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Technical Supplement
S- 7
Watchdog Timer — U21 ensures that the U3 microprocessor does not
operate when the +5 volt supply falls below 4.5 volts minimum. This chip
holds U3 in reset until the power supply is back within an acceptable range of
voltage levels. Whenever the power supply falls out of regulation, this chip
resets the microprocessor.
The watchdog timer also resets the microprocessor if the CLRWD signal is
not toggled within the timeout period. When the watchdog times out, a
WDOL signal is transmitted, causing the alarm oscillator to emit a tone to
the Piezo speaker and blank the display. This signal remains active until the
AUDIBLE ALARM OFF button is pressed.
Display drivers — The QSM serial port is used to communicate with the
two display drivers on the Display board through the serial line. The driver
data lines are daisy-chained together so that only PCS0 is needed to
communicate to both drivers.
S4.2.1.3 Power Supply
The UIF/Power Supply PCB contains three DC-DC converters that convert
voltages from the SPS power supply or the N-3000 battery to the +5 voltage
needed in the N-3100. There are additional DC-DC power converters that
6 volts to the pump valves and the
±
10.5 volts that power the analog
circuits and dump valves in the Blood Pressure module.
Power on/off — A latch composed of two gates of CMOS U20 controls the
power-on FET. The input voltage is filtered and reduced by CR15 to
approximately 9.5 volts when the unit is on and approximately 9.8 volts when
the unit is off.
Switch U19 controls the destination of the front-panel ON/STANDBY button.
If the +5 volt supply is off, the button press sets the latch, allowing the power
to the DC-DC converters. When the supply is on, a button press generates an
level 6 interrupt to the HC16 and the latch is turned off by the software.
Battery backup — The real-time clock, the power-on reset and watchdog
timer, and the power loss/CPU failure logic are the only elements on the
board that have the battery backup capability. The 691 chip is used to select
the power source for the real-time clock and Piezo alarm circuitry from either
the +5 volt power supply or the lithium battery supply.
+5 volt DC-DC converter — The +5 volt supply converts the input voltage
from inductor T4 into 5 volts for the digital electronics. The switching
converter circuit is made using switching regulators. The circuit contains a
current-limiting resistor designed to stop the converter if a short circuit
exists. Capacitors are provided at the output for better load regulation.