Apple iMac G5, 20-inch - Manual (Page 252)

28 - iMac G5, 20-inch Troubleshooting Ports
consumption are monitored by the operating system which communicates with the SMU,
which in turn controls and monitors fan operation. If Mac OS X is not booted, thermal
management must be provided by the alternate development operating system.
Note: If Mac OS X is not booted and the alternate development operating system does not
manage the fans, the fans go into an unmanaged state and run at full speed.
The SMU controls the fans and regulates the speeds to run each fan. The SMU derives
fan speed from sensors in each thermal zone. Some of these sensors monitor thermal
only, while others monitor both thermal and power.
If the SMU does not receive an update from the operating system within two minutes, it
begins to ramp up the speed of the fans to full speed.
Ports
The iMac G5 ports are shown below.