Novametrix Medical Systems MARS PO2 TECH 2001, Руководство по обслуживанию

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2775 Main Board
Section 4
Theory of Operation
Rev. 01 Model 2001 Service Manual
17
Sensor Status Decoding and Conversion
The microprocessor monitors several sensor parameters in addition to the Red and Infrared
data channels. It monitors the status parameters, as well as the voltage of the monitor’s internal
battery.
The 8-to-1 multiplexer, IC33, decodes the A0AUX-A2AUX input address lines and connects
one of eight status parameter inputs (labeled channels 0-7 at IC33) to the multiplexer output at
IC33 pin 3. Resistor R29 and diode D13 prevent negative voltages from reaching the input to
the analog-to-digital converter, IC31.
IC31 is an 8-bit analog-to-digital converter with a serial data output. While the IC31 Chip Select
(CS) input is high, the CLK input and DOUT output are in 3-state mode. When CS is brought
low (under processor control), the most significant bit (D7) of the PREVIOUS data conversion
becomes available at the DOUT pin. The remaining bits (D6-D0) are shifted out on subsequent
falling edges of the CLK input. On the clock pulse following the one that shifts out the least
significant bit (D0), the CLK and DOUT lines are returned to 3-state and the ADC performs a
new conversion based on the input it receives from the IC33 channel selected by the A0AUX-
A2AUX input address lines.
The ADC sample/convert/store-result cycle is based on internal chip timing and not the CLK
input which (along with CS) only controls serial data output. Thus the CS line is free to return
high once the ADC cycle begins.
Sensor Status Parameters
The sensor (and battery) status parameters input to the multiplexer IC33 are described below.
Note that channel number refers not to the IC33 pin number, but to the signal label (e.g.,
channel I0 signal resides at IC33 pin 13).
Channel I0: ADCV12.
This is an extra input to the multiplexer IC33 pin 13. It is unused as of this writing.
Channel I1: Auxiliary Input.
This is an extra input to the multiplexer IC33 pin 14. It is unused as of this writing.
Channel I2: Photodiode DC Level.
Resistors R40, RP8 (pins 1, 2) and capacitor C97 form a voltage divider and low pass filter that
provide a measure of the mean DC level at the output of the photodiode current-to-voltage
amplifier IC40 pin 1. This channel (IC33 pin 15) is used in determining ambient light
interference. If this line is examined while the sensor’s Red and Infrared LEDs are turned off,
then any DC level at IC40 pin 1 must be the result of ambient light impinging on the photodiode.
If the DC shift is in excess of limits set in the software, a Light Interference message appears
on the monitor’s display.
Channel I3: Sensor LED Supply Voltage.
This channel, at IC33 pin 12, monitors the sensor LED supply voltage through a voltage divider
consisting of RP10 (pins 5, 6) and RP8 (pins 1, 8). If a fault occurs that causes the LED supply
fuse F2 to blow, or if the sensor wires are shorted, this channel reports the condition and the
Message Center displays FAULTY SENSOR.
Channel I4: Incompatible Probe Detection
The input at IC33 pin 1 provides the processor with an incompatible probe indicator (the words
probe and sensor are interchangeable). The photodiode output voltage at IC40 pin 1 will be
positive if a SuperBright™ series sensor is connected to the monitor. This positive signal
passes through the high pass filter of C73 and RP10 (pins 7, 8) to the amplifier inverting input