S3.2 Cpu Pcb Theory Of Operation - Nellcor NPB-40 Service Manual

S3.2 CPU PCB Theory of Operation
Power from the batteries is controlled and filtered by the power control and
PCB power circuits on the CPU PCB. When the On/Off key is pressed on the
front panel keypad and the unit is turned off, the power control circuits will
turn on the unit. If the unit is on when the On/Off key is pressed, the unit
will be turned off. The PCB power circuits provide filtered power for the
circuits on the CPU PCB and raw power is passed to the LCD PCB where it is
conditioned for the circuits there.
A patient sensor connected to the NPB-40 contains a resistance that is a
calibration reference for the sensor. The analog sensor circuits on the LCD
PCB then measure the signal from a photodiode in the sensor that is different
based on the light absorption characteristics of each patient. That signal is
processed by the analog sensor circuits and measured by the CPU. The LED
drive signals are then adjusted by the CPU to obtain an optimum signal from
the photodiode.
The CPU provides a drive signal for an IR LED on the LCD PCB. When this
IR LED is aligned with a receiving photodiode on an external printer, patient
data stored in the NPB-40 can be printed.
An external output port on the CPU PCB is controlled by the CPU and sends
drive signals to the beeper and LCD backlight and display data to the LCD.
An external serial data interface, used during testing in manufacturing only,
is also controlled by the CPU. This interface is only accessible with a special
test fixture.
Refer to Figure S3-2 for the CPU PCB block diagram and to Figure S4-3 for
the CPU PCB schematic diagram in the following description. The CPU PCB
hardware and related circuitry, which is illustrated in the following block
diagram, includes the following subsystems:
• CPU — A 16-bit microprocessor that includes: a serial port, watchdog
timer, A/D converter with an 8-input analog multiplexer, 3-pulse width
modulators, and a high speed I/O subsystem.
• System memory — External to the CPU and consists of an 8K x 8
static RAM and a 64K x 16 EPROM.
• Display control — The CPU provides drive and control signals for the
LCD driver and display on the LCD PCB.
• Audio output drive— Drives a piezoelectric ceramic beeper on the
LCD PCB for audio output.
• Printer drive— The CPU provides a printer drive signal that is
applied to an IR LED on the LCD PCB.
• Power supply/Power control circuitry — The NPB-40 receives
power from 4 "AA" cell batteries. The power control circuitry
discontinues power to the unit when the batteries are no longer
reliable.
Technical Supplement
S-5