Appendix A. Theory Of Operation; Overview; Figure 8: System Block Diagram - Welch Allyn SureTemp 986 Service Manual

Appendix A. Theory of Operation

A.1 Overview
Model 986 PCA
A/D Converter
Circuit
Voltage
Reference Circuit
Battery Detector
Circuit
Model 986 Thermometer
The Model 986 includes two integrated circuits which provide most
of the microcontroller and analog circuit functions.
•
All control and display functions are governed by the
microcontroller (U1).
•
All probe analog data interfacing to the microcontroller is
provided by U2. U2 functions as an A/D converter.
Probe resistance measurements are made by taking the ratio of
two pulse widths that are generated in U2 by sequentially switching
in one calibration resistor and the probe thermistor. These pulse
widths are measured by the microprocessor, which calculates the
probe resistance. The actual probe temperature is then calculated
from the probe resistance.
NEC UPD 789407 uP
16 Bit
Timer
8 Bit
A/D
Probe Warmer
Circuit
Four Conductor
Coil Cord
Probe
Thermistor
Warmer
Probe Assembly

Figure 8: System Block Diagram

CPU
LCD
Driver
Probe
Liquid Crystal
Configuration
Display
Jumpers
Configuration
Serial Serial
I/O Port
Mode
Horn
Button
Driver
Horn
2 AAA
Batteries
29