EXTERNAL SHELF CLOCK BACKUP CIRCUIT
The EMU-830 List 6A includes a backup circuit for the 2.048 MHz external shelf clock. This circuit will continue to provide a 2.048 MHz clock to the DSL cards in the event the 2.048 MHz external shelf clock is lost. The software reports the status of this circuit and allows the user to set the severity of the alarm generated when the clock is lost. A block diagram of the clock backup circuit is shown in Figure 1-8 on page 1-11.
Under normal operation the external shelf clock and the backup circuit synchronize their respective clocks through the EMUs phase-locked loop (PLL) device. In the event the external shelf clock is lost, the oscillator in the backup circuit will continue to supply the 2.048 MHz clock. A Loss of External Clock alarm is reported to the management station.
External
Clock
Backplane input
EMS-83x Shelf
Slot 17
Reference
PLL
2.048 MHz
Source
Oscillator 
EMU-830 List 6
Figure 1-8. External Shelf Clock and EMU-830 List 6A Backup Circuit
