Thermal Head Controller; Thermal Head; Mechanical Controller; Internal Test Pattern Generator - Tektronix Phaser 440 Service Manual

5
Theory of Operation

Thermal head controller

The thermal head controller uses the information received from the heat
controller, in conjunction with timing signals, to drive the thermal head with the
data stored in the line memory buffer. The head controller generates several
different strobes. These strobes ranging from long pulses for "cold" thermal
elements, to short pulses for "hot" elements. During printing, the strobe length
is varied, based on the overall thermal head temperature and the volume of dots
printed in a line. The voltage level of the pulse is switched between 13.5 VDC
for transparency film to 15 VDC for paper.

Thermal head

The thermal head is the device that vaporizes dye dots onto paper. It contains a
row of 2816 solid-state heating elements. The actual number of elements used
during a particular print depends on the paper size (A, A4 or Letter Extra) and
the image to be printed. The thermal head prints raster lines parallel to the short
axis of the paper.
During printing, the information in the line memory buffer is latched to the
printhead elements and the elements are strobed to print a line. While the
elements are active, the data in the line memory buffer determines how long
each element is allowed to remain turned on. A line is printed in 7.7 msec (11.8
msec for transparency film). The thermal head also contains a heating element
that is activated, prior to printing, when the thermal head is cold. The heating
element quickly warms the entire thermal head to the thermal head's operating
temperature and then turns off. Three fans keep the thermal head from
overheating during printing.

Mechanical controller

The printer engine mechanical controller interfaces the CPU to the print engine's
motors, sensors, solenoids and fans. By interpreting signals from the sensors,
buffered by the mechanical controller, the CPU keeps track of the paper drum
position, transfer roll position and paper position during printing. Based on this
information the CPU determines the proper time to transmit signals to the
mechanical controller to advance the paper, energize a solenoid, or check a
sensor.

Internal test pattern generator

This circuit produces the print engine's internal test patterns. The test pattern
algorithms are stored in ROM. The internal test pattern data are routed to the
interface block and pass through the rest of the print engine just as regular image
data would.
Service Manual 5-7