Novametrix Medical Systems MARS PO2 TECH 2001, Service Manual

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Section 4
Theory of Operation 2775 Main Board
18
Model 2001 Service Manual Rev. 01
IC36 pin 6, where it is summed with the -5 volt output from IC43. The resultant voltage at IC33
pin 1 will be approxi2.81 volts with no photodiode input. This voltage drops as the
(positive) photodiode signal at IC40 pin 1 increases. If the IC40 pin 1 signal goes negative, as
would happen if a non-SuperBright™ 8600 series sensor were connected to the monitor, 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 a fault or error message to be displayed.
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, a fault or error message
is 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, 6), 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 message ERROR - FAULTY
SENSOR is generated.
Microprocessor
See sheet 1 of 4 on schematic.
A Hitachi HD64180RP microprocessor directs the actions of the Model 2001 Pulse Oximeter.
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 2001 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
See sheet 1 of 4 on schematic.
The Model 2001 system software is located at IC17, a 29C010 Flash 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 (scratch pad) 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.
The ROM at IC17 is read when its Chip Enable line (IC17 pin 22) is brought low by the ROMCS*
signal at IC25 pin 3, and the processor brings its Read line (IC16 pin 63) low-thereby activating
the ROM Output Enable line at IC17 pin 24. Under these conditions, ROM data from the
specified address bus location is made available to the data bus for use by the processor.