Mallinckrodt NELLCOR NPB-4000 Service Manual page 138

Section 13: Microprocessor Computer and Control –Theory of Operation
13.16.1 Block Diagram
13-24
Page 2 of the schematics contains the NIBP circuits, the speaker circuits, the
LCD contrast circuit, the battery voltage and temperature input circuits, the
push-switch interface circuit, and the backlight interface connector circuit. The
50-pin connector connects to the recorder, RS-232 connector, the power supply
connector, the defib connector, and the speaker.
The speaker circuit consists of a software-programmable digital potentiometer
chip, U28, which is a Dallas DS1666S-10. The software sets the tone frequency
by setting the high register with a value, setting the low register with a value,
then programming the digital potentiometer up/down according to the timing
specified in the requirement specification.
See Figure 13-15. The following diagram is a block diagram of the digital
section of the NPB-4000/C design.
Figure 13-15: Digital Section Block Diagram