Panasonic KX-FT21RS - Manual (Page 107)

6.3.7. SUPERVISION CIRCUIT FOR THE THERMAL HEAD TEMPERATURE
1. Function
The thermistor changes the resistor according to the temperature and uses the thermistor´s characteristics.
The output of pin 193 of IC1 becomes a low level.
Then when it becomes a high level, it triggers point (A).
In point (C), according to the voltage output time, the thermal head´s temperature is detected.
After the thermal head temperature is converted to voltage in (B), it is then changed to digital data in the A/D converter inside
IC1. The CPU decides the strobe width of the thermal head according to this value. Therefore, this circuit can keep the thermal
head at an even temperature in order to stabilize the printing density and prevent the head from being overheated.
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KX-FT21RS