{"id":12920,"date":"2016-05-17T08:07:36","date_gmt":"2016-05-17T12:07:36","guid":{"rendered":"http:\/\/scruss.com\/blog\/?p=12920"},"modified":"2025-04-12T14:22:39","modified_gmt":"2025-04-12T18:22:39","slug":"pen-plotters-not-just-output-devices","status":"publish","type":"post","link":"https:\/\/scruss.com\/blog\/2016\/05\/17\/pen-plotters-not-just-output-devices\/","title":{"rendered":"Pen plotters: not just output devices \u2026"},"content":{"rendered":"\n

Pen plotters were pretty expensive and complex pieces of electromechanical equipment. While they often earned their keep in the CAD office, they also had a function that’s almost forgotten: they could be used as input <\/em>devices, too.<\/p>\n\n\n\n

As a kid, we sometimes used to drive past the office of Ferranti-Cetec in Edinburgh. They specialized in digitizers<\/strong>: great big desk or wall mounted devices for capturing points from maps and drawings. Here’s one of their 1973 models:<\/p>\n\n\n\n

\"Ferranti<\/a>
Ferranti EP210 Freescan Digitiser. Source: Grace’s Guide, http:\/\/www.gracesguide.co.uk\/File:Im1973IME-Ferranti.jpg<\/figcaption><\/figure>\n\n\n\n

While the technology and size have changed a bit, these huge bits of engineering kit are the ancestors of today’s track pads and touch screens.<\/p>\n\n\n\n

Realizing that their plotters had very precise X-Y indexing and that they had two-way communications to a computer, HP made a drafting sight that fitted in place of a pen on their plotters:<\/p>\n\n\n\n

\"HP<\/a>
HP drafting sight, part no 09872-60066.<\/figcaption><\/figure>\n\n\n\n

This is a very pleasing piece of kit, all metal, thick plastic and polished optical glass. They show up on eBay occasionally, and aren’t cheap. With a bit of coercion, it fits into my HP plotter like this:<\/p>\n\n\n\n

\"Drafting<\/a>
Drafting sight in HP7470A plotter<\/figcaption><\/figure>\n\n\n\n

The image is very bright and clear:<\/p>\n\n\n\n

\"Drafting<\/a>
Drafting sight near an axis label<\/figcaption><\/figure>\n\n\n\n
\"Drafting<\/a>
Drafting sight over a point, showing cursor dot<\/figcaption><\/figure>\n\n\n\n

If one has a digitizing sight, one needs to find something to digitize post haste<\/em>. I’m sure everyone<\/em> can sense the urgency in that. So I found this, a scan from my undergraduate project writeup (centrifugal pump impeller design ftw, or something), which was probably made on an Amiga or Atari ST:<\/p>\n\n\n\n

\"It's<\/a>
It’s a graph, with pointy bits on it<\/figcaption><\/figure>\n\n\n\n

I printed this as large as I could on Letter paper, as it’s the only size my HP7470A plotter can take. Now all it needed was a small matter of programming to get the data from the plotter. Here’s a minimally-useful digitizer for HP and compatible serial plotters. Although I ran it on my little HP grit wheel plotter attached to a Raspberry Pi, I developed it with my larger Roland plotter. The only fancy module it needs is pySerial<\/a>.<\/p>\n\n\n

\n#!\/usr\/bin\/env python\n# -*- coding: utf-8 -*-\n# a really crap HP-GL point digitizer\n#  scruss - 2016\n\nfrom time import sleep\nfrom string import strip\nimport serial\n\nser = serial.Serial(port='\/dev\/ttyUSB1', baudrate=9600, timeout=0.5)\nlbl = ''\npoints = []\nlabels = []\nk = 0\nretval = 0\n\nser.write('DP;')                # put in digitizing mode\nwhile lbl != 'quit':\n    ser.write('OS;')\n    ret = strip(ser.read(size=5), chr(13))\n    print ('Retval: ', ret)\n    if ret != '':\n        retval = int(ret)\n    if retval & 4:              # bit 2 is set; we have a point!\n        print ('Have Point! Retval: ', retval)\n        retval = 0\n        ser.write('OD;')\n        pt = strip(ser.read(size=20), chr(13))\n        print ('OD point: ', pt)\n        lbl = raw_input('Input label [quit to end]: ')\n        points.append(pt)\n        labels.append(lbl)\n        k = k + 1\n        ser.write('DP;')        # put in digitizing mode again\n    sleep(1)\nser.close()\n\nf = open('digit.dat', 'w')\nfor i in range(k):\n    f.write(points[i])\n    f.write(',')\n    f.write(labels[i])\n    f.write('\\n')\nf.close()\n<\/pre><\/div>\n\n\n
In the unlikely event that anyone actually uses this, they’ll need to change the serial port details near the top of the program.<\/p>\n\n\n\n
The program works like this:<\/p>\n\n\n\n
    \n
  1. Move the drafting sight to the point you want to capture using the plotter’s cursor keys, and hit the plotter’s ENTER<\/strong> key<\/li>\n\n\n\n
  2. Your computer will prompt you for a label. This can be anything except<\/em> quit<\/strong>, that ends the program<\/li>\n\n\n\n
  3. When you have digitized all the points you want and entered quit <\/strong>as the last label, the program writes the points to the file digit.dat<\/em><\/li>\n<\/ol>\n\n\n\n
    I didn’t implement any flow control or other buffer management, so it can crash in a variety of hilarious ways. I did manage to get it to work on the lower trace of that graph, and got these data:<\/p>\n\n\n\n
    9649,2428,1,300,0\n357,2428,1,0,0\n357,7217,1,0,0.60\n733,3112,1,first\n826,3167,1,\n968,3256,1,\n1122,3334,1,\n1290,3405,1,\n1588,3583,1,\n1891,3725,1,\n2215,3880,1,\n2526,4051,1,\n2830,4194,1,\n3143,4280,1,\n3455,4516,1,\n4077,4767,1,\n5008,5229,1,\n6543,5954,1,\n8067,6548,1,\n8740,7195,1,\n8740,7195,1,last\n8740,7195,1,quit<\/pre>\n\n\n\n
    The first two columns are X and Y, in HP-GL units \u2014 that’s 1\/40 mm, or 1\/1016 inches. The third column will always be 1 if you have the sight down. The last columns are the label; if you put commas in them, opening the file as CSV will split the label into columns. I used it to fudge axis points. You’ll also note that the last three lines of data are my valiant attempts to quit the program.<\/p>\n\n\n\n
    Assuming the axes are not skewed (they are, very slightly, but shhh<\/em>) some simple linear interpolation gives you the results below:<\/p>\n\n\n\n
     12.1    0.086\n 15.1    0.093\n 19.7    0.104\n 24.7    0.114\n 30.1    0.122\n 39.7    0.145\n 49.5    0.162\n 60.0    0.182\n 70.0    0.203\n 79.8    0.221\n 89.9    0.232\n100.0    0.262\n120.1    0.293\n150.2    0.351\n199.7    0.442\n248.9    0.516\n270.7    0.597<\/pre>\n\n\n\n
    Good enough for a demo.<\/p>\n\n\n\n
    (For prettier things to do with plotter digitizing commands, Ed Nisley KE4ZNU has made some rather lovely Superformula patterns<\/a>)<\/p>\n\n\n\n
    If you don’t have a plotter, or even if you do and you don’t have hours to waste mucking about with Python, obsolete optics and serial connections, Ankit Rohatgi’s excellent WebPlotDigitizer<\/a> (or Engauge<\/a>, as I found out when this article hit HackerNews<\/a> in 2021) gets numbers out of graphs quickly. It handles all sorts of graphs rather well.<\/p>\n\n\n\n
    Update, 2025<\/strong>: eek, but WebPlotDigitizer is now login-only and has been infested by AI shite. Run away now, run away fast … or try to find an old mirror online, such as WebPlotDigitizer at utk.edu<\/a>.<\/p>\n","protected":false},"excerpt":{"rendered":"
    Pen plotters were pretty expensive and complex pieces of electromechanical equipment. While they often earned their keep in the CAD office, they also had a function that’s almost forgotten: they could be used as input devices, too. As a kid, we sometimes used to drive past the office of Ferranti-Cetec in Edinburgh. They specialized in […]<\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_jetpack_memberships_contains_paid_content":false,"footnotes":"","jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":true,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2}},"categories":[7],"tags":[2975,2788,2789,2540,2510],"class_list":["post-12920","post","type-post","status-publish","format-standard","hentry","category-computers-suck","tag-digitizer","tag-hpgl","tag-plotter","tag-python","tag-raspberrypi"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_shortlink":"https:\/\/wp.me\/pQNZZ-3mo","jetpack_sharing_enabled":true,"_links":{"self":[{"href":"https:\/\/scruss.com\/blog\/wp-json\/wp\/v2\/posts\/12920","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/scruss.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/scruss.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/scruss.com\/blog\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/scruss.com\/blog\/wp-json\/wp\/v2\/comments?post=12920"}],"version-history":[{"count":14,"href":"https:\/\/scruss.com\/blog\/wp-json\/wp\/v2\/posts\/12920\/revisions"}],"predecessor-version":[{"id":17820,"href":"https:\/\/scruss.com\/blog\/wp-json\/wp\/v2\/posts\/12920\/revisions\/17820"}],"wp:attachment":[{"href":"https:\/\/scruss.com\/blog\/wp-json\/wp\/v2\/media?parent=12920"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/scruss.com\/blog\/wp-json\/wp\/v2\/categories?post=12920"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/scruss.com\/blog\/wp-json\/wp\/v2\/tags?post=12920"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}