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2 System Board
Devices on the SM Bus
LM75 Chip
The LM75 chip is a temperature sensor and alarm located on the
motherboard. It is used to measure the temperature in one area of the PC
Workstation, and to send an alarm to the processor in case of overheating.
This chip includes a security mechanism which prevents the system fan
from being disabled using software controls so long as the temperature
measured by the sensor is above the maximum operating temperature.
LM78 Chip
The LM78 chip is a general purpose hardware monitoring chip, connected to
the SM bus. It monitors several hardware parameters: several critical
voltages in the CPU, main fan (CPU fan, disk fan, card area fan) speed, and
several general purpose I/Os.
Main PLL
The registers of the main PLL are accessed through the SM bus. These
registers control the PLL clock signal outputs and are write-only.
WARNING: Writing over the SM bus may be destructive to the PC Workstation, as it
allows to access information necessary to the System BIOS, without which
the system will not run.
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