Pen plotter as digitizer

The HP 7475 plotter actually had this as a function. Instead of a pen, you could place a plastic lens and the press one of the keys and the HP-GL co-ordinates would be sent over the RS-232 ports.

Look for a download of the manual. They should be out there. If not, I have the manuals for both the 7470 and 7475 on the shelf.

b. Farmer

To answer your question, more specifically, we did do this.

And at one point we investigated replacing the gun sight with a photo receptor and digitizing the contrast changes. We slewed the pen across the paper side to side and then advanced it an amount vertically. (More than one step due to image field size) The ultimate goal was to digitize and reconstruct schematics from copies of circuit boards.

b. Farmer

Yes, that was my whole point. It is common on *most* (all?) of the HP plotters. Getting the data from the plotter is easy -- the plotter *gives* it to you!

The problem is getting that data *into* something (e.g., AutoCAD) where it can be of use.

I can write a small routine to collect the incoming data from the serial port. And put it in a file. Big deal. Now I have a bunch of (X,Y)'s but no *context*! I.e., if digitizing traces on a PCB artwork, how do I know which "points" belong to each "foil"? Which points represent pads and which represent "corners" of traces? Etc.

If digitizing blueprints for a house, how do I know which points belong to each "wall"?

Sure, I could import all of the points into AutoCAD and then play "connect the dots" -- and *hope* I connect them properly. :-/ I need to be able to replace the tablet driver with a penplot driver, so-to-speak.

But, since HP made a point of including this feature in their plotters *and* since large format digitizers were expen$ive during the pen plotter era, one would think there was a mechanism already in place to use this data!

I have service manuals for all of the pen plotters. As I said, they haven't been helpful. Nor has the AutoCAD documentation (I didn't keep any of the documentation for releases 10 and earlier -- though I think I still have some version *2* floppies somewhere around here :-/ )

In our work, the reconstruction of the image was the hardest part.

First attempt was to build a follower. We would advance in the direction that maintained the contrast. This sort of worked, but failed when we hit Tees and branches.

Since we were rasterizing the data, we ended up just building a row and column matrix that held the contrast values. Once a section was done then we could follow the traces in data matrix. This was in the early/mid 80's and because memory was expensive, we could only do small sections.

I would be kind of fun to recreate that.

b. Farmer

I.e., you were just using it as an input device for a special application. I would like to use it as a pointing device in an *existing* application -- one to which it, theoretically, should be well suited... :<