Panasonic KX-FP85, Service Manual

Page: 142 / 207

6.4.3. THERMAL HEAD
1. Function
This unit utilizes the state of the art thermal printer technology.
The ink film is chemically processed. The ink film is comprised of two parts: an ink layer and a base film. When the thermalhead
contacts this ink film, it emits heat momentarily, and the ink layer is melted and transferred to the paper. If this continues, letters
and/or diagrams appear, and the original document is reproduced.
2. Circuit Operation
Refer to the block diagram and the timing chart on the following page.
There are 27 driver ICs aligned horizontally on the thermal head and each one of these ICs can drive 64 heat-emitting registers.
This means that one line is at a density of 64×27=1728 dots=(8 dots/mm).
White/Black (white=0, black=1) data in one line increment is synchronized at IC501 pin 150 (THCLK), and sent from IC501 pin
149 (THDAT) to the shift register of the ICs. The shift registers of the 27 ICs are connected in series, and upon the shift of the
1728 dot increment, the shift register becomes filled with data, and a latch pulse is emitted to each IC from IC501 pin 151
(THLAT).
With this latch pulse, all the contents of the shift registers are latched to the latch registers. Thereafter, through the addition of
strobes from the IC501 pins (142, 143), only the dot location of black (=1) among latched data activates the driver, and the
current passes to heat the emitting body to cause heat emission.
Here, the three line strobes, STB1 to STB2, impress at intervals of 9.216 msec, as required for one-line printout.
The sequence is shown on the next page. [Moreover, for the strobe width, the thermistor value inside the thermal head is
detected according to IC501 pin 2. Depending on that value, the strobe width is recorded in ROM (IC502). Accordingly, the
strobe width is determined.]
When the thermal head is not used, the IC501 (141, THON) becomes low , Q502 turns OFF, IC506 turns OFF, and the +24V
power supply for the thermal head driver is not impressed to protect the IC.
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KX-FP85