Nellcor OxiMax N-550 Service Manual page 117

Regulated DC Power Supply
Controlling Hardware
CPU
N-550
The Main PCB receives the MAIN_DC unregulated voltage of 20 volts DC
from the power supply, or 12.8 volts DC from the internal battery.
The power supply on the Main PCB generates +5 and +12 volts DC.
There is one microprocessor on the Main PCB, is a Microchips PIC17C756A.
The PIC17C756A is the main controller of the N-550. The PIC17C756A
controls the front panel display, data storage, N-550 status, sound generation,
monitor, and controls the N-550's power. The PIC17C756A also controls data
port communication and communicates with the MP506 SpO
Battery voltage is checked periodically by the processor. If the processor
determines that the battery voltage is below 9.5 ± 0.1 volts DC, a low battery
alarm is declared by the CPU. If battery voltage on the Main PCB is measured
below 8.5 ± 0.1 volts DC, the N-550 will automatically turn off. The user will
be unable to begin monitoring a patient if the battery voltage remains below
this point. If either event occurs, plug the N-550 into an AC source for 11
hours to allow the battery to fully charge.
The PIC17C756A also controls a set of dry contacts provided by a relay on
the Main PCB. The relay will function normally on AC power or on internal
battery power.
When the CPU sends a tone request, three items are used to determine the
tone that is sent to the speaker. First, pulse tones change with the %SpO2
value being measured. The pulse beep tone will rise and fall with the
measured %SpO2 value. Second, three levels of alarms, each with its own
tone, can occur: 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.
The PIC17C756 controls the display driver and the buttons. By pressing any
of seven keys on the keypad the operator can access various functions of the
N-550. The PIC17C756A will recognize the keystroke and change the display
as required. Any changes made by the operator (such as: alarm limits, pulse
beep volume) are used by the N-550 until it is turned off. Default values will
be restored when the N-550 is turned back on.
Technical Discussion
Module.
2
107