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Plotting with the HP7475A

Summary: This document outlines a workflow for offline (non-interactive) plotting with an RS-232 HP7475A, using Processing and from a Mac computer. This has been tested in OSX 10.15.7, 11.5.2, and 14.1.2. Last update: 1/6/2024.

Contents

  1. Standalone Testing the HP7475A
  2. Testing Connectivity of a Mac to the HP7475A
  3. Generate an SVG with Processing
  4. Convert the SVG to HPGL with vpype
  5. Preview Your HPGL File
  6. Transmit HPGL to HP7475A with CoolTerm

(TODO: Workflow for ofxHPGL.)

About the HP7475a

7475A_9836_1984-PromoPhoto-27.jpg

The HP7475a was the most popular pen plotter ever made. Introduced in 1983, it was not obsoleted until 1994, an amazing 11 year product life. In 1984, the 7475 was the most profitable single product within Hewlett-Packard. OEM versions of this machine were also sold as the AT&T 435 and the IBM 7372.

HP7475A Tips

  • Don't leave pens in the plotter carousel, and especially don't leave pens in the plotter head. They dry out quickly.
  • When the plotter is not in use, raise the paper clamp lever.

1. Standalone Testing the HP7475A:

Summary: In this section we will have the HP7475A execute a built-in test plot. For complete information about this device, see this repository of manuals.

  1. Plug in the HP7475A plotter to 120VAC wall power.
  2. Load the plotter carousel with 6 pens. You'll need to depress the carousel's spring-loaded black rubber pen-caps in order to insert the pens.
  3. Load the plotter with letter (8.5x11) paper. Push the paper load lever up; align the paper against the left edge; and push the paper just past the short white line in the upper left of the paper bed. Lower the lever.
  4. Execute the plotter's "Demonstration Plot (Confidence Test)" as described on page 2-31 of the HP7475A Operation and Interconnection Manual. This is achieved by holding down the plotter's P1 and P2 buttons while powering it on, and continuing to hold them down "until the tapping noise begins". This should produce a multicolor plot which looks something like the following:

HP7475A demonstration plot

2. Testing Connectivity of a Mac to the HP7475A:

Summary: In this section we will make sure that we are able to send signals from our computer to the HP7475A plotter.

NOTE: These instructions assume you are connecting to an RS-232 version of the plotter, using a DB25 male cable. To connect with an HP-IB version of the HP7475A plotter, see this document from the current repository.

7475a_interfaces.jpg

  1. Connect the USB to Serial Cable Adapter (USB-A to DB9 male) to your Mac. We have a Sabrent cable which is known to work. (If your Mac uses USB-C, you may need to pre-pend a USB-C to USB-A adapter.)
  2. Verify that the serial adapter is visible to your Mac's operating system, by opening the Terminal app and typing ls /dev/tty.*. You should see something similar to /dev/tty.usbserial-A101768Y among the listed results.
  3. Connect the serial adapter into the HP 24542G cable (DB9 female to DB25 male). This cable has the following wiring, according to page A-6 of the HP7475A Operation and Interconnection Manual (or you can wire one yourself with these instructions):
    HP_24542G 9-to-25 pin cable wiring
  4. Ensure that the DIP switches on the rear of the plotter are set to 9600/8-N-1, and US letter (8.5x11") paper, as described on page 2-21 of the HP7475A Operation and Interconnection Manual. Note that the DIP switches control the machine defaults for paper size and measuring system (imperial/metric), but these can be changed from the front panel as well. The switches should look like the following:
    HP 7474A DIP switches.jpg.
  5. Turn on the plotter.
  6. Launch a serial port terminal program, such as CoolTerm. (A list of other possible serial port apps can be found here and includes RealTerm, Screen, Minicom, ZTerm, goSerial, Serial Tools, etc.) CoolTerm is distributed for macOS, Windows, Linux, and Raspberry Pi; a backup copy of version 1.9.0 (5/31/2021) for Mac is stashed here.
  7. Configure the serial port terminal program so that it matches the communication settings of the plotter, 9600/8-N-1. You will need to turn OFF the "Software Supported Flow Control". For example, in CoolTerm, go: Connection→Options->Serial Port:
    CoolTerm configured for 9600/8-N-1
  8. Configure the serial port terminal program so that it is in "line mode", meaning that commands are transmitted when you press return:
    CoolTerm configured for line mode
  9. In CoolTerm, Connect! Go Connection→Connect.
  10. Transmit HPGL to the plotter. Type the command IN;SP1; into CoolTerm and press return:
  11. In response to the command, the HP7475A plotter should initialize itself and Select Pen #1. To put the pen back, type SP0;.

3. Generate an SVG with Processing

Summary: Processing "is a flexible software sketchbook and a language for learning how to code within the context of the visual arts." In this section, we will generate an SVG file using Processing and its built-in SVG Library. Total time for this section is about 5-10 minutes.

  1. Download Processing from here and install it in your Applications folder. This tutorial was tested with Processing v.3.5.4 on Mac OSX 10.15.
  2. Create a new sketch (⌘N) and paste in the code below, or download the code from here. Save your sketch (⌘S).
  3. Run the sketch (⌘R). This will generate and export an SVG file of a Lissajous curve. You can locate this SVG file by opening the folder containing your sketch (⌘K).
// Generate a Lissajous curve, and export it as an SVG file.
// For more information on the Processing SVG library, see:
// https://processing.org/reference/libraries/svg/index.html

import processing.svg.*;

void setup() {
  size(1056, 816); // Letter: 11"x8.5" at 96 DPI.
  noLoop(); // Just execute once!
}

void draw() {
  background(255); 
  beginRecord(SVG, "lissajous.svg");
  
  stroke(0); 
  noFill(); // Don't create duplicate shapes.

  int nPoints = 100;
  float cx = width/2; 
  float cy = height/2; 
  float radius = width/4; 

  beginShape(); 
  for (int i=0; i<nPoints; i++) {
    float theta = map(i, 0, nPoints, 0, TWO_PI);
    float px = cx + radius * sin(2.0 * theta);
    float py = cy + radius * cos(3.0 * theta); 
    vertex(px, py);
  }
  endShape(CLOSE); 
  endRecord();
}

The resulting SVG file, when examined with a text editor, begins something like this:

<?xml version="1.0"?>
<!DOCTYPE svg PUBLIC '-//W3C//DTD SVG 1.0//EN'
          'http://www.w3.org/TR/2001/REC-SVG-20010904/DTD/svg10.dtd'>
<svg xmlns:xlink="http://www.w3.org/1999/xlink" 
<!-- etcetera -->

This SVG file depicts a vector graphic resembling the following:

Screenshot of Processing program generating an SVG of a Lissajous curve

Helpful tips (explained here):

  • When exporting shapes (like ellipses and rectangles, etc.) to an SVG file, you'll probably want to issue the command noFill(). Otherwise, Processing will export the shape data twice (once for the stroke, once for the fill).
  • You'll probably want to use noLoop() or exit() to terminate the program after you export the SVG.
  • You can also export a single frame from an animation as an SVG.
  • Using Processing's 3D renderer, beginRaw() and endRaw(), you can export SVG Files from 3D geometry.

4. Convert SVG to HPGL with vpype

Summary: vpype, by Antoine Beyeler, "is an extensible CLI pipeline utility which aims to be the Swiss Army knife for creating, modifying and/or optimizing plotter-ready vector graphics." In this section, we will use vpype to convert the SVG to HPGL—in particular, using its export settings for the HP7475A as described here. For more information on the HPGL language, here's the complete HPGL specification (PDF).

Image: A generic example of a vpype command.
(Image: A generic example of a vpype command.)

This section should take about 10-15m.

  1. The preferred way to install vpype is in a dedicated virtual environment running Python 3.9+. We will follow the vpype installation instructions for end-users, here, which are duplicated below. Make sure your computer has at least 1GB of available hard drive space for this virtual environment.
  2. For starters, in MacOS, you'll need to have the Homebrew package manager installed. You can achieve this with: /bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/HEAD/install.sh)"
  3. Using your Mac's Terminal app, install Python 3.9: brew install python@3.9
  4. Change directory to the folder in which you'd like to create your virtual environment, e.g. cd /Users/golan/Desktop/myVpype
  5. Create a new virtual environment in that directory: python3.9 -m venv vpype_venv . This will create a subdirectory (vpype_venv) containing various files.
  6. Activate the newly created virtual environment: source vpype_venv/bin/activate. (You can exit the virtual environment later by typing deactivate.)
  7. Within the virtual environment, install pip: pip install --upgrade pip
  8. Within the virtual environment, install vype: pip install vpype[all]
  9. You should now be able to run vpype: vpype --help. This may take a moment the first time.
  10. Move a copy of your SVG file into your myVpype directory. Verify that vpype can load and display your SVG: vpype read lissajous.svg show. (Note that this viewer exclusively uses metric units.)
    The vpype viewer
  11. VPYPE TO CONVERT SVG TO HPGL:
    In your Terminal, execute the vpype command to read the SVG and write HPGL formatted for the HP7475a. I used this command: vpype read lissajous.svg write --device hp7475a --page-size letter --landscape lissajous.hpgl but you can find a cookbook of additional recipes here.
  12. This will produce an HPGL file (lissajous.hpgl), which, when examined with a text editor, contains:
IN;DF;PS4;SP1;PU5213,1334;PD5565,1384,5911,1532,6247,1771,6566,2095,6863,2492,7135,2947,7376,3444,7583,3966,7753,4494,7883,5009,7971,5494,8015,5931,8015,6305,7971,6602,7883,6812,7753,6927,7583,6944,7376,6861,7135,6682,6863,6413,6566,6064,6247,5646,5911,5175,5565,4668,5213,4142,4861,3616,4515,3108,4180,2638,3861,2220,3563,1871,3291,1602,3050,1423,2843,1340,2673,1357,2543,1472,2455,1682,2411,1979,2411,2352,2455,2789,2543,3274,2673,3790,2843,4318,3050,4840,3291,5337,3563,5792,3861,6188,4180,6512,4515,6752,4861,6900,5213,6949,5565,6900,5911,6752,6247,6512,6566,6188,6863,5792,7135,5337,7376,4840,7583,4318,7753,3790,7883,3274,7971,2789,8015,2352,8015,1979,7971,1682,7883,1472,7753,1357,7583,1340,7376,1423,7135,1602,6863,1871,6566,2220,6247,2638,5911,3108,5565,3616,5213,4142,4861,4668,4515,5175,4180,5646,3861,6064,3563,6413,3291,6682,3050,6861,2843,6944,2673,6927,2543,6812,2455,6602,2411,6305,2411,5931,2455,5494,2543,5009,2673,4494,2843,3966,3050,3444,3291,2947,3563,2492,3861,2095,4180,1771,4515,1532,4861,1384,5213,1334;PU10365,7962;SP0;IN;

Helpful vpype tips (explained here):

  • The online documentation for vpype is here.
  • vpype can also layout existing vector files with precise control of position, scale and page format.
  • vpype can also optimize existing SVG files for faster and cleaner plots.
  • vpype can also be used to create generative artwork directly, generating HPGL from your own Python code.
  • The speed of the HP7475A plotter can be adjusted from 0.38 to 38.1 centimeters/second, using the HPGL VS command. This is documented here and in the HP7475A programming manual on page 3-3. Vpype allows for an (overall) speed command to be emitted with a file, as documented here, use -vs to emit a velocity value in cm/sec for the HPGL file. The HP7475A ignores the FS (Pen Force) command.
  • Note that the HP7475A always aligns the internal X axis with the paper's long edge, even though A4 is loaded landscape and A3 is loaded portrait. So both are defined with their long edge as the first dimension.
  • As usual, Paul Bourke has some excellent HPGL documentation.

5. Preview Your HPGL File

If desired, it is possible to test-preview your HPGL file using ShareCad.org. Just drag the HPGL file into the ShareCad browser window. The purpose of this might be to check that the conversion from SVG to HPGL was successful.

sharecad_hpgl_preview.png

6. Transmit HPGL to HP7475A with CoolTerm

Here are instructions for executing the plot, by sending the HPGL file over the serial connection to the plotter using CoolTerm (on the Mac). A similar workflow exists for Windows with RealTerm.

  1. Have your HPGL file already generated.
  2. Have the HP7475a connected to your computer.
  3. Launch CoolTerm.
  4. Under Connections→Options→SerialPort, enable CTS and DTR flow control, and turn OFF Software-Supported Flow Control:
    CoolTerm serial port settings
  5. Under Connections→Options→Terminal, set the Terminal Mode to "Line Mode":
    coolterm_7475a_terminal_configuration2.png
  6. Activate the serial connection to the plotter using Connections→Connect.
  7. Load and Transmit the HPGL file using Connections→SendText/BinaryFile:
    Connections->SendText/BinaryFile
  8. The plotter will plot the file!

successful HP7475A plot

Some Alternate Workflows

Vpype is an extremely powerful tool for SVG manipulation, but there are several alternative workflows, especially if you need real-time control of the HP74785a or if you prefer to work in Java or C++.

  • Real-time control of the HP7475A in Processing. Processing can be used to directly control the HP7475A by sending HPGL commands over the serial port, effectively unifying steps 3-6 above. Critically, this also allows for the possibility of interactive (real-time) control of the plotter. An example of real-time cursor control in Processing is here, in which the plotter copies the user's mouse drawings.

Less commonly:

  • Nick Hardeman's ofxHPGL is an openFrameworks (C++) addon which can generate and/or load SVGs, and control the HP7475a directly. It can also allow for interactive real-time control of the HP7475A plotter.
  • Inkcut is a standalone application, also available as an InkScape extension extension, which can load SVG files and control plotters directly (unifying Steps 2 and 3 above). However, it may be challenging to install.
  • Chiplotle is a mostly obsolete Python toolkit for controlling the HP7475A, last actively developed around 2012. A guide to controlling the HP7475A with Chiplotle can be found in this repository, here.

Miscellaneous

HP7475A Plotter Pens