Power Distribution System in the Cisco 12008
Cisco 12008 Gigabit Switch Router Installation and Configuration Guide
1-72
The switching fabric of the SFC is identical to that of the CSC. However, the SFCs do not
perform any of the system services native to the CSC (see the section entitled “Clock and
Scheduler Card” on page44). The SFC merely augments the switching capacity of the
router.
Power Distribution System in the Cisco 12008
In the Cisco12008, source AC or source DC power is converted by the installed power
supply(ies) into the +5 VDC and –48 VDC required for router operation. These voltages
are delivered to the backplane through the blind mating Elcon connector at the rear of the
power supply enclosure. The backplane then distributes these operating voltages to all of
the installed components in the system (see Figure1-23).
The +5 VDC is fed to the MBus module on each installed card, and the –48 VDC is fed to
a DC-DC converter on each card.
The DC-DC converter on each card operates under control of the card’s MBus module.
When directed by the GRP or system software during normal system startup, the DC-DC
converter on each card is activated to convert the –48 VDC from the backplane into the
voltages required to power the card’s electronic circuitry.
The card cage fan tray and the power supply fan tray derive their operating power from a
DC-DC converter on the CSC. This converter takes the –48 VDC from the backplane and
converts it into the +24 VDC operating voltage required by the fan trays.
If an overtemperature condition is sensed anywhere within the router, or if any one of the
fans fails in either the card cage fan tray or the power supply fan tray, the DC-DC converter
on the CSC increases the voltage being delivered to the fan trays. This causes the fans to
run at maximum speed to increase the volume of cooling air flowing through the router.
Once the overtemperature condition is resolved, the fans revert to their normal operating
speed.
Because the fans must operate continuously to prevent thermal damage to router
components, they cannot be turned off by software.