Mallinckrodt Nellcor N-20 Service Manual page 58

Technical Supplement
BZ1, a piezo ceramic sounder, is the audio output device. Due to its low drive current of 2 mA
maximum, no drive circuitry is needed, and the audio output device is driven directly from the
external output port. It is differentially driven with 2 square waves 180 degrees out of phase. The
drive frequency is approximately 1480 Hz or 740 Hz and is generated by the CPU. BZ1 is
differentially driven to obtain maximum audible volume.
9.10.13 Display Control Circuitry
The display control circuit is illustrated in Figure 9-22, at the end of this section.
The Taliq display is controlled by the display control circuitry. A photosensor measures ambient light
and automatically switches on the electroluminescent display backlight during low light conditions.
The display control circuitry is divided into the following subsections:
9.10.13.1 Control Conditioning Circuit
The control conditioning circuit, located on the main PCB (Figure 9-6), processes signals generated
by the CPU to produce timing signals for the display drivers.
9.10.13.2 Display Driver ICs
The display driver ICs are located on the auxiliary PCB (Figure 9-6). Each of the two display driver
ICs have 32 high-voltage outputs that enable individual segments of the display to be turned on or off.
9.10.13.3 High Voltage Control Circuit
The high voltage control circuit is located on the auxiliary PCB (Figure 9-6). The high voltage control
circuit allows the CPU to switch on or off the display's high voltage input.
Main PCB
Microprocessor
9-20
BEEP_1
BEEP_2
Figure 9-21: Audio Output Circuit
Main PCB
Control
conditioning
circuit
(generates
timing
signals)
AUX PCB
High voltage
control circuit
(enables
+70 VDC
to display)
Figure 9-6: Display Control Block Diagram
BZ1
TP74
TEST
TP75
TEST AT17
BEEPER
AUX PCB
Display driver
(1)
Display driver
(2)
70 volts
Display
Display
backlight