Theory of Operations
2–3
2.4 System board
System board
Supply voltage
+4.8 VDC
ComWheel
Speaker
4 to 8 ohm
Oximetry sensor
Battery power
Line power
Display board
+12 VDC
•
LED
•
Photodetector
•
Sensor ID
•
Power button
•
Alarm Silence button
•
Alarm LEDs
•
AC power LED
•
I
2
C bus
•
Backlight LCD
Trend Download board
•
RS-232
•
SPI bus
•
Alarm annunciation
Figure 2-2. System board block diagram
The System board provides patient isolation, voltage regulation, over-current
protection, watchdog features, and other functions related to system monitoring and
control. An ID circuit on the board can output an analog voltage indicating the
hardware version of the board. The System board also communicates printer
functions to an installed Trend Download board.
The System board conditions power from the power supply and the battery. When a
DC voltage is supplied from the power supply, the board charges the battery at a
constant current while simultaneously supplying power to the rest of the circuit.
The board transmits data for the display and monitors the Alarm Silence and power
switches. It controls the backlight and drives the speaker.
The System board drives the sensor LEDs and receives photodetector signals from the
sensor. The output from the signals is used to determine patient oxygen saturation,
perfusion, and pulse rate. The electrically-isolated side of the board contains the
oximetry analog hardware, CPLD, and A/D converters. A +3 VDC rail input to a step-up
isolation transformer pr5.3 VDC isolated voltage to the oximeter side of the
board. Digital Signal Processing (DSP), which processes the oximetry data, requires a
core voltage of +1.5 V and an I/O voltage of +3.3 V.
An EEPROM contains the DSP boot code; a FLASH device contains the application
code. The DSP is the master of the memory devices during the boot sequence; the
microcontroller is master of the FLASH during normal operation.
The System board contains a connector for reprogramming the FLASH and for
installing software upgrades, including boot, DSP, and system software.
