GE MAC 1200 Service Manual page 33

Thermal Printhead Temperature Monitoring
Motor Control
Revision D
Functional Description: PCB Control CS_CI
The thermal printhead temperature monitoring measures the temperature of the
thermistor, located on the printhead. A constant current source effects a
temperature-dependent voltage drop.
If a printhead temperature of 55°C is exceeded, the signal REC_OVHEAT is
activated and the heating of the printhead is prevented by disabling the signal
STROBE.
Only when the printhead temperature drops below 50°C is the signal REC_OVHEAT
disabled and the heating via the signal STROBE reenabled.
In addition, a continuous reduction of the heating duration occurs with increasing
printhead temperature, resulting in a regular typeface throughout the entire
temperature range.
Paper transportation for the speeds 5 mm/s, 25 mm/s and 50 mm/s is driven by a
stepping motor. The stepping motor is controlled by an integrated stepping motor
driver circuit. The current for the motor is adjusted by the sense resistors of the
stepping motor driver.
For the speed 5 mm/s, the motor current is reduced, triggered by the signal
LOW_SPEED.
The motor speed is controlled by the frequency of the signal RECTMR_STEP, which
is generated by the timer unit of the MPC855T. The driver circuit is enabled by the
signal REC_MOTEN, the driver is powered from +22.5 V.
MAC 1200 resting ECG analysis system
2012250-095
2-15