User manual

95.08.23.11.29
Color LaserWriter 12/600 PS Basics & Theory 1–35
Laser Intensity
Control
The CPU in the DC controller controls the intensity of the laser beam
by using the
APCUP signal for rough control and the APCLW signal for
fine control. The APC-IC (IC103) adds the
APCUP and APCLW signals
to produce the laser beam intensity control signal (
LLCNT), which is
then output to the laser driver circuit in the laser/scanner assembly. The
CPU performs automatic power control (APC) of the laser diode and
adjusts the
LLCNT signal so that the laser diode will emit the laser beam
at the constant intensity. The APC is composed of initial APC and
between-pages APC.
Initial APC The printer performs the initial APC in the following steps while it is
executing the initial rotations.
1 When the APC enable signal
(/APCENB) goes “L,” the CPU
decreases the output values of the laser power control signals
(
APCLW and APCUP) to zero. After resetting the laser current, the
CPU rotates the scanner motor.
Note: The
/APCENB signal goes “L” when the 5 V and 24 V power
supplies are at the specified voltage.
2 The CPU sets the forcible laser-on signal (
/LSON) to “L” and the
video-enable signal (
/VDOENB) to “L” via the gate array. It changes
the D/A output value of the
APCUP signal and gradually increases
the laser diode current. When the laser current increases, the laser
diode starts emitting the laser beam. The intensity of the laser beam
emitted by the laser diode is detected by the photodiode (PD) and its
output voltage is fed back as the laser power detect signal (
PDIN) to
the APC-IC.
3 The CPU monitors the
PDIN signal. Until its value reaches the spec-
ified value stored in the CPU, step 2 is continued.
4 The between-pages APC described below is subsequently
performed during the remaining period of the initial rotations to
correct the offset from the target value.
5 The CPU sets the
/LSON signal to “H” to complete the initial APC
and then proceeds to the between-pages APC.