Printer Control Block Diagram
Printer circuitry is divided into two subsections: the printer interface and the printer flex circuit. The printer
interface circuitry is present on all models, but is disabled by software in the N-20. The printer flex circuit is
added when a printer is present.
Definition of Terms
Analog to Digital (A/D) converter
– The CPU has a 10-bit A/D converter on board. Up to eight different
analog inputs can be provided to the A/D for measurement.
Central Processing Unit (CPU)
– An intel 80C196KC 16-bit microcontroller. The CPU sends and receives
control signals to the SpO
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analog section, display, and optional printer.
Content Addressable Memory (CAM)
– The CPU controls the HSO lines with the CAM. CAM is software
controlled and programmed with events scheduled relative to one or two internal timers.
High Speed Outputs (HSO)
– The 6 HSO lines control most of the timing of the LED signal pulse and the
demodulation of the received signal.
Input and Output (I/O)
– Digital lines that are used by the CPU to read in data and output data.
Light Emitting Diodes (LEDs)
– Two LEDs are used in Nellcor oximetry sensors. Light is transmitted through
body tissue and received by a photodetector circuit that converts it to photocurrent. The two wavelengths, which
are used for calculation of pulse rate and oxygen saturation in blood, are transmitted at the following wave
lengths:
infrared (IR) light at = 915 nanometers
red light at - 660 nanometers
Pulse width Modulation (PWM)
– The three 8-bit PWM outputs can be software controlled; their duty cycle
can be changed from 0-255/256 of the total pulse duration. PWM frequency is the crystal frequency of the
CPU, which is 10 MHz divided by 1024. The PWMs control the gains within the analog circuit.
RCal
– Sensor RCal value is a resistance value specific to an individual sensor. This value is used by the
software during oxygen saturation computations to maximize accuracy.
Real-Time Clock (RTC)
– The RTC is used with the optional printer to track time and date for printouts.
Overall Block Diagram
Exclusive of covers, button, and external connectors, the N-20P consists of three main components: the main
PCB, the auxiliary PCB, and the display assembly and analog shield.
Main PCB
– contains the SpO
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analog circuitry; the CPU; support memory circuits; sensor circuits for ambient
light, temperature, and battery voltage; the check battery circuit; a serial data port; and some display control
circuits.
Auxiliary PCB
– contains the power supply circuitry; the display driver circuits; the real-time clock; the interface
circuitry for the printer flex circuit board (which is not used unless a printer is present); and audio output
hardware.
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