Ambulatory Blood Pressure Monitor 90217 Service Manual
3-3
Theory
Watch Dog Timers
The watch dog timers ensure that the cuff cannot remain inflated because of a software crash.
There are two watch dog timers in the Model 90217: one resides inside the processor; the other
inside the real-time clock. Each counts 180 seconds before timing out. Both timers start at
converter power up.
The real-time clock watch dog timer triggers the hold off that disables the pump, opens the bleed
valve, and informs the CPU of its action. In 16 seconds after the hold off is asserted (if the software
does not end the reading) fault shutdown resets the system. The hold off is also asserted at the
end of every reading. The hold off always stays active for 32 seconds or until a manual reading is
started.
Fault Shutdown
The fault shutdown circuitry resets the system because of two conditions:
• There is an over-pressure condition not detected by software.
• Pressure reading takes longer than 180 seconds (this indicates a software crash since a
software time out should have already stopped the reading).
Over-Pressure Detector
In addition to software over-pressure detection, there is also a hardware over-pressure detector
that activates at approximately <295 mmHg. A one-half second delay in initiating an over-pressure
response prevents motion artifacts from causing a false over-pressure detection.
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