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Gentle edits of plotter post

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wildconceits 2019-03-12 20:40:36 -04:00 committed by Dhananjay Balan
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blog/2019-02-23-resurracting-an-hp-7440a-plotter.markdown
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@ -11,12 +11,12 @@ I finally decided to give in to FOMO on
an old plotter off ebay. Me being in batch at [Recurse
Center](https://recurse.com) helped a lot, from the decision to get one to
[nerdsnipe](https://xkcd.com/356/) [Alex](https://github.com/wildconceits) into
collaberating with me. All of the work below is done with him.
collaborating with me. All of the work below is done with him.
## What is a plotter anyway?
Plotters are graphics devices that can transfer vectors onto to a physical
medium. Core mechanism of a plotter consists of an arm that can move pen in 2
axes (w.r.t the medium) and ability to put the pen "Down" (draw) and "Up",
Plotters are graphics devices that can transfer vector images onto a physical
medium. The core mechanism of a plotter is an arm that can move a pen in 2
axes (w.r.t the medium) and ability to pick up or place down the pen to draw.
Versions of plotters exist where paper is replaced with other flat materials
like vinyl or pen with a knife to make it a cutting plotter.
@ -27,11 +27,6 @@ made working with raster images hard, and plotters didn't need much operating
memory.
## HP7440A
HP was on top of the plotter game when plotters were popular, so much
that other manufacturers started to support
[`HP-GL`](https://en.wikipedia.org/wiki/HP-GL) (short for `HP Graphics
Langauge`) as the way to talk to their plotters as well.
HP7440A _"ColorPro"_ was an affordable plotter manufactured by HP, it can hold
and switch between 8 pens simultaneously and draw on surfaces as large as A4.
[HP Museum has a longer post about this
@ -44,9 +39,9 @@ easy it is to open, take that 2018 tech!
![7440A Top cover open](/images/7440a_open.jpg){ width=600px }
Internal mechanism is pretty simple, There are two servos. One for moving paper
back and forward, and one for moving the pen left and right. There is also a
solenoid based lever to switch pen down and up.
The internal mechanism is pretty simple, There are two servos. One for
moving the paper back and forward, and the other for moving the pen left and
right. There is also a solenoid based lever to switch pen down and up.
## Talk To Me
@ -65,30 +60,40 @@ Communication turned out be just standard serial, however our plotter has a
`DB-22` adaptor, so we had to use a `DB-22` to `DB-9` adpator and then `DB-9` to
`usb` adaptor.
The final step was writing in the only language the plotter can
understand, [`HP-GL`](https://en.wikipedia.org/wiki/HP-GL) or
`HP Graphics Langauge`. Lucky for us, HP was on top of the plotter game when
plotters were popular, so `HP-GL` has become a de facto standard
for talking to plotters.
.. and finally our plotter moves!
<video width="600" height="450" controls>
<source src="/images/7440a_printing.mp4" type="video/mp4">
</video>
## Gooo faster.
## Goooooooo faster.
HP7440A has a limited amount of buffer space (about `60 bytes`), so if we send a
longer command list to the interface, it will just drop bits after 60 and crash.
Our first naive solution was to add `1s` sleep between sending subsequent
commands, however this made drawings really slow and there was ink bleeding on
the paper when the plotter is waiting for the next command.
commands, however this made drawings really slow and added artifacts from
ink bleeding while the plotter is waiting for the next command.
Another recurser Francis pointed us at the wait [function in
hpgl.js](https://djipco.github.io/hpgl/hpgl.js.html#line1535).
Another recurser Francis pointed us to a clever hack in the [wait function]
in [hpgl.js]. This function uses the HPGL command `OA;` to block execution
until the plotter is finished with the current instruction. When the plotter
executes `OA;` it sends the current pen position, but it first needs to
wait until the pen has stopped moving. Thus we can batch a bunch of commands
and append it with `OA;`. As soon as we see the position over the serial,
we know that the previous batch is consumed and we can send the next batch of commands.
It is a clever hack, the idea is to send the HPGL command `OA` as a marker, `OA`
sends the current pen position back from plotter, so we batch a bunch of
commands, append it with `OA;`, and as soon as we see the position on the line,
we know that the current set is consumed and we can send the next batch again.
[wait function]: https://djipco.github.io/hpgl/hpgl.js.html#line1535
[hpgl.js]: https://github.com/djipco/hpgl
The code for it looks like this:
The code for it looks like this
```python
import serial