112 Theory of operation EN
Writing the image
The laser/scanner of this printer has two diodes in the laser unit. During the writing process, the
modulated laser diodes project two beams onto the rotating six-sided scanning mirror. As the mirror
rotates, the beams reflect off the mirror, through a set of focusing lenses, through a slot in the top of
the toner cartridge, and onto the photosensitive drum. The beams sweep the drum from left to right,
discharging the negative potential wherever the beams strike the surface. This creates a latent
electrostatic image, which later is developed into a visible image.
Figure 23. Writing the image
Because the beams are sweeping the entire length of the drum and the drum is rotating, the entire
surface area of the drum can be covered. The speed of the scanner motor (which turns the scanning
mirror) and the speed of the main motor (which turns the drum) are synchronized, and each
successive sweep of a beam is offset by 1/1200th of an inch. The beams can be turned on and off to
place a dot of light every 1/1200th of an inch. This is how the printer achieves its true 1200 by 1200
dpi resolution. After the writing process, the drum surface has an invisible (latent) electrostatic image.
At the beginning of each sweep, the beams strike the beam detect mirror and PCB, generating the
beam detect signal (BD). The BD signal is sent to the engine controller board, where it is converted
to an electrical signal used to synchronize the output of video data for one sweep (two scan lines)
and to diagnose problems with the laser diode or scanner motor.
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