Philips M3921A, Service Manual

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Theory of Operation and System Architecture
Chapter 10 - Training Program 123
SpO2, and temperature measurements to digital format and to connect this
information to the processor. The respiration detection is obtained from two
of the three electrodes of the ECG connections.
Galvanic isolation of these circuits from the remainder of the monitor is
accomplished by utilizing an isolated power supply and by incorporating
opto-isolators between the Front End outputs and inputs to the microprocessor
computation and control circuitry.
NiBP Front End
The NiBP section contains the pumps, valves, pressure measurement
circuitry, and control circuitry for the noninvasive blood pressure
measurement. Pressure data is converted to digital format and conveyed to the
processor section.
Power System
The power system section contains a power supply capable of operating the
monitor and charging the battery from either an AC source of 100 to 240 volts
AC at 50 to 60 Hz or a DC input of 10 to 16 volts. This section also contains
the circuitry. The battery provides the operating power for the monitor.
Power system outputs of ±12 volts DC, +5 volts DC, and –24 volts DC are
developed for use throughout the non-isolated portions of the monitor.
Microprocessor,
Memory and
Control
The microprocessor (µP), Memory and Control section contains the system
CPU and all digital support circuitry. The latter includes the RAM,
nonvolatile memory, and real-time clock. This section also contains the
display logic, keypad (switch) interface logic, RS-232 I/O control,
defibrillator synchronization control, and printer logic.
Display
The display is a cold-cathode, backlit, fluorescent LCD unit. The pixel
resolution of 0.23 mm provides a 640 x 480 line display. For the A3 models
M3926A and M3927A, the display is presented in black and white mode only
(no gray scale). The A3 models M3928A and M3929A have color a display.
Keypad (Switches)
and Wheel
The keypad circuit contains five push-switch membrane switches and two
green LEDs. The LEDs are driven by the power supply system and indicate
(AC or DC), the charging of the batteries, and the condition of the source
cable in-line fuses.
Signals from the LEDs are returned to the microcomputer for processing and
control as required.
The power switch, connected directly to the supply, toggles the power
between Standby and On modes. When in Standby, the display is blank, and