Canon LBP-2000 Service Manual page 72

4 . Controlling the emission of the laser diode
In keeping with the paper size information from the CPU, the ASIC generates the image formation
enable signal (/ENABLO), and sends it to the logic circuit of the laser driver IC. The /VDOUT
signal is masked by the /ENABLO signal in the logic circuit so that the length of time during which
the laser diode emits light is controlled.
The paper size information is sent to the CPU in the form of the paper size selection command
from the video controller in the case of the multifeeder or form the cassette size detection switch in
the case of the cassette.
From the contents of the paper size information, the ASIC sends readings on leading edge erase/
trailing edge erase and left/right image mask settings to the ASIC.
In response, the ASIC generates the image leading edge erase/trailing edge erase (TOPE) and left/
right image mask signal (MSK); TOPE and MSK are used within the ASIC, and are not sent outside
i t .
To prevent writing by the laser beam outside the non-image area except during the un-blanking
period), it causes the image formation enable signal (/ENBLO) to go 'L' while the TOPE signal and
the MSK signal remain 'L'.
The laser driver turns on the laser diode when the /ENBLO signal is 'L' and, in addition, the /
VDOUT signal is 'L'.
Note: 1 . The shading indicates the area in which writing by the laser beam is possible.
2 . T1 and T2 lengths of time vary according to paper size.
If no specific paper is identified by the paper size selection command coming from the
video controller when the multifeeder tray is used, the printer will have no means of
knowing the paper width (T1), automatically identifying the width to be 293 mm (which
is the result of subtraction of 2 mm (left and right each) from the width assigned to the
universal size paper cassette (maximum width allowed by the machine: 297 mm)).
/BD
T1
MSK
2mm
Figure 2-3-3
2mm
CHAPTER 2
2 - 33