Welch Allyn 9000 User manual

Model 9000 and Model 9020
Infrared Thermometer
Technical Manual
Thermo Menu
9000 Operation Manual
9020 Operation Manual

© 1997 by Welch Allyn, Inc. All rights reserved. No part of this manual may be reproduced or transmitted in any
form or by any means, electronic or mechanical, including photocopy, without prior consent in writing from Welch
Allyn. Printed in the U.S.A.
Welch Allyn is a trademark of Welch Allyn, Inc. All rights reserved.
The CE mark on this product indicates it has been
tested to and conforms with the provisions noted
within the 93/42/EEC Medical Device Directive
0050
European Regulatory Manager, Medical Division
Welch Allyn Ltd., Navan, Co. Meath
Republic of Ireland

Technical Manual
i
TABLE OF CONTENTS
INTRODUCTION........................................................................................................................ 1
Changing the Probe Cover Dispenser..................................................................................... 2
CLEANING THE THERMOMETER............................................................................................ 3
Routine Cleaning .................................................................................................................... 3
ETO Gas Sterilization Procedure.........................................................................................3
Cleaning the Probe Sensor Lens ............................................................................................ 4
Method 1: Removing Light Dust.......................................................................................... 4
Method 2: Removing Smudges or Excessive Debris........................................................... 5
CHANGING THE PROBE TIP.................................................................................................... 7
REPLACING THE BATTERIES.................................................................................................. 7
THEFT PROTECTION SYSTEM................................................................................................ 9
Security Time-out function ......................................................................................................9
Programming the Security Timer ............................................................................................ 9
TROUBLESHOOTING............................................................................................................. 11
ERROR CODES ...................................................................................................................... 13
THERMOMETER DISASSEMBLY ........................................................................................... 15
THERMOMETER REASSEMBLY ............................................................................................ 16
THEORY OF OPERATION (ELECTRICAL)............................................................................. 17
Overview............................................................................................................................... 17
Microprocessor ..................................................................................................................... 17
Power Up Reset.................................................................................................................... 18
Microprocessor Clocks.......................................................................................................... 18
Display Circuit....................................................................................................................... 18
EEPROM.............................................................................................................................. 18
Analog IC U1 ........................................................................................................................ 18
A/D Converter Circuit............................................................................................................ 19
Internal A/D Calibration Circuit.............................................................................................. 19
Calibration Check Circuit.......................................................................................................19
Thermometer Wakeup.......................................................................................................... 19
IR Temperature Measurement Cycle .................................................................................... 20
HORN CIRCUIT....................................................................................................................20
BACKLIGHT CIRCUIT.......................................................................................................... 21
Wall Mount Security Switch (Model 9000 Only)..................................................................... 21
THEORY OF OPERATION (MECHANICAL)............................................................................ 21

Welch Allyn Model 9000/Model 9020
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MODEL 9000 CALIBRATION CHECK & ADJUSTMENT ......................................................... 22
MODEL 9020 CALIBRATION CHECK & ADJUSTMENT ......................................................... 25
FACTORY SERVICEABLE ONLY REPAIRS ........................................................................... 27
FIELD SERVICEABLE REPAIRS............................................................................................. 27
FIELD SERVICEABLE PARTS LIST........................................................................................ 28
SUGGESTED SPARE PARTS (PER 100 UNITS).................................................................... 31
OPTIONS & ACCESSORIES................................................................................................... 32
LIMITED WARRANTY - MODEL 9000..................................................................................... 40
LIMITED WARRANTY - MODEL 9020..................................................................................... 41

Technical Manual
1
INTRODUCTION
This manual provides technical information about the electronic and mechanical
operation of the Welch Allyn Model 9000 series infrared thermometers. It is intended for
use by trained service representatives.
This manual is applicable for Model 9000 thermometers with serial numbers equal to
and greater than 5262151 and for Model 9020 thermometers with serial numbers equal
to and greater than 4160001. If you have an earlier serial number, please contact our
Customer Service Department to obtain the correct technical manual.
Please refer to your
Users’ Manual
supplied with your thermometer for operating
instructions.

Welch Allyn Model 9000/Model 9020
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Changing the Probe Cover Dispenser
The thermometer handle will not slide if a dispenser is not installed or if it is empty. A
vertical window in the handle of the thermometer allows you to see when the probe
cover dispenser is empty. To replace the dispenser:
:
Figure A
1. Ensure that the probe is fully retracted, then
remove the probe cover dispenser from the
thermometer’s handle by pulling on the tab at the
end of the dispenser (Figure A). Discard the
empty dispenser.
2. Check to see that the first probe cover in the
new dispenser is positioned properly (Figure A).
3. Insert the new probe cover dispenser (flat end
first) into the handle of the thermometer. Align
the edges of the dispenser with the slots inside
of the handle, then slide the dispenser upward
until it snaps into place (Figure B). The
thermometer is now ready for use.
You should routinely take a moment to inspect
the sensor lens for clarity and debris, and it is
recommended that the sensor lens be cleaned
as needed. Refer to “Cleaning the Probe Sensor
Lens” on page 4.
Figure B

Technical Manual
3
CLEANING THE THERMOMETER
Routine Cleaning
Clean the exterior of both the thermometer and the wall mount as needed. Wipe all
surfaces with a clean cloth, dampened with warm water and a mild detergent or
germicidal cleaner (see the table below for acceptable cleaning agents).
• As some solutions may stain, try a small test area first.
• Make sure that the cloth is damp, but
not
wet.
• Do not allow cleaning solution to run inside of the thermometer.
• Never immerse the thermometer in the cleaning solution.
CLEANING AGENT LENS THERMOMETER WALL MOUNT
Ammonium ✔✔
Bleach (50%) ✔✔
Cidex ✔✔
Sporicidin® ✔
MetriSpray™ ✔
Detergent (mild) ✔✔
Isopropyl Alcohol ✔✔ ✔
Lysol ✔✔
ETO Gas Sterilization Procedure
When no other form of decontamination such as a germicidal wipe is acceptable, a low
temperature (not to exceed 130°F) ETO gas sterilization cycle may be used. Refer to
your hospital standard operating procedure for length of ETO cycle.
This type of sterilization may cause some hazing of glossy plastic surfaces and should
be used only when absolutely necessary.
1. Ensure that the probe is retracted, then remove the probe cover dispenser from
the thermometer’s handle.
2. Remove the screw that secures the battery compartment door, press the release
notch to remove the door, then remove the batteries.
NOTE: Leaving batteries in the thermometer during the sterilization procedure may
present an explosion hazard.
3. Wrap the thermometer in a standard sterilization type packaging, such as the
Baxter Tower Dualpeel Sterilization Pouch.

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4. ETO gas sterilize (temperature not to exceed 130°F) and aerate.
5. Remove the sterilization packaging.
6. Follow the steps for “Cleaning the Probe Sensor Lens – Method 2” described in
the next section.
7. Insert the probe cover dispenser back into the thermometer’s handle.
8. Reinstall the batteries in the
exact
order and position as shown in the diagram
inside of the battery compartment, then secure the battery compartment door.
9. Before use, allow the thermometer to stabilize to room temperature for at least
one
hour,
then verify calibration using a Welch Allyn Model 9600 Calibration Tester.
Cleaning the Probe Sensor Lens
Dust and other minute particles of environmental debris can build up on the probe
sensor lens during normal use of the thermometer.
The probe sensor lens should always be shiny and clear. Inspect the lens frequently
and clean as necessary. The lens should also be cleaned as routine maintenance
according to the two cleaning methods below.
NOTE: A dirty lens will affect the accuracy of temperature readings.
Method 1: Removing Light Dust
(Recommended every 24 hours)
1. Advance the slide to extend the probe tip.
2. Leave the probe cover in place. Using a dry cotton swab, hold the slide and gently
press the probe cover against the sensor lens (Figure C). Loose debris on the lens
will adhere to the probe cover.
Figure C
3. Move the slide back to its original position to remove the probe cover and retract
the probe tip. The thermometer is now ready for use.

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Method 2: Removing Smudges or Excessive Debris
(RECOMMENDED AS NEEDED OR IF LOW TEMPERATURE READINGS PERSIST
AFTER METHOD #1)
MATERIALS NEEDED • Isopropyl Alcohol
• Cotton Swab
1. Advance the slide to extend the probe tip.
2. Manually remove the probe cover to expose the probe sensor lens.
3. Using a cotton swab slightly moistened with isopropyl alcohol, gently wipe the
surface of the lens to remove debris (Figure D).
4. Wipe the lens with a dry cotton swab to absorb moisture and pick up any
remaining debris (Figure D).
5. Move the slide back to its original position to retract the probe tip.
6. Allow 30 minutes before using the thermometer.
Figure D

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CHANGING THE PROBE TIP
A damaged probe tip can tear the ultra-thin film in the probe covers. A temperature
taken with a torn probe cover may result in a measurement that is higher than the
actual temperature and cause debris to collect on the lens which may then result in
lower temperature readings.
Periodically inspect the plastic probe tip for nicks or other damage, and replace if
necessary.
MATERIALS NEEDED Probe Tip Replacement Kit (Welch Allyn Part #20688-000)
1. Advance the slide to extend the probe tip.
2. Manually remove the probe cover to expose the probe tip.
3. Remove the probe tip by turning it counterclockwise (Figure E). Discard the
damaged probe tip.
4. Install the new probe tip by turning it clockwise, then hand-tightening (5 inch-lbs.)
until it is positioned snugly against the base of the probe. Do not overtighten the
tip.
5. As a precaution, clean the sensor lens to remove any dust or debris that may have
adhered to the exposed lens during replacement of the probe tip.
Follow the steps for “Cleaning the Probe Sensor Lens – Method 2” described
earlier in this section of the manual.
Figure E
REPLACING THE BATTERIES
By returning the thermometer to the wall mount after use, a set of batteries will provide
power to the thermometer for up to 20,000 temperatures, or two years (this will vary
with backlight usage and time outside of the wall mount). When battery power is low, a
Battery symbol appears on the display and the thermometer cannot be used until they
are replaced.

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1. Remove the screw that secures the battery compartment door, then press the
release notch to remove the door (see Figure F).
2. Remove the four depleted batteries and dispose of properly.
3. Press the SCAN button to reset the thermometer (this discharges the battery
circuit capacitor C1).
4. Install four new AAA size Alkaline batteries in the
exact
order and position as
shown in the diagram inside of the battery compartment.
The thermometer will automatically perform a diagnostic self-test of the electronics
(indicated by a segment cycling through the display). If not, reinstall the batteries
to ensure correct positioning.
Figure F
5. Replace the battery compartment door, then reinstall the screw to secure the door
(Figure F).
6. Note that the Model 9000 will be in a security time-out condition when the Theft
Protection System is enabled (“SEC” will appear on the display). Return the
thermometer to the wall mount to reset the security timer, and resume normal
operation.

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THEFT PROTECTION SYSTEM
MODEL 9000 ONLY
The wall mount serves as a base for the Model 9000
when it is not in use. While in the wall mount, the
thermometer remains in a “sleep” mode to preserve
battery power.
Upon return to the wall mount, the 9000 “clicks” into
place, held by a spring latch mechanism. To remove
the Model 9000, move the latch mechanism’s release
key to the right while lifting the thermometer out of the
wall mount (as shown).
For additional theft protection, lock the Model 9000
into the wall mount by rotating the release key 90° to
the right. Once in this position, the key can be
removed by pulling downward.
An optional security cable (Welch Allyn Part #20796-000) may be added to the
thermometer and wall mount for single area use.
Security Time-out function
The wall mount also includes a programmable time-out
function. If the Model 9000 is not returned to the wall mount
within a specified time period, the instrument enters a security
time-out condition and ceases to function.
There is a 15-minute warning period prior to the security time-
out condition. During this time, the Model 9000 continues to
operate normally and “SEC” appears when a temperature is not
displayed.
Once in a security time-out condition, when any button is pressed, “SEC” is displayed
and the Model 9000 beeps rapidly to indicate a security status.
After the 9000 is returned to the wall mount, the security timer is reset and the
thermometer is ready for use.
Programming the Security Timer
The built-in security timer may be programmed in one-hour increments from 0 to 8
hours. The timer is preset to 8 hours at the factory.
NOTE: Setting the timer to zero (0) disables the time-out function.

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If you wish to change the current security time-out setting, follow these steps to initialize
the security programming mode:
1. Place the thermometer into the wall mount, then advance the slide to extend the
probe (the display will remain blank).
2. Simultaneously press the SCAN button and MODE switch (insert a small diameter
object, such as a paper clip, into the small hole in the MODE switch).
3. Remove the thermometer from the wall mount to activate the display. The current
time-out setting will be displayed.
4. The setting is incremented by one hour each time the °F/°C switch is pressed; it is
decremented by one hour each time the PULSE TIMER button is pressed.
5. When the number of hours desired for security time-out is displayed, retract the
probe and return the thermometer to the wall mount.
The Model 9000 will automatically perform a diagnostic self-test of the electronics
(indicated by a segment cycling through the display). Once this test is complete,
the thermometer is ready for use.

Welch Allyn Model 9000/Model 9020
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TROUBLESHOOTING
PROBLEM SOLUTION
Low Temperatures Debris on probe sensor lens. Clean lens. See “Maintenance” section of this manual for
procedure
Improper technique–refer to the Users’ Manual. Insert probe snugly into ear, completely
closing off ear canal..
Check current Operation Mode . Oral mode obtains lower temperatures than core and
rectal modes (test mode is used for calibration only).
Probe tip not installed securely; hand tighten tip (clockwise) until snug against base of
probe. See “Maintenance” section of this manual for procedure.
Unit out of calibration. Refer to the Calibration Testing section of this manual.
High Temperatures Check current operation mode. Core and rectal modes obtain higher temperatures than
oral mode (test mode is used for calibration only).
Improper technique–refer to the Users’ Manual. Insert probe snugly into ear, completely
closing off ear canal.
Detected warmer temperature prior to inserting probe into ear, or heat source (such as a
heat lamp) nearby when taking temperature. Insert probe into ear, wait 2 seconds, then
press SCAN button.
Check integrity of probe cover prior to taking temperature. Damaged (or missing) probe
cover can result in temperature reading up to 1.5°F higher than actual temperature.
Slide Will Not Move Probe cover dispenser missing or empty. Check position of dispenser in thermometer’s
handle. If empty, install new dispenser.
Probe covers positioned improperly within probe cover dispenser. Remove dispenser
from thermometer handle. Pull on first probe cover to remove any misaligned or bent
covers. Reposition first probe cover to extend 1/8" beyond top of dispenser, then
reinstall dispenser to thermometer handle. If problem persists, replace probe cover
dispenser. See "Maintenance" section of this manual for procedure.
Probe tip not installed securely and may be lodged against reference arm (shields lens
when probe retracted). Looking into front of thermometer, press down on reference arm
while moving slide forward. Hand tighten tip (clockwise) until snug against base of
probe, then retract probe. See "Maintenance" section of this manual for procedure.
Inconsistent Results
of Repeated
Temperatures
Not enough time elapsed since last temperature taken (same ear). Insertion of probe
results in slight “draw down” of ear canal temperature. Wait 2 minutes to allow
temperature to stabilize between repeated temperature measurements in the same ear,
or use patient’s opposite ear.
Improper technique–refer to the Users’ Manual. Insert probe snugly into ear, completely
closing off ear canal. If ear canal is curved significantly, sensor's view of the tympanic
membrane may be obstructed. Gently pull up and back on external ear to straighten ear
canal before inserting probe.

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PROBLEM SOLUTION
Improper Delivery of
Probe Cover
or
Probe covers positioned improperly within probe cover dispenser. Remove dispenser
from thermometer handle. Pull on first probe cover to remove any misaligned or bent
covers. Reposition first probe cover to extend 1/8" beyond top of dispenser, then
reinstall dispenser to thermometer handle. If problem persists, replace probe cover
dispenser. See “Maintenance” section of this manual.
Probe Cover
Positioned
Incorrectly on Probe
Obstruction between probe and probe cover dispenser. Remove dispenser, inspect
thermometer handle and remove debris, if any. Ensure probe covers are properly
positioned in probe cover dispenser, then reinsert dispenser into handle.
Probe tip is damaged. Replace probe tip. See “Maintenance” section of this manual for
procedure.
Damaged probe covers. Replace probe cover dispenser.
Probe Cover Film
Breakage Probe tip is damaged. Replace with new probe tip. See “Maintenance” section of this manual for procedure.
Slide moved forward too quickly.
Probe covers positioned improperly within probe cover dispenser. Remove dispenser
from thermometer handle. Pull on first probe cover to remove any misaligned or bent
covers. Reposition first probe cover to extend 1/8" beyond top of dispenser, then
reinstall dispenser to thermometer handle. If problem persists, replace probe cover
dispenser. See "Maintenance" section of this manual for procedure.
Continuous or
Intermittent Audible
Tone
Probe is partially extended. Ensure slide is fully retracted.
Improper operating condition. Check display for error status. See “Error Codes” section
of this manual for explanation.
“SEC” Displayed 9000 locked in security time-out condition. Return thermometer to wall mount to reset
Theft Protection timer, then resume normal operation.
Blank LCD Display Probe extended for over 5 minutes. Retract probe.
Remove thermometer from wall mount. Move the slide handle forward to activate the
display.
9000 locked in security time-out condition. Return thermometer to wall mount to reset
Theft Protection timer, then resume normal operation.
Battery power depleted. Replace batteries. See “Maintenance” section of this manual for
procedure.
Ensure that all switch lines are at Vcc.
Unable to Remove
the Thermometer
from Wall Mount
Wall mount’s spring latch mechanism is engaged. Move release key (underside of wall
mount) to the right while removing thermometer.
Release key for spring latch mechanism (underside of wall mount) is missing or locked.
Insert key into mechanism, then rotate key clockwise until tab is perpendicular to front
panel of wall mount. Move release key to the right while removing thermometer.

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ERROR CODES
When the thermometer detects an improper operating condition, the malfunction
symbol (X) will appear with one of the numeric error codes (Er) below.
Set the thermometer aside for one hour to allow the
electronics to stabilize to room temperature.
• If the thermometer is faulty, the error status will remain to
indicate that the thermometer needs service.
• If the condition is transient in nature, the error status will
disappear. The thermometer will then self-calibrate, and
be ready for use.
ERROR EXPLANATION
Er0
Er1
Er2
Er6
These error codes indicate possible thermistor reference circuit problems.
1 The resistance of the thermistor should read about 10k when measured from J2-2 to
ground with power removed, thermometer requires service.
2. The voltage from J2-2 to ground with power applied and display test complete is about 1.2
volts with the thermistor in the circuit at room temperature, thermometer requires service.
Er3 Sensor pulse width is not between PTB and Hical values. Check PTB and Hical digital signals
(U3-13, 12). Check C1 and C2. Internal error; thermometer requires service.
Er4 Sensor pulse width exceeding 95 milliseconds. Usually a basic A/D converter circuit fault (non-
operating). Check 1.2 and 2.4 volt references. (U1-3, 4). Check U2 power (U2-7). Possible
open IR sensor circuit or 4-conductor flex strip. Infrared sensor may be detecting nearby
temperature source greater than 110°F. If not, thermometer requires service.
A/D circuit voltages should be as follows after applying power and after the display test is
complete:
1. Power @ U2-7 = Vcc
Q1-C = Vcc
J8-8 = 0 volts, U4-53 = 0 volts
2. Reference voltages
U2-2 = 1.2 volts (C2)
U2-3 = 1.2 volts (should be equal to U2-2)
U2-3 lower than U2-2 may indicate that C1 is bad
U3-2 = 2.4 volts
U1-14 = 1.2 volts
U4-56 = Vcc
3. U2 output reset prior to starting A/D conversions
U2-6 = 2.4 volts (must be equal to 2.4 volt reference)
U1-6 = 2.4 volts
U1-5 = 1.2 volts
4. Microprocessor I/O prior to starting A/D conversions
U1-7 = Vcc, U4-51 = Vcc
U1-10 = Vcc, U4-13 = Vcc
U1-11 = 0v, U4-18 = 0v

Technical Manual
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ERROR EXPLANATION
Er5 Sensor resistance pulse width exceeding 95 milliseconds. Ambient temperature is above
110°F. If not, thermometer requires service.
Er6 Thermistor reference pulse width exceeding 95 milliseconds. Check thermistor reference
circuit. If not, thermometer requires service.
Er7 Hical pulse width exceeding 95 milliseconds. Check probe switch circuit including 4-conductor
flex cable. If not, internal error; thermometer requires service.
Er8 LoCal pulse width exceeding 95 milliseconds. Check probe switch circuit including 4-conductor
flex cable. If not internal error; thermometer requires service.
Er9 Calibration self test is out of specification, thermometer requires service.
X only Microprocessor or EEPROM test failed, thermometer requires service
In assembled thermometers with the probes extended, error codes 3-8 may be caused
by a bad probe switch circuit. Check both the probe switch and the 4-conductor flex
strip.
THERMOMETER DISASSEMBLY
NOTE: Extreme care must be taken at all times to prevent kinking or damage to the
sensor or reference arm flex cables. Creases may cause premature failure of these
cables.
Follow these steps to disassemble the thermometer (see Figure G at the back of this
manual).
1. Remove the battery compartment screw (34), battery compartment cover (2), and
batteries (1).
2. Remove the slide handle (35) by gently pulling outward on the bottom of one side
of the slide.
3. Remove four labels (24) from the thermometer: the label on the slide, the label
around the display window, the label below the SCAN button, and the diamond-
shaped label on the left side of the case.
4. Remove the thermometer case screws. Using a flat surface, lay the thermometer
on the left case half (8) and carefully separate the case halves (7) (8), keeping the
internal thermometer components in the lower case half.
5. At this level, the ring gear (14), bell crank (6), pawl (28), actuating lever (57), and
lever spring (36) may be inspected and replaced without total removal of the circuit
board assemblies. The actuating lever and spring must be lifted out before
removing the ring gear, bell crank, and pawl. The ring gear, bell crank, and pawl

Welch Allyn Model 9000/Model 9020
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can then be removed by lifting slightly on the front edge of the analog circuit board
to allow the assembly to be lifted out.
6. The entire electronics assembly may be removed by gently lifting the display and
analog boards out of the lower case half while also lifting the sensor assembly (52)
and reference assembly (41), being careful not to stress or kink any of the
connecting flex cables. The scan switch (38) and its flex cable may be removed
by sliding the cable out from underneath the cable retainers along the groove and
then lifting out the switch. The scan switch cable may be desoldered from the
analog board and left in the lower case half.
THERMOMETER REASSEMBLY
NOTE: Extreme care must be taken at all times to prevent kinking or damage to the
sensor or reference arm flex cables. Creases may cause premature failure of these
cables.
Follow these steps to reassemble the thermometer (see Figures G, H).
1. Reassemble the thermometer into the left case (8). Install the scan switch
assembly (38) into its cavity.
2. Route the connecting flex cable along the channel and under the cable retaining
clips.
3. Assemble the ring gear (14), O ring (27), bell crank (6), pawl (28), and pawl spring
(50) and install onto the case boss.
4. Install the actuating lever (51) and spring (36) onto the boss; position the straight
spring end into the slot on the top of the boss as shown in Figure H.
5. Install the display and analog circuit board assemblies into the case, taking care to
locate the board's positive battery contact in the case channel and ensuring that
the 22 conductor flex strip is not being kinked by the rear analog PCB case stop.
6. Position the reference arm assembly (41) around the sensor assembly with the
rear reference tabs located in the side grooves of the sensor slide body.
7. Insert the sensor and reference assemblies into the case (at its rear most position)
making sure that the ratchet spring (49) is positioned as shown.
8. Locate the tabs on the reference assembly into the oval shaped guides in the
case.
9. Install the window (17) into its cavity in the case.
10. While rotating the position of the lever arm as necessary to allow the reference
arm to travel downward, move the slide assembly forward and locate the pawl into
the position shown in Figure I.
11. Move the slide assembly back towards the display about 1/8" to cause the ratchet
spring to ride against the left case half.

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12. Install the right case half (7) applying gentle pressure until the two case halves are
together. If the case does not close completely, carefully remove the right half and
check the position of the PCB’s, battery contacts, cables, and reference arm tabs.
13. Turn the thermometer over on the right case half. Install the two case fastening
screws (33).
14. Install the slide handle (35) by spreading it slightly and pressing it down onto the
tabs extending out of the thermometer case slide while lining them up with the
matching recesses in the slide handle.
15. Install a probe cover dispenser with probe covers.
16. Test the thermometer slide action to ensure that it moves normally.
17. Install the four AAA batteries (1) according to the diagram on the inside of the
battery compartment.
18. Install the battery compartment cover and screw (2, 34), then observe the power
up display test.
19. Install new labels
THEORY OF OPERATION (ELECTRICAL)
Overview
(Refer To Schematic & Block Diagram)
The infrared temperature measuring circuitry consists of infrared detector SEN1,
thermistor reference TH1, and an integrating A/D converter consisting of integrator U2,
and various support circuitry including U1 and U3. The A/D control and display
functions are performed by the microprocessor U4 and the display DS1. Calibration
and additional thermometer characteristic data are stored in the nonvolatile EEPROM
U5. The backlight system consists of an inverter circuit including Q2 and transformer
T1 which provide drive for the electroluminescent panel DS2. The audible tones are
created by a signal generated by microprocessor U4 driving horn LS1.
Microprocessor
The microprocessor U4 is an 8 bit low power CMOS microprocessor. It is a Mitsubishi
type M38203 containing a custom proprietary program in its 16,384 [8192] byte ROM
and 512 byte RAM. This device also contains several internal timers and the LCD
driver.
Power Up Reset
The power up reset for the microprocessor is generated by C13, D1, and R19 which
provides a low reset with about a 10 millisecond time constant to return high. This will
occur each time batteries are installed. Following the reset, the microprocessor
performs internal RAM and ROM, and external EEPROM checksum tests. A visual

Welch Allyn Model 9000/Model 9020
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display test is then initiated which sequentially activates and then deactivates each
segment on the LCD.
Microprocessor Clocks
A 4 MHz resonator and r/c components are connected to U4-30, 31 to provide the
primary system clock. This 4 MHz clock output can be monitored on U4-31. The clock
is active during all analog conversion functions such as temperature taking and
calibration. The clock will be seen to turn off between calibrations.
A 32.768 KHz resonator and r/c components are connected to U4-28,29 to provide a
low frequency clock for timing and low power operation. This clock turns off only while
the thermometer is in the security wall mount, which places all internal circuitry into a
micropower sleep mode.
Display Circuit
The Diplex type liquid crystal display (DS1) is directly driven by the microprocessor
segment outputs SEG0-SEG23 and common outputs COM0,COM1. The display
operating voltages are generated by a voltage divider (R23, R24, and R25) which
supplies 1/3 Vcc to U4-7, 8 and 2/3 Vcc to U4-6 when the display power is turned on
by microprocessor output U4-49 being low.
EEPROM
The EEPROM U5 is nonvolatile memory which stores calibration and characteristics
data that are entered during factory calibration. The constants contained in memory
are unique to each thermometer and require programming for the thermometer to be
functional. The EEPROM is also used to record the current operating mode, °F/°C
setting, and security time-out setting. A change in operating mode, °F/°C, or security
time-out is written to the EEPROM each time the thermometer is placed into the
security wall mount. At power up (batteries installed), the EEPROM constants are
copied into the microprocessor RAM for frequent use. Each time a temperature is to be
taken, a test of the checksum for the EEPROM data transferred to the microprocessor
is performed and, if an error has occurred, the data are reloaded into the
microprocessor. To conserve battery power, the ground and all data and clock lines for
the EEPROM are set high by the microprocessor when the EEPROM is not being read.
Analog IC U1
U1 is a Welch Allyn custom bipolar IC which contains a 1.2 and 2.4 volt reference, a
low battery detector, and circuits associated with performing the A/D converter function.
The A/D power line U1-14 is pulled to about 1.2 volts by microprocessor output U4-56
going high through R13 to activate this IC. The low battery detector output U1-12 goes
low when the battery voltage drops below approximately 4.4 volts.