Novametrix Medical Systems OXYPLETH 520A, Service Manual

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Section 9
Electronic Theory of Operation
28
Model 520A Service Manual 5693-90-01
PRELIMINARY
the cumulative effect of the -5 volts and the negative photodiode signal passing
through D14 and R34 would be to send the IC36 pin7 output to its positive rail (+12
volts). The processor recognizes this higher voltage and causes an
INCOMPATIBLE
PROBE
message to displayed in the Message Center.
Channel I5: Infrared LED Cathode Voltage.
A low pass filter consisting of RP10 (pins 3, 4), RP8 (pins 1, 7) and C89 provides
a means to measure the cathode voltage of the sensor’s Infrared LED. If the channel
at IC33 pin 5 is sampled the monitor can determine if the LED is open circuit (zero
volts at IC33 pin 5) or operational (approximately 2.5 volts at IC33 pin 5). If not
operational, display messages such as
PROBE I.R. LED FAILED
or
ERROR -
FAULTY PROBE
are generated.
Channel I6: Battery Supply Voltage
The monitor’s internal battery voltage is divided down by RP3 (pins 7, 8) and RP8
(pins 1, 5). The voltage at IC33 pin 2 is monitored and if its magnitude is less than
a predetermined value (encoded in the software) the monitor lights and/or flashes
its front panel battery indicator. This provides the user with a low battery warning.
Channel I7: Red LED Cathode Voltage.
A low pass filter consisting of RP10 (pins 1, 2), RP8 (pins 1, 8), and C92 provides
a means to measure the cathode voltage of the sensor’s Red LED. If the channel at
IC33 pin 4 is sampled the monitor can determine if the LED is open circuit (zero
volts at IC33 pin 4) or operational (approximately 2.5 volts at IC33 pin 4). If not
operational, display messages such as
PROBE RED LED FAILED
or
ERROR -
FAULTY PROBE
are generated.
Microprocessor
9.3.14
A Hitachi HD64180RP microprocessor directs the actions of the Model 520A Pulse
Oximeter. (See sheet 2 on 2471 schematic, sheet 1 on 2775 schematic.) The
processor, IC16, is operated at 6.144 MHz (half the12.288 MHz frequency of
crystal Y1), has an 8-bit data bus and a 19-bit address bus (the 520A uses only 18-
bits). The microprocessor also provides two asynchronous serial communication
channels, a clocked serial I/O port and various interrupt and control signals. The +5
volt VCC supply to the processor is first sent through inductor FB1, a ferrite bead,
before powering the chip at IC16 pin 32.
Memory
9.3.15
The Model 520A system software is located at IC17, a 27C010 EPROM. The 32 K
byte RAM, IC20, stores trend data, system power up settings (averaging times,
serial output parameters, etc.), and provides an area for system (scratchpad)
memory requirements. Since IC20 is powered from the VBACK supply, RAM
memory is retained when the monitor is turned off and it becomes available again
when the monitor is turned back on.