Nellcor NPB-190 Service Manual page 70

Technical Supplement
S8.3 Sensor Output/LED Control
S-6
monitor until it is turned off. Default values will be restored when the unit is
power-on again.
Patient data is stored by the NPB-190 and can be downloaded to a printer
through the serial port provided on the back of the monitor. An in-depth
discussion of the serial port is covered in the Appendix of this manual.
PIC
Primary responsibilities of the PIC include monitoring and controlling the NPB-
190ís power, and generating sounds.
Since the PIC monitors and controls system power, the Power On/Standby
switch is interfaced with the PIC. When the Power On/Standby switch is pressed,
the PIC sends power to the circuits within the NPB-190. The PIC will determine
if the unit is running on AC, or battery power, and illuminate the proper
indicator. The serial port and nurse call functions are disabled by the PIC if the
unit is running on battery power.
Battery voltage is checked periodically by the processor. A signal from the
processor turns the charging circuit off to allow this measurement to be taken. If
the processor determines that the battery voltage is below 5.85 ± 0.1 VDC, a low
battery alarm is declared by the PIC. If battery voltage on the UIF PCB is
measured below 5.67 ± 0.1 DCV, the monitor will display an error code and
sound an audible alarm. (Voltages measured at the battery will be slightly higher
than the values listed above). The user will be unable to begin monitoring a
patient if the battery voltage remains below this point. If either event occurs,
plug the unit into an AC source for 14 hours to allow the battery to fully
recharge.
When the NPB-190 is powered by AC, the nurse call function is available. If no
alarm conditions exist, the output will be -5 to -12 VDC. Should an alarm
condition occur, the output will be +5 to +12 VDC.
When the CPU sends a tone request, three items are used to determine the tone
that is sent by the PIC to the speaker. First, pulse tones change with the %SpO
value being measured. The pulse beep tone will rise and fall with the measured
%SpO value. Second, three levels of alarms, each with its own tone, can occur,
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High, Medium, and Low priority. Third, the volume of the alarm is user
adjustable. Alarm volume can be adjusted from level 1 to level 10, with level 10
being the highest volume.
A time clock is provided by the NPB-190. The PIC is powered at all times to
support this function. To conserve power, the PIC enters a low-power sleep
mode when the instrument is powered down.
The SpO analog circuitry provides control of the red and IR LEDs such that the
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received signals are within the dynamic range of the input amplifier. Because
excessive current to the LEDs will induce changes in their spectral output, it is
sometimes necessary to increase the received signal channel gain. To that point,
the CPU controls both the current to the LEDs and the amplification in the signal
channel.
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