plotters again

[Tony Duell]

> > I assume this plotter ahs a 'listen only' mode, where it's selected at
> > power-on and doesn't need ot be addressed. Many HP plotters do.
> > 
> > Of course you lose some functionality if you do that. Most HP plotters
> > are HPIB talkers too, they can report the current plot size and pen
> > position -- you could move the pen around with the buttons on the plotter
> > and use the machine as a primitive digitiser (put a trace from some other
> > instrument on the plotter bed, then move the carriage to points on that
> > trace and read them into the computer). You might well not need this though.
> 
> The 9815 interface supports the digitising functions regardless of the
> (non-)addressability issue, so that functionality is not lost. (My unit
> even came with the digitising sight.)

You _will_ lose the digitising functionality if you use the HPIB 
connector and set the machien to 'listen only' (if you can do that, most 
HP HPIB plotters do have that feature). In that mode there is no way the 
plotter can output data.

> 
> >From the schematics, the 9815 interface asserts a signal on the special
> connector on the plotter, said signal ends up on the plotter microprocessor bus,
> so the plotter microproc is distinguishing 9815 vs HPIB and acting appropriately.
> 

I don't know the 9872 that well. I have soem experience of the internals 
of the 7245, which from what I've seen has somewhat similar electronics 
(the motor driver interpolator is almost identical circuitry, for 
exampl). Now on the 7245 the HPIB interface is very hardware-intensive -- 
even the HPIB bus commands (serial poll enable/disable, etc) are decoded 
in hardwre. Of course the handshanke and addressing are handles in 
hardware too. All the processor does is get a signal that says 'read a 
byte from the HPIB data register' and the processor does so (and 
interpets the bytes read as HPGL commands, of course).

If the 9872 is like that, it should be very easily possible to work out 
what the 9815 interface is doing, since presumanbly it's also mostly 
handled in hardware, possbily even much the same hardware. From what 
you've said, it must be more than just a listen-only (unaddressable) HPIB 
interface, since the plotter can send data back to the 9815

-tony