171 lines
6.3 KiB
Markdown
171 lines
6.3 KiB
Markdown
---
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layout: post
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title: "Resurrecting an HP 7440A plotter"
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date: 2019-02-23
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comments: true
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tags: plotter, art, python, hardware, recurse
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---
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I finally decided to give in to the FOMO of
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[#plottertwitter](https://twitter.com/hashtag/plottertwitter?lang=en) and bought
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an old plotter off ebay. Me being in batch at [Recurse
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Center](https://recurse.com) helped a lot, from the decision to get one to
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[nerdsnipe](https://xkcd.com/356/) Alex into collaberating with me. All of this
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work below is done with him.
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## What is a plotter anyway?
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Plotters are devices that can transfer vector graphics onto to a physical
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medium. Core of a plotter is the arm that can move in 2 axes around the medium,
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and ability to put the pen "Down" (draw) and "Up", to position before starting
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to draw. Versions of plotters exist where paper is replaced with other flat
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materials like vinyl and pen with a knife to cut rather than to draw.
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I like to think plotters as the naive solution to the problem that computers
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should be able to draw. Smaller, expensive memory chips (or [magnetic
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cores](https://en.wikipedia.org/wiki/Magnetic-core_memory)) in earlier computers
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made working with raster images hard.
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## HP7440A
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HP was on top of the plotter game when they were popular, so much
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that other manufacturers started to support
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[`HP-GL`](https://en.wikipedia.org/wiki/HP-GL) (short for `HP Graphics
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Langauge`) as the way to talk to their plotters as well.
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HP7440A _"ColorPro"_ was an affordable plotter produced by HP, it can hold 8
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pens at the same time and switch between them automatically and draw on surfaces
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as large as A4. [HP Museum has a longer post about this
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plotter](http://hpmuseum.net/display_item.php?hw=80)
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Ours came pretty unscathed, with the original manuals!
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First thing we did was to open it and clean it. I was quite surprised by how
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easy it is to open, take that 2018 tech!
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{ width=600px }
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Internal mechanism is pretty simple, There is two servos. One for moving paper
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back and forward, and one for moving the pen left and right. There is also a
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solenoid based lever to put the pen down and up.
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## Talk To Me
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{ width=600px }
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However, our plotter didn't come with any cables to either power it or to send
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commands. Power was the biggest mystery <power picture>
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After digging through the manuals, and the [hand drawn schematics from HP
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Museum](http://hpmuseum.net/exhibit.php?hwdoc=80), we managed to identify power
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supply to be a `10-0-10` AC to AC. Luckily someone was selling one in ebay, but
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we couldve built one ourselves if not.
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Communication turns out be standard serial, however our plotter has a `DB-22`
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adaptor, so we had to use a `DB-22` to `DB-9` and `DB-9` to `usb` adaptor.
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.. and finally our plotter moves!
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<video width="600" height="450" controls>
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<source src="/images/7440a_printing.mp4" type="video/mp4">
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</video>
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## Gooo faster.
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HP7440A has a limited amount of buffer space (about 60 bytes), so if we send a
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longer command list to the interface, it will just drop the overflow bits and
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stop to respond.
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Our first naive solution was to add `1s` sleep between sending subsequent
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commands, however this made drawings really slow and there was ink bleeding on
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the paper when the plotter is waiting for the next command.
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Another recurser Francis pointed us at the wait [function in
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hpgl.js](https://djipco.github.io/hpgl/hpgl.js.html#line1535).
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It is a clever hack, the idea is to send the HPGL command `OA` as a marker, `OA`
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sends the current pen position back from plotter, so we batch a bunch of
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commands, suffix it with `OA`, and as soon as we see the position on the line,
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we know that the current set is consumed and we can send the next batch again.
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The code for it looks like this
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```python
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import serial
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# combine command together with maxlen buflen and expose as an iterator
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def stitch(body, buflen=40):
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start = ["IN;PU;", "SP1;"]
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end = ["SP0;"]
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final = start + body + end
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## read in 20 bytes at a time or boundary
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count = 0
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buf = []
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for ins in final:
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if count + len(ins) >= buflen:
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yield "".join(buf)
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buf = []
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count = len(ins)
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else:
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count += len(ins)
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buf.append(ins)
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# send rest of the code
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yield "".join(buf)
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# cmds is a list with semicolon attached to the command
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def exec_hpgl(cmds):
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port = "/dev/cuaU0"
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speed = 9600
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body = stitch(cmds)
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with serial.Serial(port, speed, timeout=None) as plt:
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for ins in body:
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# size (Esc-B) returns bufferlen
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plt.write(ins)
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# For block sent, end with OA, which reports back current
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# position on the pen
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plt.write("OA;")
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c = ""
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data = ""
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while c != '\r':
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c = plt.read()
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data += c
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print("read: {}".format(map(ord, c)))
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print("OA return: {}".format(data))
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# We got data, mean OA got executed, so the instruction buffer
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# is all consumed, ready to sent more.
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```
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This made the plotter super fast!!!
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## Everything's a line anyway!
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After initial success we quickly realized our plotter does not respond to any
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commands that involved HP-GL instructions to draw basic geometry, like `CI` for
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circle.
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A re-reading of the manual made us realize HP sold these functionality as an
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[hardware adaptor board that plugs in the
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bottom](https://support.hp.com/us-en/document/bpp01354), which we don't have.
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But everything in computer graphics is a line anyway right? With some [roots of
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unity](https://support.hp.com/us-en/document/bpp01354) we came up with a circle
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drawing routine.
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<video width="600" height="450" controls>
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<source src="/images/7440a_circle.mp4" type="video/mp4">
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</video>
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## This is only the beginning
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Hardware wise, the only thing remaining to fix is our pens, which expired in
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1996! We are yet to come up with a strategy to refill/replace them.
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{ width=600px }
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I am also going to leave this plotter at RC, so that other recursers continue
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hacking on it, and get another one for me and actually to some generative art.
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:-)
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## Notes
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1. Big thanks for Amy and Francis and Alex S for their help at various points.
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2. All of our python code is here: [https://github.com/dbalan/plotter-scripts](https://github.com/dbalan/plotter-scripts)
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