GE MAC 1200

GE Medical Systems MAC 1200 V 1.2 Page 18
Information Technologies 227 492 21 RevF
Thermal Printhead Dot Control:
The MPC821 prepares a complete dot column and sends it to the FIFO. A complex PLD
reads out a complete column of the FIFO and generates the digital control signals for the
printhead, which are shifted in series.
The duration of heating a dot column is defined through the pulse width of the PWM Signal
HEAT that is generated from the complex PLD too.
In addition, the duration of heating a dot column is influenced by the thermal printhead
monitoring.
Thermal Printhead Temperature Monitoring:
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.
lf 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 re-enabled.
In addition, a continuous reduction of the heating duration occurs with increasing printhead
temperature, resulting in a regular typeface throughout the entire temperature range.
2.3.10 Motor contro}
Paper transportation for the speeds 5mm/s, 25mm/s and 50mm/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 5mm/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 MPC821.
The driver circuit is enabled by the signal REC_MOTEN, the driver is powered from
+22,5V.