Minolta Di551 Service Manual page 55

WRITE UNIT
1. Operation
a. Image processing
The following processing is performed by the ICB
(image control board):
(1) AOC (Automatic Offset Control)
The IC on the ADB (A/D converter board) auto-
matically adjusts the analog offset voltage of the
CCD sensor output so that it is at the lower limit
of the A/D converter level.
(2) AGC (Automatic Gain Control)
During shading correction, the white reference
plate is read to adjust the analog amplification
factor of the CCD sensor output so that the read
level is at the upper limit of the A/D converter
level.
(3) Shading correction
<Timing>
• When SW2 (sub power) is ON
• At job start
(4) Brightness/density conversion
(5) AE processing
(6) Text/dot pattern judgment
(7) Filtering
(8) Magnification change processing
(9) Copy gamma correction
(10) Skew correction
(11) Error diffusion processing
(12) Data compression/expansion processing
(13) Write density control
b. Write
The ICB (image control board) sends image data
on a pixel basis to LDB (laser driver board)
according to the control signals from the PRCB
(printer control board).
LDB causes the laser light to be emitted for a
period corresponding to the image data. This
laser light is radiated onto the drum surface.
(1) MPC (Maximum Power Control)
ICB (image control board) informs LDB (laser
driver board) of the maximum output value and
sets that value for the laser beam emission. LDB
store this value and maintain the laser beam level
using the APC (Auto Power Control).
<MPC timing>
When SW2 (sub power switch) is turned ON
(2) APC (Automatic Power Control)
After MPC is set, the ICB (image control board)
outputs an APC start instruction to LDB (laser
driver board) at the following timing:
<APC timing>
LDB (laser driver board) automatically monitor
the laser drive current one line at a time, and con-
trols it so that the light intensity remains the MPC
value.
(3) Write timing
a) Main scanning direction
Using INDEX signal (/IND) from INDXSB (index
sensor board), the laser write reference position
is determined for each scan in the drum rotation
direction, and the image is written onto the copy
paper according to the copy paper position
detected by PS70 (paper mis-centering).
b) sub scanning direction
Specified interval after PS44 (registration)
detects the leading edge of the copy paper.
2. Signals
a. Input signals
(1) /IND (INDXSB to ICB)
This is an index signal used to detect deviation of
main scanning.
(2) /INDPR (INDXSB to ICB)
This signal monitors the INDXSB (index sensor
board) power supply.
[H]: Abnormal
[L]: Normal
(3) /ALM1 (LDB to ICB)
This signal indicates the state of the laser 1 drive
current.
[H]: Normal
[L]: Abnormal
(4) LPR5V (LDB to ICB)
This signal monitors the LDB (laser driver board)
power supply.
[H]: Normal
[L]: Abnormal
(5) /ALM2 (LDB to ICB)
This signal indicates the state of the laser 2 drive
current.
[H]: Normal
[L]: Abnormal
2-D-4