(OT) Laserjet III laser scanner module

Tony Duell ard at p850ug1.demon.co.uk
Wed Jun 30 15:08:26 CDT 2010


> Having played with a the Canon II printer series scanners (the guts of =
> both the Laserjet II and Apple Laserwriter II), some comments:
> 
> > Well, working backwards from DPI and print speed gives about 3MHz dot =
> clock:
> >=20
> >   300DPI * 11 inches (paper height) * 3ppm =3D 9900 lines per minute
> >     (thus, the line rate is 9900Hz, or 9.9kHz)
> >   9900lpm * 300 dots per line =3D 2.97e6
> >     (thus, the pixel rate is 2.97MHz)
> 
> Unless you are using the control board, the above calculation is moot: =
> the scanner is a DC brushless motor - the speed is controlled by the =
> input voltage. The control board closes a motor loop based on the =

Not quite...

I was mis-rememebring last night, and thinking more of the CX, which I 
did a lot more work with...

The CX scanner motor has a control board on the bottom, This contains the 
brushless motor control chip and a PLL circuit [1]. The feedback is taken 
from an FG coil in the motor, not from the beam detect pusel. The motor 
runs fine -- and at the right speed -- without the laser bring presnet.

The SX is actually similar, but the PLL is on the DC cotroller board (I 
think it's a separate chip, not in the gate array, but In would have to 
check the schematics). Again it takes the feedback from an FG coil in the 
motor assembly (IIRC a wiggly track on the motor PCB)

[] The PLL chip, at least the one used in the CX (and IIRC it's used 
there both in the scanner motor controller and the main motor controller) 
was designed for use in record player turntables (phonographs?). 
According to the data sheet, one of the pins is called '33/45' :-)

-tony




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