Sharp AM-900 Service Manual page 89

3.1.3 Laser scanning unit
This unit controls the laser beam power and laser beam scanning.
The control is performed with the signals inputted outputted to or from
the CPU and Logic circuit.
Laser scanning unit (LSU)
Laser start
position sensor
Photodiode
Lens
Laser
Lens
LD PWB
Polygon mirror
Laser diode
Laser Scanning Unit (LSU)
LD PWB
Amp.
Photodiode
nHSYNC 8
Laser diode driver
4
nLDEN
nVIDEO 3
Laser diode
nS/H 2
+5V
6
1, 5, 7
GND
Polygon motor
Motor driver
Motor
CLOCK 1
nREADY
2
nSTART 3
GND
4
+24V 5
Fig. 10
Scan
Lens
Polygon motor
Fig. 9
Laser
Laser
Polygon motor
Polygon mirror
1) Signal functions
PMCLK
Clock signal for driving the scanning motor. (2659.57Hz)
PMD
Scanning motor ON/OFF signal.
HSYNC
This signal is outputted when the laser beam scanned by the laser
beam sensor signal is sensed by sensor (Photo diode).
Used for the left margin control.
VIDEO
This signal is used to control the laser diode emitting.
Not only when the laser beam is emitted to perform the LEND pro-
cess, but also when the laser beam is emitted as image data, 16bit
CPU (IC21) controls and the signal is outputted from video termi-
nal.
2) Laser beam power control
The laser beam power is controlled in the laser scanning unit.
This circuit functions to keep the laser beam output power at a con-
stant level.
The laser beam output is monitored with photo diode for monitor.
When the laser beam output rises above the specified value, the
impedance of photo diode is decreased to decrease the monitor
input voltage of the laser diode control IC.
Then the laser diode drive voltage is decreased to decrease the
laser beam output to the specified level.
When the laser beam output is decreased below the specified
level, the contrary operation are performed.
3) Timing of scanning
ON
Laser
OFF
16.26mm
27.88µs
Non-Active
nHSYNC
Active
Polygon rotation speed/Rotation speed:26595.7min -1
a) Scanning motor interface
Pin No. Terminal
1 CLOCK
2 nREADY
3 nSTART
4 GND
5 +24V
5 – 16
AM-900U
Effective printing width
Effective width of printing data
>3.75mm
216mm
>6.43µs
370.34µs
1 LINE
328.95mm
564.0µs
F-number/Focal Length:104.7mm
VIDEO CLK:13.7774MHz
Fig. 11
I/O Function
Input Clock input
Output Motor rotation detect signal
"L" : Synchronous
"OPEN" : Asynchronous
Input Motor control signal
"L" : Start
"OPEN" : Stop
- GND
- Power supply (+24V)
1.0µ s max.