(OT) Laserjet III laser scanner module

[Tony Duell]

> > The origian laser controller was on/off only (but not a simple design due
> > to the speed reuired).
> 
> Thankfully fast opamps aren't as rare as they used to be. Fast MOSFETS 
> moreso :)

No, but fast op-amps have this habit of turning into oscillators (== adio 
transmitters) if you look at them wrong. Layout, decoupling, ground 
planes, careful design of the feedback network, etc are not optional.

> 
>  > There was a DAC (IIRC discrete resistors in the
> > SX engine) to steh laser diode current,
> 
> It was current-regulated? That's an odd choice if you're going to switch

Of course. The laser current is critical. Too low and it doesn't lase. 
Too high and it never lases again!. Within that range it had to be set 
according to the sensitivity of the drum.
 
> a diode on and off at ~20MHz. Would have thought the current regulation 
> loop would have had kittens every time the current was turned off, and 
> put a current spike through the diode at every off-to-on transition 
> (unless it was (*really* carefully designed).

It was cerefully designed (I did say it was complicated). IIRC it 
switched the current source between the laser diode and a dummy load 
accordinmg to the state of the drive input. It had to work at nearly 30MHz.

And BTW, overshoot on the drive currend will do nasty things to the laser 
diode! 

> > Would uofficial scheamtics of the SX engine be any use?

That's _unofficial_ of course...

> 
> If they include schematics of the laser driver and scanner motor driver? 
> Hell yeah.

Includes every part of the printer. Even the PSU and EHT block. I will 
see what I can do. 

> > Not AFAIK (or at least the CX enginer controllers I've worked on don't).
> > The motor speed is controlled by a PLL-tpye thing (the chip on the motor
> > PCB in the scanner unit, with an 'FG' coil on the PCB). Any slight errors
> > in the bir rate would slightly cheange the size of the printout, but who
> > actually checks that (rememebr a crystal almost certainly better than 1
> > part in 1000, so the erors will not be large).
> 
> Right, so it's locking the motor off of the incoming bit stream? When 

No. The motor runs a a constant speed (controlled by the PLL taking the 
feedback from the FG coil in the motor). The bistream runs at any speed 
you like (within reason), controlled by a crystal on the formatter board 
(or the VDO interface, or ...) The motor will ru at the right speed even 
if the rest of the machine is malfunctiuoing.

The CX makes this a bit clearer. since the motor controller for that is a 
separate PCB on the bottom of the scanner module. It has (IIRC) 4 
connecitons --power, ground, motor on/off and an 'up to speed' feedback 
signal.


> you explain it like that, it makes far more sense than locking the 
> driver off of the motor.
> 
> > I think the PERQ scheamtics books are on bitsavers. Take a look at the
> > OIO board schematics,  you should find a CX-VDO printer interface there.
> > Agian I shouldbe able to provide details of the state machine ROMs, etc.
> 
> I think I've found it... PERQ T2 schematics, volume 2. The PERQ 1A 

Sounds right. 

> schematic set doesn't seem to include the OIO board, but there's a Canon 
> LBP-only OIO board for the T2.

The OIO boards fit all classic PERQs (it's the only board that is totally 
universal like that). AFAIK the 3 OIO versions (ethernet, cannon, 
etherent and canon) were all built on the same PCB, with just different 
ICs missing.

> 
> I had a quick look at the PCB photos too... I've never seen anything 
> quite like them. What are they, wire-on-epoxy? They look like someone 

Some of the prototypes/early ones were. But AFAIK all later PERQ boards 
were notmal PCBs. Certainly every OIO I've seen has been a normal PCB.

-tony