Welch Allyn PneumoCheck 61000, Service Manual

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30 Pneumocheck Handle and Printer/Charger Service Manual 610132 Rev. E
This board is based on the Mitsubishi 50734 processor
which has an integral stepping motor driver, a UART
and a four channel 8 bit A/D converter. This makes the
digital circuitry very simple.
The printer mechanism has the thermal head driver
internal to it, so the interface to the processor is simple
and direct. The head temperature must be monitored
by the processor so that the head pulse width can be
controlled to produce a consistent end image at any
temperature. In this circuit the head thermistor is
balanced against an adjustable resistor which allows us
to trim each unit for variations in the head and
thermistor without trimming the timing constants within
the software. The software is designed to prevent
driving the head when the thermistor reading is out of
range. By using other analog channels for “Head-up”
and “Papersense” we avoid the need for logic level
converters on these lines.
The printer mechanism prints 256 dots per line in four
distinct banks of 64 dots. The signals DST1 through
DST4 select the bank. The data for each bank is loaded
serially from the processor using the CLK, DIN, and
LATCH signals. The ENABLE signal protects the
element heaters during CPU reset cycles.
In order to achieve maximum printing speed, the
printing actually occurs while the paper is moving
smoothly. Thus the printed horizontal lines are actually
less diagonal.
The UDS 5707 serves to buffer the paper drive motor
signals. D12 clamps the inductive surges. A port bit is
used to disable the motor drivers when they are not
being used. CPU pin 22 must be high to enable the
motor. On newer boards this buffer is an SN 75437
(P/N 761181-501), and performs the same function.
The front panel is a simple matrix of switch closures
isolated with doides. The options DIP switch is handled
in the same way.
3.2 Spirometer - Printer
Circuit Description
1. Digital Circuit Desription
Section 3 - Theory of Operation