GMU:Algorithmic Art/HP 7475A

Working with the HP 7475A

The plotter is placed in room 204, in the shelf behind the projection screen. Take it out, place it on a desk and connect the serial adapter to your computer. Please do not forget to return it to the shelf when you are done! You have to bring your own pens and paper. So far it seems that some trial and error is involved when working with the HP7475A.

Working Times

If you want to work with the plotter, first check if the room is currently unoccupied. If you want to be sure that no one else is working with the plotter, make sure to send a message to the whole class, with the date and time that you intend to use the plotter (Email, Telegram, WhatsApp!?)

Class Schedule in M7 Room 204

Paper Setup

The HP7475A either takes A4 or A3 paper. The thickness of the paper is very flexible.

Pen Setup

To use your own pens with the plotter, you can simply extend your pen’s diameter, by wrapping some paper around it. This way it will fit the pen mount of the plotter. A better way would be to 3D print adaptors, that exactly fit the original HP pen measurements:

Original HP pen measurements and openSCAD code

Unless you have a pen that mimmicks the original HP pen shape, avoid the HPGL commands SP; SP1; SP2; SP3; SP4; SP5; SP6; as they attempt to change the pen.

Serial Connection

You need to find out the COM port, as which the USB/Serial adaptor was registered in your computer. In Windows, you can find out in the Device Manager, in Mac OS, I don’t know yet.

HPGL

The plotter only understands HPGL (Hewlett Packard Graphics Language). You can try the “HP7475A_live_input” sketch, to test some HPGL commands. You can find documentation about that language here:

Simple but incomplete reference
Full original reference (PDF)
Full original reference (txt)

Fortunately, there is a HPGL library for Processing, which can convert your sketches to HPGL files.

Another approach is to export a .svg file with Processing and to convert it to .hpgl with InkScape. (The files produced by inkscape are not compatible with my “HP7475A_feed_hpgl_file_01” sketch yet). InkScape also has the option to directly send graphics to the plotter, but I had no good experience with that. It also needs original HP Pens (or 3d printed adaptors), as it sends SP; commands, which let the plotter attempt to change the pen, before beginning to draw.

How to install the HPGL library in Processing:

Sketch → Import Library… → Add Library… → search for: HPGLGraphics → Install → restart Processing

You can now open the examples of the library (File → Examples → Contributed Libraries → HPGLGraphics → …). Try the examples and adapt the code for your own sketches.

How to use the HPGLGraphics library in a Processing sketch?

import hpglgraphics.*;
HPGLGraphics hpgl;

void setup(){
  size(1104, 772); // 1616 x 1104 for A3(!?)
  Hpgl = (HPGLGraphics) createGraphics(width, height, HPGLGraphics.HPGL);
  hpgl.setPaperSize("A4"); //either A4 or A3
  hpgl.setPath("yourFileName.hpgl"); //file will appear in sketch folder
  noLoop();
}

void draw(){
  beginRecord(hpgl);
  // your code here (the graphics you wnat to export as hpgl file)
  endRecord();
}

How to send a HPGL file to the plotter?

Use the “HP7475A_feed_hpgl_file_01” or “HP7475A_feed_hpgl_file_02” sketches, which are almost the same (only the buffer handling works differently). Both do not work perfect yet for each case, but almost. For very detailed graphics (for example the “noise.hpgl” file), version 02 works better. Instructions how to use the programs are in the comments