4-82
120 Series Maternal/Fetal Monitor
Revision B
2015590-001
Theory of Operation: Recorder Board
Recorder Board
NOTE:
A newer, surface-mount board replaces earlier versions. Functionally, the
boards are equivalent. Refer to
“Chapter 14, Replacement Parts”
for further
information.
The recorder board provides the interface between the Main Motherboard and the
strip chart recorder. The board performs three functions. First it powers the recorder
head through an adjustable regulator. Second it drives the 4-phase stepper motor,
and third it provides the paper-out, paper-low, and paper-misloaded interface
circuitry.
The monitor’s power supply module provides a regulated 20 Vdc which enters the
Recorder Board at J1 (pins 9, 10) and powers the input to a voltage regulator (VR1).
Voltage regulator VR1 is adjustable and provides a regulated DC output from 13 V
to 18 V depending on the position of the wiper on potentiometer R2. This output
powers the heating elements in the thermal printhead. Since printheads very in
average resistance, this voltage is calibrated to the value required by the particular
printhead installed.
A variety of safeguards shut down the printhead heating element supply (VHD)
under various conditions to protect the printhead. When the 5 V supply to the logic
circuitry is low, diode D1 will not be forward biased and transistor Q3 will be off.
Transistor Q1 will then saturate and bring pin 3 of VR1 to ground. The output of
VR1 will then be approximately 1.3 V. also, when transistor Q2 is turned on, the
current through resistor R4 to the base of Q1 will turn on Q1 and shut the output of
VR1 sown to 1.3 V. The first condition that can turn on transistor Q2 is when the
RE/ logic signal is low. This occurs during power-up reset of the microprocessor or
when the microprocessor brings HDSHUT/ low by writing to the appropriate output
port. The microprocessor brings HDSHUT/ low immediately after it comes out of
reset and brings it high before printing. Another condition than can turn on Q2 is
when POUT/ is low.
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