Mallinckrodt NELLCOR NPB-4000 Service Manual page 191

Section 15: Drawings
15.12.1 Watchdog Shutoff
Another shutdown path exists for the system supply . When the system supply
begins operation, PWRUP goes high, which begins to charge C49 through R65.
Eventually (0.5 second), C49 will charge up (through U12) and the system
supply will shut off.
The way that the supply is kept running is through the receipt of a
SYNC/ALARM pulse from the processor board. This 50 kHz square wave from
the processor has two purposes: it synchronizes the switching frequency of the
system power supply, and its absence indicates a time-out of the watch dog timer
in the processor section. Transistor Q16 produces a 5-volt pulse for synching the
supply, and the periodic pulsing allows Q15 to keep C49 discharged. If the sync
pulse disappears, the system supply will keep running for a while (based on its
own R/C oscillator), but the rising voltage on C49 will shut the unit down in a
few hundred milliseconds.
15-11