Philips M3921A Service Manual page 136

Theory of Operation and System Architecture
Navigation Wheel
124
no monitoring is performed. However, the batteries are charging if either an
AC or DC power source is connected to the rear panel.
The alarm silencing switch is connected directly to the processor and to the
system power supply. Pressing this switch turns off the battery fuse alarm in
the system power supply. Response of the processor depends upon the action
in pressing this switch. If momentarily pressed (less than 2 seconds), alarms
are silenced temporarily for a preset interval determined by the menu
selection. If held pressed for 2 seconds or more, the Alarm Suspend condition
is initiated.
The NiBP switch output is connected to the processor. Response of the
processor depends upon the state of NiBP operation at the time and the action
in pressing this switch. If momentarily pressed (less than 2 seconds), a single
NiBP measurement is obtained. If pressed for 2 seconds or more, the
processor initiates a STAT monitoring sequence. Pressing the NiBP switch at
any time a pressure measurement is in effect causes the processor to terminate
the measurement and to deflate the cuff.
The display Contrast switch operates in conjunction with the navigation
wheel to determine the apparent black/white contrast setting in the display.
Changing contrast is actually a change in the viewing angle. Outputs of the
switch and wheel are connected to the processor.
Momentarily pressing the switch sets the contrast to mid-range, factory-
default value.
Momentary pressing of the switch, followed within 3 seconds by a rotation of
the wheel are processed to vary the contrast of the display. When there has
been no wheel rotation for three seconds, the contrast control function is
terminated by the processor. The contrast control function is also terminated if
the wheel is pressed any time within this 3 second interval.
Operation of the Volume switch accomplishes similar functions for the
volume of the heart rate audible tone as the display contrast control switch
does for the display. Pressing this switch enables the wheel to vary the tone
volume.
This is a rotating, push-switch wheel. The associated wheel circuitry
generates a pulse when pressed and generates a digitally encoded pair of
quadrature signals whose relative magnitudes and polarities represent the
angular position of the wheel. These outputs are connected to the processor
where they are interpreted as required for the functions involved. Successive
angular positions determine the direction of wheel rotation.
In addition to the functions performed when in conjunction with the keypad
switches as described above, the wheel operates in conjunction with the
display to select menus and lists of parameter variables.
Chapter 10 - Training Program