Vent 101 User manual

HAND-HELD RESPIRATORY
MECHANICS MONITOR
Service Manual
Model 101
April 17, 2000
Catalog No. 6800-90-01
Novametrix Medical Systems Inc.
PO Box 690
5 Technology Drive
Wallingford, Connecticut, U.S.A. 06492

Revision History
Rev. 01 Model 101 Service Manual
iii
Revision History
22-May-98 Release
17-Apr-00 Revision 01, R-N746

Revision History
iv
Model 101 Service Manual Rev. 01
[This page intentionally blank.]

Rev. 01 Model 101 Service Manual
v
Table of Contents
Safety ..................................................................................................... 1
Introduction ........................................................................................... 3
Ke
y
pad Controls and Indicators ........................................................................4
S
y
mbols .............................................................................................................4
Theory of Operation ............................................................................. 5
Main Board ........................................................................................................5
Power Supply ..............................................................................................................5
Reference Voltages .....................................................................................................5
Battery Charger ...........................................................................................................5
On/Off Control .............................................................................................................6
Digital Control and Microprocessor .............................................................................6
Serial Communication .................................................................................................7
Audio ...........................................................................................................................7
Interface ......................................................................................................................8
Analog Control Signals ................................................................................................8
Sensor Identification ....................................................................................................8
Flow Zeroing ...............................................................................................................8
Flow Circuitry ..............................................................................................................9
Barometric and Airway Pressure ...............................................................................10
Interface Board ................................................................................................11
Key panel Interface ...................................................................................................11
Display interface ........................................................................................................11
LED control ...............................................................................................................11
Power Supplies .........................................................................................................11
Functional Tests .................................................................................13
Equipment Required ........................................................................................13
Procedure ........................................................................................................13
Accuracy Tests ................................................................................... 15
Equipment Required ........................................................................................15
Procedure ........................................................................................................15
Electrical Tests ...................................................................................17
Equipment Required ........................................................................................17

Table of Contents
vi
Model 101 Service Manual Rev. 01
Procedure ........................................................................................................ 17
Power Supplies .........................................................................................................17
Airway Pressure Calibration ......................................................................................18
Differential Pressure Calibration ...............................................................................19
Barometric Pressure Calibration .....................................................................19
Maintenance ........................................................................................21
General ........................................................................................................... 21
Maintenance Schedules .................................................................................. 21
Cleanin
g
and Sterilization ................................................................................ 22
Batter
y
and AC Operation ............................................................................... 22
Battery Installation ....................................................................................................23
External Battery Charger ..........................................................................................23
Features Connector ..................................................................................................23
Assembl
y
Exchan
g
es ......................................................................................24
Disassembling the Monitor ........................................................................................24
Reassembling the monitor ........................................................................................26
Accessories .........................................................................................27
Parts Lists ............................................................................................29
6800-00 Final Assembly, Model 101 .........................................................................29
6800-01 Main Assembly, Model 101 .........................................................................29
2740-01 01 Main Board Assy, Model 101 .................................................................30
2740-17 01 Main Board Subassy, Model 101 ...........................................................30
2739-01 00 Battery & Comm Interface Board Assy ..................................................32
2741-01 00 Interface Board Assy, Model 101 ...........................................................32
6935-48 00 Test Fixture, Flow Leak Test .................................................................33
Drawings and Schematics .................................................................35

Rev. 01 Model 101 Service Manual
vii
Guarantee
Equipment manufactured or distributed by Novametrix Medical Systems Inc., is fully guaranteed,
covering materials and workmanship, for a period of one year from the date of shipment, except for
certain disposable products and products with stated guaranteesother than one year. Novametrix reserves
the right to perform guarantee service(s) at its factory, at an authorized repair station, or at the customer’s
installation.
Novametrix’ obligations under this guarantee are limited to repairs, or at Novametrix’ option,
replacement of any defective parts of our equipment, except fuses, batteries, and calibration gasses,
without charge, if said defects occur during normal service.
Claims for damages during shipment must be filed promptly with the transportation company. All
correspondence concerning the equipment must specify both the model name and number, and the serial
number as it appears on the equipment.
Improper use, mishandling, tampering with, or operation of the equipment without following specific
operating instructions will void this guarantee and release Novametrix from any further guarantee
obligations.
Caution: Federal (U.S.A.) law restricts this device to sale, distribution, or use by or on the order of a
licensed medical practitioner.
Copyright 1998, 2000 Novametrix Medical Systems Inc. This document contains information which
is proprietary and the property of Novametrix Medical Systems Inc., and may not be reproduced, stored
in a retrieval system, translated, transcribed, or transmitted, in any form, or by any means, without prior
explicit written permission from Novametrix Medical Systems Inc.
Declaration of Conformity with European Union Directive
The Authorized Representative for Novametrix equipment is:
D.R.M. Green
European Compliance Services Limited
Oakdene House
Oak Road
Watchfield
Swindon, Wilts SN6 8TD
Service Department
For factory repair service, call toll free
1-800-243-3444
In Connecticut, call Collect [phone]
FAX [phone]
http://www.novametrix.com
Email [e-mail]

Guarantee
viii
Model 101 Service Manual Rev. 01
Service Policy
Novametrix Medical Systems Inc. provides 24-hour a day access to technical support through its
Technical Support Department in Wallingford, Connecticut, and company Service Representatives
located throughoutthe United States. (Outside the U.S., primary technical support is handled through our
qualified international sales and service distributors.)
Novametrix will provide Warranty Service support within 48 hours of receiving a request for assistance.
Contact the Technical Support Department by telephone toll free at 800-243-3444, or 203-265-7701; by
facsimile at 203-284-0753; or, by e-mail at [e-mail]. After hours telephone support
requests (before 8:00 AM and after 5:00 PM Eastern Time) will be responded to promptly by the
Technical Support on-call staff. After hours facsimile and e-mail requests will be answered the next
business day. It is suggestedthat any person calling in for technical support have the equipment available
for product identification and preliminary troubleshooting.
Novametrix reserves the right to repair or replace any product found to be defective during the warranty
period. Repair may be provided in the form of replacement exchange parts or accessories, on-site
technical repair assistance or complete system exchanges. Repairs provided due to product abuse or
misuse will be considered “non-warranty”and invoiced at the prevailing service rate. Replaced or
exchanged materials are expected to be returned to Novametrix within 10 days in order to avoid
(additional) charges. Return materials should be cleaned as necessary and sent directly to Novametrix
using the return paperwork and shipping label(s) provided. (Transferring return materials to a local sales
or dealer representatives does not absolve you of your return responsibility.)
Novametrix manufactures equipment that is generally field serviceable. When repair parts are provided,
the recipient can call Technical Support for parts replacement assistance and repair assurance. In the
event a replacement part requires increased technical capability, Technical Support may request
Biomedical assistance, provide on-site technical support or complete replacement equipment. If the
customer requires the return of their original product, the exchange material will be considered “loaner
material”and exchanged again after the customer equipment is repaired.
Novametrix promotes customer participation in warranty repairs, should they become necessary. A
longer useful product life, and quicker, more cost-effective maintenance and repair cycles—both during
and after the warranty period, are benefits of a smooth transition into self-maintenance. The Technical
Support Department can provide technical product support at a level appropriate to your protocol and
budget requirements.
Please contact Technical Support for information on these additional programs and services:
•Focus Series Technical Training Seminars
•Test Equipment and Test Kits
•Service Contract / Parts Insurance Plans
•On-Site Technical Support
•“Demand Services”including
Flat rate parts-exchange
Flat rate return for repair
Time and Material
•Full warranty, discounted replacement sensors

Rev. 01 Model 101 Service Manual
1
Section 1
Safety
The VENT✔Handneld Respiratory Mechanics Monitor, Model 101, is electrically isolated. Patient
leakage current flowing from the instrument to ground is limited to less than 10 uA.
•Keep the VENT✔and its accessories clean. Do not operate the VENT✔when it is wet due to spills
or condensation.
•Connect only Novametrix Series 3 Flow Sensors to the VENT✔. For maximum performance; keep
the pressure sensor ports oriented upward, and keep the sensor clear of moisture and secretions by
proper breathing circuit maintenance.
•Connect the sensor first to the VENT✔and then to the patient breathing circuit in order to limit
circuit volume loss and to avoid excessive moisture build-up in the flow sensor tubing.
•VENT✔has electrically isolated inputs. Patient leakage current flowing from the instrument to
ground is limited to less than 10 µA at 120 V, 60 Hz. Patientisolation is greater than 10 MΩ, 2500V
rms at 60 Hz.
•Where electromagnetic devices (i.e., electrocautery) are used, patient monitoring may be interrupted
due to electromagnetic interference. Electromagnetic fields up to 3 V/m will not adversely affect
system performance.
•VENT✔contains no user serviceable parts. Refer servicing to qualified service personnel.
•This product and its accessories which have patient contact are latex free.
For maximum patient and operator safety, observe the following warnings and cautions.
•Explosion Hazard: Do NOT use the VENT✔in the presence of flammable anesthetics. Use of this
instrument in such an environment may present an explosion hazard.
•Electrical Shock Hazard: Always turn the monitor off before cleaning it. Do NOT use a damaged
monitor or sensor. Refer servicing to qualified service personnel.
•Fire Hazard: The VENT✔should not be exposed to elevated oxygen levels at elevated pressures.
Use in such an environment may present a fire hazard.
•Failure of Operation: If the monitor fails to respond as described, do not use it until the situation
has been corrected by qualified personnel.
•Do not apply tension to thesensor tubing while connected to a patient breathing circuit, as accidental
extubation may result.
•Do not position the flow sensor’s tubing in any manner that may cause entanglement or
strangulation.
•Use the optional external battery charger in non-patient areas only.
WARNINGS
Indicates a potentially harmful condition that can lead to personal injury.
!

Section 1
2
Model 101 Service Manual Rev. 01
•Federal (U.S.A.) law restricts this device to sale, distribution, or use by or on the order of a licensed
medical practitioner.
•Electrical Shock Hazard: Always turn the monitor off before cleaning. Do NOT use a damaged
monitor.
•Do NOT use a damaged flow sensor.
•Do NOT immerse the monitor or sensors in liquids.
•Do NOT sterilize the monitor or the sensors.
•No user serviceable parts inside. Refer servicing to qualified service personnel.
•Operate at temperatures between +10°C to +40°C (50-104°F), < 90% relative humidity (non-
condensing).
•Avoid storing the monitor at temperatures less than -20°C or greater than +55°C (<-4°F or >131°
F).
•The VENT✔operates with Novametrix Series 3 Flow Sensors only.
•The VENT✔performs an automatic zero (self calibration) periodically and as needed. During this
time, monitoring is interrupted for less than three seconds.
•The automatic zero can be manually initiated by simultaneously pressing the DATA and GRAPH
keys. After changing the sensor from Adult to Neonatal (while the VENT✔is operational), wait 30
seconds then perform an automatic zero.
•This product and its accessories which have patient contact are free of latex.
•The C20/C Compliance Ratio (neonatal) parameter is not supported.
•To determine the VENT✔software version, turn the monitor on. During the self test performed at
power up, the software level is shown on the third line as “main-101-xx”, where “xx”is the software
version.
•Some VENT✔monitors were produced with the statement “Use only Novametrix approved devices,
13 VDC, 1A”located on the label at the Flow Sensor Input Connector. This erroneous statement
does NOT apply to the VENT✔monitor and should be ignored.
CAUTIONS
Indicates a condition that may lead to equipment damage or malfunction.
NOTES
Indicates points of particular interest or emphasis for more efficient or convenient operation.

Rev. 01 Model 101 Service Manual
3
Section 2
Introduction
The VENT✔Handheld Respiratory Mechanics Monitor, Model 101 is shown below.
Connect Series 3
ToPatient
ToVentilator
Neonatal Pediatric/Adult
Quick Guide Battery Access
Features Compartment
Flow Sensor
Keypad &
Display
Sensor
#6718/6720 Sensor #6717
Connector

Section 2
Keypad Controls and Indicators
4
Model 101 Service Manual Rev. 01
2.1 Keypad Controls and Indicators
2.2 Symbols
# Key Action Function
1 Press Turns
VENT
✔on/off.
2Press
Display Graph Screens.
Additional presses causes
VENT
✔to sequence
throu
g
h available Graph Screens.
3Press
Display Data Screens.
Additional presses causes
VENT
✔to sequence
throu
g
h available Data Screens.
4Press Turns display backli
g
ht on/off.
Press &
Hold Adjusts contrast/viewin
g
an
g
le of display
(1 step/sec.)
A None
Illuminated if powered from battery
Green; battery char
g
ed
Yellow flashin
g
slowly; capacity
g
ettin
g
low
Red flashin
g
quickly; exhausted in 10-15 min.
B None Illuminates when a Neonatal sensor is connected.
Symbol Description
Patient Isolation
Identifies patient isolation connection as type BF.
Attention
Consult manual for detailed information.
POWER GRAPH DATA NEO
1
2
3
4B
A
POWER
GRAPH
DATA
NEO
!

Rev. 01 Model 101 Service Manual
5
Section 3
Theory of Operation
3.1 Main Board
3.1.1 Power Supply
Refer to page 4 of the schematic 2740-03. An external power source can be connected to J403 to power
the monitor and charge a rechargeable battery (only Novametrix approved rechargeable batteries and
chargers should be used with the model 101). The internal battery connects to the cathode of D11, fuse
F1 provides overload protection. Since the monitor has the possibility of being powered by the internal
battery or an external charger, diodes D4 and D11 isolate each of these sources from one another.
3.1.2 Reference Voltages
Refer to page 5 of the 2740-03 schematic. A 2.5 volt reference (AVCC_2) is generated by U10 and
buffered by U1A. This voltage is amplified by U1B and Q1 and supplies a 5 volt line (AVCC). The 2.5
volt reference is divided down by resistors R6 and R7, buffered by U2B to supply a 1.5 volt reference
(A_1_5V). A negative 2.5 volt reference (NEG_AVCC_2) is generated by U9B and Q9 which are set up
as a unity gain inverting amplifier fed by the 2.5 volt reference (AVCC_2).
3.1.3 Battery Charger
Refer to page 9 of the 2740-03 schematic. The internal NiMH battery will charge when the monitor is
connected to the DC wall mount adapter or installed in the cradle option. Battery charging is controlled
by U11, a frequency modulated fast charge controller. U11 monitors temperature, voltage, and time
throughout the charging process to safely and effectively charge the internal battery. The charger is
configured to terminate charging using the ∆T/∆t (delta temperature/delta time) method of charge
termination. Charging is maintained at the C/4 (600mA) rate while current to the battery is controlled by
Q11, Q2, Q12, and the MOD output of U11. Q2 provides base drive for Q11 while Q12 serves to shut
Q11 off very quickly on a cycle by cycle basis allowing the large currents required for charging to pass
through Q11 which is a surface mount SOT-23 package capable of 500mW power dissipation.
Temperature is monitored using the battery’s internal thermistor, R115, R116, andR118. Resistors R115,
R116, and R118 set the ∆T/∆t charge termination parameter to 1ºC per minute. R39 and R106 set the
maximum temperature for charge termination (a safety override) to 45ºC. Battery charging is initiated in
one of two ways. Either by applying 13 VDC to +VCHG, therefore providing power (BVDD) to U11, or
by inserting a rechargeable battery into the battery compartment (provided external power is available).
Resistors R34 and R36 set up a divider which determines whether the installed battery is within the
correct voltage range for charging. BVDD is regulated by D1, a 5.1V zener diode while R35 keeps D1

Section 3
Main Board
6
Model 101 Service Manual Rev. 01
operating in the knee region and C68 and C69 provide filtering. Over current protection is provided by
F2, a 1A slo-blo replaceable fuse. Reverse leakage protection is provided by D2 and D7 which prevents
the battery from trying to power BVDD and +VCHG in the battery operation state.
3.1.4 On/Off Control
Refer to page 4 of the 2740-03 schematic. When the power key is pressed the cathode of D3 is brought
to ground, bringing the anode low, this biases Q3B on and allows VDCIN to power the unit. Pressing the
power key again will signal the processor that the unit must power down. The processor will assert the
POWER_HOLD line (low) to bias Q4 off, this will turn Q3B off and power the unit down.
The voltage level of the battery (or power source) is monitored by U13A. If the VSWDC level drops
below a certain threshold then comparator U13A will go low, this will pull the POWER_HOLD line low
through D19 and shut the monitor down.
3.1.5 Digital Control and Microprocessor
Refer to page1 of the2740-03 schematic. The system iscontrolled by microprocessor U18. Eight analog
channels are monitored and processed and several digital lines are generated for various operations. The
analog channels are monitored and converted to digital information by the microprocessor. These
channels are listed below:
The digital lines are listed below:
ANALOG
CHANNEL DESCRIPTION
ACH_0 Airway flow channel X1
ACH_1 Airway flow channel X10
ACH_2 Airway flow channel X100
ACH_3 Airway flow channel X1000
AWPRESS Airway pressure zero
ABPRESS Barometric pressure
F5V CPU power plane measurement
VBATTADC Battery voltage measurement
DIGITAL LINES DESCRIPTION
8255_CS Chip select for programmable peripheral interface, U22
DAC_SDI Serial data input line for U8 and U25
DAC_LD Load assert line for U8
DAC_CLK Clock signal for U8
OCDRV_0 Drive signal for opto isolator 1
OCDRV_1 Drive signal for opto isolator 2

Main Board
Section 3
Rev. 01 Model 101 Service Manual
7
Crystal Y1 sets the operating frequency at 12MHz. System software is stored in U21, a flash ROM
device, and system RAM in U19. Decoding for the RAM and ROM is handled by U27, U28 and U20.
Address lines A0-A7 are shared withdata lines D0-D7, U15 decodes the data lines as address lines when
needed through the ALE line.
The digital supply is monitored by U26, if the voltage drops below a certain threshold then the SRST*
signal will reset the system. The SRST* line will also bias Q8 off, this will assert the RESET line that
will reset other chips on the board at the same time that the microprocessor is reset.
3.1.6 Serial Communication
Refer to page 2 of the 2740-03 schematic. Serial communication is handled by U17, the transmit
(MPU_TXD) and receive (MPU_RXD) lines are converted into RS232 levels by U17. Diodes D13 and
D14 are for protection on the processor side of U17. Capacitors C62-C65 are used by U17 for generating
the required voltages for the RS232 levels. L1 and L2 are high frequency filters for immunity and
susceptibility. The RS232 signals appear at the rear connector of the unit and at J401 (circuit board
mounted header connector). The COMMPWR line from the microprocessor enables U17 when high,
when set low U17 is put in sleep mode to conserve power.
3.1.7 Audio
Refer to page 2 of the 2740-03 schematic. Audible tones are generated by LS1 when driven by Q13. The
AUD_CLK line from the microprocessor biases Q13 on and off creating the desired tone output from
LS1. A 750KHz clock signal is generated by U31 from the 6MHz system clock. This clock input is
OCDRV_2 Drive signal for opto isolator 3
POWER_HOLD Signal line asserted low for powering down the monitor
VLV_CNTL Valve control line used for zeroing pressure sensors
CLKOUT 1/2 of the system clock output (6MHz)
MPU_TXD Serial communication transmit line
MPU_RXD Serial communication receive line
750KHz Clock input signal for audio output pulse width modulated signal
EE_OUT Erasable EPROM data output signal
LED_PWM Clock output signal for display back light
EE_CS Erasable EPROM chip select
COMMPWR Enable/sleep control for RS232 Transceiver, U17
AUD_CLK Audio drive output signal
ALE Address line enable (A0-A7)
INST Control line for RAM and ROM access
WR Write enable
RD Read enable

Section 3
Main Board
8
Model 101 Service Manual Rev. 01
required by the processor for generating the audio tones of AUD_CLK. The RESET line on U31 prevents
any audio from inadvertently being generated by disabling the chip during power up and power down.
Audio is used for low battery only. There are no other alert outputs in this monitor.
3.1.8 Interface
Refer to page 3 of the 2740-03 schematic. The microprocessor interfaces to the display, key panel, and
LED’s on the 2741 board through U22. U22 also interfaces to the optical isolators (located on the 2740
board), which determine which type of flow sensor is installed (adult, neonatal, or neonatal/CO2). The
microprocessor communicates to U22 via the A0, A1 address lines, D0-D7 data lines, 8255_CS, RD and
WR lines. The PA0-PA7 lines are a buffered data bus which go to the 2741 board where they are used as
latched I/O. The PB5-PB7 lines decode the appropriate latch on the 2741 board for display and LED
indicator output and key-panel input and the PC5-PC7 lines drive the opto isolators on the 2740 board.
3.1.9 Analog Control Signals
Refer to page 5 of the 2740-03 schematic. The serial to DAC (digital to analog converter) U8 contains
four independent DACs for control signals in the system. A 1.5 volt reference (A_1_5V) is used as the
reference input to all four DACs via U12A, an amplifier with a gain of 2. The clock (DAC_CLK), load
(DAC_LD) and data (DAC_SDI) lines are directly controlled by the microprocessor.
DAC_A and DAC_B’s outputs are used for gain control in the flow measuring circuitry. DAC_C’s output
is used for offset adjustments in the airway pressure measuring circuitry. DAC_D controls the VDISP
voltage through U9A and Q10, this varies the contrast on the monitor’s display.
3.1.10 Sensor Identification
Refer to page 8 of the 2740-03 schematic. When a flow sensor is plugged into the monitor it is
automatically identified by means of a reflective/non-reflective label that is read by the monitor’s
circuitry. Three opto isolators are used to emit a light and measure any reflection from the label. There
are three bands that can reflect or absorb light, this enables eight distinct possibilities. The opto isolators
are driven by OCDRV_0 through OCDRV_2 which drive Q5-Q7 respectively. If an opto isolator receives
a reflection then the transistor portion will conduct and trigger a comparator output (U13) to go low. The
outputs OCRD-0, OCRD_1 and OCRD_2 will be read by U22 and allow the processor to determine the
type of flow sensor connected.
3.1.11 Flow Zeroing
Reference page 3 of the 2740-03 schematic. The zero process begins when the CPU brings the
VLV_CNTL line high, thisbiases Q3A on energizing valves V1 and V2. This disconnects the differential
pressure transducer U29 (via V1 and V2) and the absolute pressure transducer U30 (via V2) from the

Main Board
Section 3
Rev. 01 Model 101 Service Manual
9
patient airway, and opens all pressure transducer ports to atmosphere. Diodes D16 and D17 are for
protection against back EMF from the valves coils.
The pressure transducers are “zeroed”by adjusting the amplified and conditioned pressure output signals
so that each reading reads approximately mid-scale (512 counts) using a successive approximation
algorithm. With a reference voltage of 5.0 Volts, each count returned by the 10-bit ADC is equal to 4.883
mV. Centering the no flow (ambient) signal to the ADC's mid-scale allows the sensor to report both
positive andnegative airway pressures. U8, a Digital toAnalog Converter(DAC) provides the adjustment
under microprocessor control. The DAC maintains each adjustment voltage obtained during the zeroing
process until a new zero cycle is initiated.
The patient airway pressure transducer is "zeroed" first by adjusting the UB_DAC_A output of the DAC
until the Airway Pressure signal reads mid-scale. The barometric (ambient) pressure as sensed by the
processor is recorded after the airway pressure zero is completed. Next the flow channels are zeroed. A
non-inverting summing amplifier U6A combines two of the DAC's outputs and a constant voltage from
AVCC equal to the mid-scale of the ADC. The output voltage produced by the summer is fed into U14,
a monolithic instrumentation amplifier, which takes the differential output of the pressure transducer,
U29, andadds an offset equal to the reference voltage input. DACoutputs UB_DAC_A and UB_DAC_B
serve to provide the flow channels with a fine and a course adjustment. The result from each channel is
stored in SRAM and used as an offset in the flow calculations. Valves V1 and V2 are then de-energized,
reconnecting the pressure transducers with the patient airway.
3.1.12 Flow Circuitry
Reference page 6 of the 2740-03 schematic. Differential Pressure Transducer, U29, is a silicon-based,
piezoresistive bridge with four active elements. When pressure is applied between transducer ports P1
and P2, a differential output voltage proportionalto the applied pressure isproduced. Thefull-scale input
pressure range for the transducer is 0 to 4 inches of water (P1>P2). By setting the 0 differential pressure
(no-flow) point to mid-scale (during the zeroing process described earlier), negative pressure readings
(P2>P1) are also available. The transducer is temperature compensated at 25 degrees Celsius and
designed to be driven by a constant current source (U6B).
In the normal system operating mode, all valves are de-energized. Transducer ports P1 and P2 are
connected to the patient airway. As air flows through the flow sensor, a pressure difference between P1
and P2 is created. This signal is dependent on both the magnitude and the direction on the air flow. The
greater the flow volume, the larger the pressure difference created between the two transducer ports. The
transducer senses an inspired flow as a positive pressure difference (P1>P2), while an expiratory flow is
VALVE 1
VALVE 2
BAROMETRIC PRESSURE
TRANSDUCER
AIRWAY PRESSURE
TRANSDUCER
PNEUMATIC TUBING

Section 3
Main Board
10
Model 101 Service Manual Rev. 01
seen as a negative pressure (P2>P1). With a source voltage of approximately 5.0V, the sensor transforms
this pressure difference into an electrical signal with a nominal absolute magnitude of 50 mV Full-scale
Output. This signal is conditioned and amplified by U14, which is a monolithic Instrumentation
Amplifier (IA). The flow IA U14 also offsets the signal to the mid-range of the ADC obtained during the
zeroing process. A positive pressure difference (inspiratory flow) creates a signal above the offset
(approximately 1.25 to 2.5V). A negative pressure difference (expiratory flow) becomes a 1.25 to 0V
signal. The nominal gain of U14 is set by fixed resistor R59 and variable resistor VR1. The output for the
transducer is adjusted using VR1 and a known pressure input as a calibration reference. With an input
differential pressure of 10 inH2O, the gain of the amplifier is set to give an ADC count of 3498.
The signal out of the flow U14 is taken through a two-pole low pass filter U7B with a 31Hz cutoff
frequency to remove unwanted high frequency electronic noise before it is passed on to the four gain
stages (U4 and U5). The four flow differential gain amplifiers provide signal gains of 1 (ACH_0), 10
(ACH_1), 100 (ACH_2) and 1000 (ACH_3). The gain of 1 amplifier is used to buffer the flow signal and
provide signal conditioning consistent with the other channels. The x10, x100 and x1000 channels
amplify the flow signal according to the following equation:
Vout = (Vflow-Vrefo/2)(Av) + (Vrefo/2)
where Avis the amplifier gain, Rfb/Rv(1, 10, 100 or 1000)
Rfb is the feedback resistor (R51,R58, R68, R78)
Rvis the reference resistor (R131,R61, R73, R82)
The circuit is designed to amplify the difference between the flow signal into each gain stage and the
reference voltage so the zero point of each stage remains at mid-scale.
The output from each gain stage appears at the microprocessor for conversion into digital information.
An alternate pressure transducer, U29A may be installed in place of U29. The principal of operation is
the same as described above with the exception that it’s full scale pressure input range is 0 to 10 inches
of water and it is excited by a constant 5.0 volt (AVCC) reference.
3.1.13 Barometric and Airway Pressure
Refer to page 7 of the 2740-03 schematic. U30 is a piezoresistive differential pressure transducer with
port P2 held at a vacuum (0 psi). It measures the absolute pressure difference at port P1 relative to the
vacuum at port P2. The transducer is calibrated for a full scale output of 0 to 30 psi, has internal
temperature compensation and is designed to be driven by a constant current source. Resistor R99 is used
to set the current through the sensing bridge by amplifier U7A. Instrumentation amplifier (IA) U23
conditionsthis signal to correspond to the current barometric pressure, which is set by adjusting VR2 for
span and VR3 for offset. The nominal gain of this amplifier is 68.75. The output signal from U23 appears
as an input to both the 10-bit ADC and a second IA, U24. U24 provides gain adjustment via VR4 and
offsets the output signal from the barometric amplifier to mid-scale during the zeroing state. This is
handled by the AW_DAC line from U2A (page 5 of the 2740-03 schematic), the output is then fed to the
low pass filter circuit of U12B. The nominal gain of the airway pressure amplifier is 2.1. This signal
connects to the P1 (proximal to the patient)side of the differential pressure transducerduring monitoring
and provides patient airway pressure sensing (AWPRESS).

Interface Board
Section 3
Rev. 01 Model 101 Service Manual
11
3.2 Interface Board
3.2.1 Key panel Interface
Referto page 1 on 2741-03 schematic. The key panelkeys are monitored by thesystem through latch U4.
When any key is depressed the associated pull-up resistor (R9-R13) is brought low and the appropriate
line will appear as a low on the PD0-PD7 line. The latch is read by U22 from the main board (lines PD0-
PD7) when the 0xF800 line is asserted.
3.2.2 Display interface
Refer to page 1 on 2741-03 schematic. Communication to the display is handled by latches U1 and U2,
these in turn are controlled by U22 on the main board. U1 is enabled by the oxF806 line and handles the
display data lines DS0-DS7. The remaining control lines are handled through U2 and the 0xF807 line.
3.2.3 LED control
Refer to page 1 on 2741-03 schematic. The LEDs on the front panel display are controlled by latch U2
and the 0xF807 line. LEDs D1 and D2 are bicolorLEDs that are controlled by two lines each. D3and D4
are unicolor LEDs.
3.2.4 Power Supplies
Refer to page2 on 2741-03 schematic. The VSWDC supply fromthe main board is regulated to a +5 volt
supply by U3, a VALVE_SUPPLY supply by U7 and ±12 volt supplies by U5 and U6. U7 is used as a
separate5 volt regulator for the main board valves. The other supplies are connected to the main board
through J2.

Section 3
Interface Board
12
Model 101 Service Manual Rev. 01