Technical Supplement
S-17
S3.3.6 Display Control Circuitry
The LCD CE, LCD CLK, and LCD DATA from external output port U9 and
LCD EN from the microprocessor on the CPU PCB are connected to the LCD
drive IC, U1. U1, in turn, drives the individual segments of the LCD. The
microprocessor drives LCD CE and LCD EN high, connected to the U1 CE
and INH inputs, respectively. At that time, data on the LCD DATA line is
clocked into U1 by the LCD CLK line. U1 then decodes the data input and
turns the proper segments on or off on the LCD. The INH inhibit input of U1
is held low by resistor R9 until the line is driven high by the LCD EN signal
from microprocessor on the CPU PCB. This assures that the display is
blanked until it is under microprocessor control. Should the microprocessor
be reset, the LCD EN line goes to a floating state, R9 then pulls the INH
input to U1 low, and the display is blanked.
S3.3.7 Power Conditioning Circuitry
Unfiltered positive (RAW+10V) and negative (RAW-5V) voltages from the
CPU PCB are applied to the power conditioning circuit. RAW+10V is filtered
by a circuit consisting of R6, C1, and C2 to provide a clean and stable +10 V
dc operating voltage for LCD PCB SpO
2 analog and other circuits. Likewise,
RAW-5V is filtered by a circuit consisting of R11, C8, and C10 to provide a
clean and stable -5 V dc operating voltage for the LCD PCB circuits. A
filtered +5Vdc (VCC) from the CPU PCB is also used as VCC by circuits on
the LCD PCB.
S3.3.8 Analog Reference Voltage
RAW +10V from the CPU PCB is applied to a filter consisting of R7 and C6 to
create VIN that is used by voltage converter U2 to create VREF. U2 provides
an accurate, regulated voltage that is used as the reference voltage for the
A/D inside the microprocessor U4 on the CPU PCB. Filtering is provided by
C7, C9, and R12. The voltage output VREF is + 5 Vdc.
S3.3.9 LCD Backlighting
RAW+10V from the CPU PCB is used as power for the LCD backlight. The
LCD backlight consists of LEDs DS1 through DS6. Resistors R3 and R4
provide current limiting for the LEDs. Transistor Q2 is turned on when the
LCD LIGHT signal from external output port U9 on the CPU PCB goes high
which, in turn, illuminates the backlight LEDs.
S3.3.10 Printer Drive Circuit
The printer drive circuit consists of R1, R2, Q1, and DS7. The IR OUT signal
from microprocessor U4 on the CPU PCB controls transistor Q1 which, in
turn, controls infrared LED DS7. DS7 is used to provide a data signal that is
transmitted to an external printer with an infrared receiver.
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