Welch Allyn PneumoCheck 61000, Service Manual

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28 Pneumocheck Handle and Printer/Charger Service Manual 610132 Rev. E
The output of U11B (TP5) depends upon the particular
transducer, the charge pump and the adjustment of R33
and R36. The nominal output is 0.125 volt with no
pressure applied and 2.10 volts with a pressure
corresponding to 12 L/S of airflow (approximately 6cm
H
2
O). With no pressure and no charge pump pulse train
TP5 will be in the neighborhood of 1 volt.
U15A provides the final stage of amplification (2x). D9
clamps its input to prevent latch up when the output of
U11B goes negative.
With VA on, the charge pump operating and R33 and
R36 adjusted. U15A drives the A/D channel 0 with 0.5
- 4.5 volts as the airflow in the sensor ranges from 0 to
14 L/S.
The CPU, U2, has an address and data bus which
connects it to a 32K RAM U4, and 8K RAM, U5. Control
for these is performed by U3.
D1 and C4 provide a power-on reset. Residual power is
quickly discharged through SW1 and R3 to ensure that
reset occurs every time the power is cycled.
The A/D, EEPROM and LCD controller (U6) are serial
devices connected to the CPU ports.
The “TEST” key is connected directly to a CPU interrupt
input. The other interrupt input is shared by the “DATA”
key and the DC sense line from the power supply.
In order to allow both positive and negative transitions
of DC input to cause interrupts. U1B can be used to
invert the DC sense signal under software control. U1A
forms a latch which can be forced high or low by
software. R2 keeps it latched which can be forced high
or low by the software. R2 keeps latched high or low
when the CPU goes to sleep and the P4,4 goes to the
tri-state condition.
5. Spirometer Digital
Circuit Description
Section 3 - Theory of Operation