HP V8.5 - Manual (Page 53)

5
T H E O R Y O F O P E R A T I O N
5 -9 V8.5e Rev 2.2.2002
5.4 THERMAL PRINTHEAD
The thermal printhead is comprised of a row of 1728 heater elements
(nibs) . Each bit within a scan addresses an individual nib. Plot patterns
are generated one scan at a time, each scan consisting of a horizontal
row of dots. A nib produces a dot if the corresponding scan bit in
the buffer is a logic level “1”. Nibs in the printhead are heated, causing
a thermo-chemical reaction with the coating on the media. The
intensity or period for which the nib is heated is called the burn
strobe.
Automatic Head Temperature Feedback
The thermal printhead rises in temperature as it plots images on
paper, thus, less energy is required to activate each nib as the plot
proceeds to create an image of the same intensity. A thermistor
embedded in the head measures the head temperature and provides
feedback information to the main logic board. The main logic board
decreases the burn strobe accordingly to keep the contrast of the
plotted image consistent as the plot proceeds.
If the printhead temperature reaches 60º C, the Automatic Head
Temperature Feedback circuit stops the plotter in order to let the
head cool down. The control panel display reads hot
(Ht) and the
error light flashes. When the thermistor measures that the printhead
temperature has cooled to 45º C, the plot resumes with no loss of
data.