S3.3 Lcd Pcb Theory Of Operation - Nellcor NPB-40 Service Manual

S3.2.5 Serial Interface
S3.2.6 Printer Control
S3.2.7 External VCC Monitor and Watchdog Timer
S3.3 LCD PCB Theory of Operation
S3.2.4.4 Power Supply Circuits
When the PWR ON signal at the S/S input of U3 is high, U3 generates a
square wave signal that drives the primary winding of transformer T1. The
three secondary windings of T1 are rectified and filtered to provide
RAW+10V, VCC (+5Vdc), and RAW-5V. The VCC signal is resistor divided by
R9 and R12 for a feedback voltage that U3 must see to continue operating.
VCC is used on the CPU and the LCD PCBs for operating circuit power.
RAW+10V and RAW-5V are supplied to the LCD PCB for backlight power
and for power conditioning for the SpO
The serial interface is only used for factory test purposes, and is not at the
RS-232 level, neither is it electrically isolated and, therefore, cannot be used
outside of the factory. The serial data port J5 is a TTL level serial interface.
RXD and TXD are configured as a standard asynchronous serial transmitter
and receiver at 19.2 Kbaud with 1 stop bit, 8 data bits, and no parity. The
serial interface can operate in full duplex mode. If no external serial data
device is connected, R19 pulls the RXD input high which prevents the input
from floating when it is not being used.
Microprocessor U4 provides a drive signal (IR OUT) at HSO5 that controls an
infrared LED on the LCD PCB which is used to interface with an external
printer.
The external VCC monitor and watchdog timer circuit consists of U6 and
associated components. U6 provides two functions. If the VCC input voltage
drops below 4.0 Vdc, U6 will drive the RST-L line low, which resets
microprocessor, U4. U6 also periodically receives the WD RST pulse output
from U4. If U6 does not receive the WD RST signal at least every 500 ms, it
will drive the RST-L line low, which resets U4.
Refer to Figure S3-3 for the LCD PCB block diagram and to Figure S4-4 for
the LCD PCB schematic diagram in the following description. This subsection
describes the SpO analog hardware. The analog circuitry has high signal
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sensitivity and reduced susceptibility to noise. Its design allows for a wide range
of input signal levels and a broad range of pulsatile modulation. The SpO
circuitry consists of four subsections:
• Sensor output/LED control —The CPU controls the gain of both
LEDs so that signals received at the input amplifier are within an
acceptable dynamic range. Signal channel gain may also need to be
increased. The CPU uses Pulse Width Modulation (PWM) lines to
control LED current level or to amplify the signal channel.
• Input conditioning —Sensor output current is converted to voltage. A
demodulation circuit minimizes the effects of other light sources and
stray frequency inputs. Because the IR and RED signals are at different
current levels, the two LED signals are demultiplexed and separately
amplified, so they can be compared with each other. Two circuits handle
the demultiplexing by alternately selecting LED signals using switches.
Technical Supplement
analog circuits.
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analog
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