI modules installed in the cardcage cool and 2)
operate as quietly as possible. These are somewhat conflicting goals because
to operate at its quietest, the fan would have to be off and hence not keep the
modules cool. Alternately, at its coolest, the fans would have to be full on
and the airflow is not quiet. Therefore, the fan control algorithm is to keep
the mainframe as quiet as possible while cooling the VXI modules
adequately.
What is adequate cooling? You specify it through your selection of slot
temperatures. For example, if you specify slot 7 temperature to be
maintained within 15 °C of the ambient temperature, the fan controller
adjusts fan speed until the cooling air coming off slot 7 is just under 15 °C
above ambient. And it will keep it there, adjusting the fan speed as needed
to maintain that temperature rise no matter what the ambient temperature
might do. That way, the fan noise is at its lowest while maintaining
appropriate cooling.
Each slot is monitored and compared to the specified slot temperature limits
every two seconds. The slot closest to its limit is allowed to approach the
limit in order to keep the fan speed as low as possible.
If you want a cooler mainframe, lower the slot temperature limits. If you
want a quieter mainframe, raise the temperature limits. Just remember that
the Enhanced Monitor Fan Controller is very accurate; if you specified
30 °C rise above ambient, the controller will maintain a limit just below
30 °C.
There is one exception. The power supply cooling overrides all other cooling
requirements. For example, you may specify a 30 °C limit for all slots, but
as you monitor the slot temperatures you notice that no slot is approaching
the limit. It is likely that the Enhanced Monitor is working to keep the power
supply adequately cooled. Power supply cooling requirements are primarily
driven by the load placed on the power supply. It needs more cooling if it is
significantly loaded. So, if the fan controller stops dropping the fan speed
even though the slot temperatures are not close to the specified limits, it is
probably because the power supply cooling requirements are dominating.
The
SYSTem:BLOWer:STATe
the fans from VARiable to FULL at any time. When software control has set
the fans to FULL, the front panel fan switch can be set to variable and the
fans will remain at FULL. Software control can not put a mainframe with its
switch set to FULL into a variable fan state. FULL means that the fans will
run at full speed no matter what the conditions in the mainframe. VARiable
means that the enhanced monitor sets the fan speed based on the temperature
conditions in the mainframe and the temperature limits set by the user.
command (page 131) can change the state of
Getting Started 35
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