OctoPrint on an Atom N270 with a USB Webcam and Serial Port DTR Control

by snm, December 28th, 2019

As mentioned in 3D Printing with 3D MARS white PLA: pass-through wallplate, paper towel holder, right-angle spool holder, shower curtain rings, and a prototype case, I always wanted to install OctoPrint as long as I’ve had this printer, but until now, I was printing by manually walking over and inserting an SD card. Following the upgrades covered in Glass bed and heater rewire mods to Monoprice Select Mini v2 3D printer, it is finally time to install OctoPrint. This post details my setup.

Initial setup

Atom N270

Lots of people run OctoPrint on a Raspberry Pi 3+, but I happened to have another embedded system lying around unused I wanted to put to good use. It runs an Intel Atom® Processor N270, 512K Cache, 1.60 GHz, 533 MHz FSB.

What is Intel Atom?

Intel Atom is the brand name for a line of IA-32 and x86-64 instruction set ultra-low-voltage microprocessors by Intel Corporation.

The N270 I have is from the N2xx series, Diamondville code name, with a single 45 nm core, hyperthreading, but no Intel 64, released June 2008. Wikipedia: List of Intel Atom microprocessors classifies it has a “netbook (sub-notebook)” processor.

An older processor, but not ancient. The Atom series is still being used, and was mentioned in Hackaday: 2019: As the hardware world turns year-end review:

But the Raspberry Pi isn’t the only SBC game in town. It’s not even the only one with a cute “Pi” name, for that matter. We saw significant interest in the Atomic Pi, which delivered the power of a quad-core Intel Atom processor at a size and price not far from that of its berry-flavored peer. We’ve since featured a number of impressive modifications to the powerful board, but the discovery that the Atomic Pi was actually surplus hardware purchased from the now defunct Mayfield Robotics raised some valid questions about the long-term viability of the product.

The Atomic Pi runs an Intel Atom x5-Z8350 (Cherry Trail) at 1.44 GHz, surprisingly a lower clock speed than my 1.60 GHz N270 (not that clockspeed is all that matters). Anyways, either are more powerful than the ARMv6 BCM2835 1GHz in the Pi Zero, and should be sufficient to run OctoPrint. Here’s a picture of my board (unknown motherboard, not Atomic) running the Atom N270:

OctoPrint

As the Atom N270 is only 32-bit, I installed Arch Linux 32, an actively-maintained 32-bit distribution based on Arch Linux. Many other distributions have dropped 32-bit support, moving onto 64-bit only, so a distribution had to be selected meeting the architectural requirements. I picked Arch due to its philosophy as a “lightweight and flexible Linux® distribution that tries to Keep It Simple”, which makes a lot of sense for this application.

After following the very informative Arch Linux installation guide, I installed OctoPrint from https://github.com/foosel/OctoPrint, following the steps as documented. Well, sort of: I used the rc/devel branch instead of master , and avoided setting up a virtual Python environment, instead installing everything for Python 3.8.0 (from the python 3.8.0-1.0 pacman package).

OctoPrint listens on port 5000. The Pi Zero camera I’ll setup in the next section is available on 192.168.7.2… a private address only available to the host PC (this Atom N270) and the Pi Zero itself. To get the camera working on the web interface, it will have to be accessible from the web. To accomplish this, I installed HAproxy, and configured it as follows in /etc/haproxy/haproxy.cfg :

global maxconn 20000 log 127.0.0.1 local0 user haproxy chroot /usr/share/haproxy pidfile /run/haproxy.pid daemon frontend http-in bind *:80 mode http option httplog timeout client 30s acl is_pizero_camera path_beg /pizero_camera use_backend pizero_camera if is_pizero_camera default_backend octoprint backend pizero_camera mode http timeout connect 5s timeout server 5s http-request set-path %[path,regsub(^/pizero_camera/,/)] server mjpgstreamer 192.168.7.2:80 maxconn 32 backend octoprint mode http timeout connect 5s timeout server 5s server mjpgstreamer 127.0.0.1:5000 maxconn 32

Now OctoPrint can be accessed on port 80, and the camera can be accessed on the same address and port, but at the /pizero_camera path. In OctoPrint settings Webcam and Timelapse, configured:

Check “Enable webcam support” Stream URL: http://172.16.0.87/pizero_camera/?action=stream

Check “Enable timelapse support” Snapshot URL: http://172.16.0.87/pizero_camera/?action=snapshot Path to FFMPEG: /usr/bin/ffmpeg



Install FFmpeg for video encoding (note, when installing FFmpeg on a Pi, for some reason a lot of guides explain how to compile it yourself, but you can just use FFmpeg Static Builds, armhf version on the Pi 3… I used this technique later, for setting up homebridge-camera-ffmpeg to allow the camera to be streamed over Homebridge. But in this case, I just installed FFmpeg using the Arch Linux repository, via pacman ).

The great advantage of OctoPrint is the vast quantity of plugins available.

I installed OctoPrint-MalyanConnectionFix to fix the connection error with the MPSM.

Raspberry Pi Zero: CSI camera, USB Ethernet gadget

To view the print, we’ll need a camera. I decided to repurpose the Raspberry Pi Zero I had previously used for these projects:

The Pi Zero is connected to a compatible camera (via the miniature Camera Serial Interface port):

However, the Pi Zero is not necessarily recommended to run OctoPrint. At least, the official site says the “Raspberry Pi Zero W is not recommended since severe performance issues were observed”, and the recommended hardware is a Raspberry Pi 3B.

Hence, I’ll only be using the Pi Zero for the camera. To do this, it will be configured as a USB Ethernet gadget, following these tutorials:

After editing /boot/config.txt to enable dwc2,g_ether , the device can be plugged in to a PC (from the “USB” port, not PWR) and the PC will both supply power and treat it as an Ethernet device. I statically configured the Pi’s address to 192.168.7.2 , and my host PC’s address to 192.168.7.1 .

Next I setup mjpg-streamer to stream from input_raspicam to output_http . To allow apt to access the Internet, edited /etc/apt/apt.conf and added:

Acquire::http::Proxy "http://USERNAME:PASSWORD@192.168.7.1:9011"; Acquire::https::Proxy "https://USERNAME:PASSWORD@192.168.7.1:9011";

then ran inaz2/proxy2 on the host machine, modified to listen on all interfaces:

--- a/proxy2.py +++ b/proxy2.py @@ -373,6 +373,7 @@ def test(HandlerClass=ProxyRequestHandler, ServerClass=ThreadingHTTPServer, prot else: port = 8080 server_address = ('::1', port) + server_address = ('', 9011) HandlerClass.protocol_version = protocol httpd = ServerClass(server_address, HandlerClass)

Once mjpg-streamer is working properly, http://192.168.7.2/?action=stream can be accessed from the host PC to view a live stream from the Pi Zero camera.

Not pretty (to say the least), but it works:

To view the Pi’s display, I connected the mini HDMI port to a VGA monitor using Mini HDMI to VGA Adapter, Benfei Gold-Plated Mini HDMI to VGA Adapter (Male to Female). To access the console, I enabled the serial console by adding enable_uart=1 to /boot/config.txt , and wired TXD, RXD through a SparkFun FTDI Basic Breakout - 3.3V serial to mini-USB adapter.

Not too happy with the complexity of the setup, but I’ll improve it in the next sections, stay tuned.

Power control with ESP8266

The other feature I wanted, besides remote monitoring, is remote power control. Forcefully turn the printer power off, if it is needed for emergency purposes or any other reason.

Similar to the design here:

I went with an Espressif ESP8266 connected to a GPIO to drive a relay, controllable over WiFi. Now I can access the web interface and turn the printer’s power off or on.

Taking stock and eyeing improvements

To make sense of this setup, I drew this diagram on draw.io:

If it looks overly complex, that is because it is. If the diagram looks messy, it is even worse in person, which I won’t show. While it is functional, and I learned a lot building this complex configuration, there is a lot to be improved. Without further ado, let’s get started.

Improvements

Power control with DTR signal on RS232 port

In the first iteration, I used a separate microcontroller to provide GPIO to control the relay to switch the printer’s AC power brick. Not too elegant having yet another system, merely for one purpose. Why not merge it with the Atom N270 system? Doesn’t it have any GPIO signals I can tap for controlling a relay?

It turns out: yes, sort of. Taking a look at the motherboard backplane:

USB, PS/2, VGA, DVI, Ethernet, audio, then (getting warmer) parallel, and two RS232 serial ports! Yes, this board actually has serial. Parallel could also be used. But it so happens RS232 has a few signals that can be used as GPIO, as this post explains:

Here’s the connector pinout (from Lammert Bies: RS232 serial cables pinout):

I decided to use the Data Terminal Ready (DTR) pin. Linux Journal: The tty Layer, Part II explains the ioctl() ’s available to control these signals, basically either TIOCMSET to change all the values, or TIOCMBIS to set (to high) and TIOCMBIC to clear (to low).

tty_ioctl(4) also documents this API quite well:

Modem control TIOCMGET int *argp Get the status of modem bits. TIOCMSET const int *argp Set the status of modem bits. TIOCMBIC const int *argp Clear the indicated modem bits. TIOCMBIS const int *argp Set the indicated modem bits. The following bits are used by the above ioctls: TIOCM_LE DSR (data set ready/line enable) TIOCM_DTR DTR (data terminal ready) TIOCM_RTS RTS (request to send) TIOCM_ST Secondary TXD (transmit) TIOCM_SR Secondary RXD (receive) TIOCM_CTS CTS (clear to send) TIOCM_CAR DCD (data carrier detect) TIOCM_CD see TIOCM_CAR TIOCM_RNG RNG (ring) TIOCM_RI see TIOCM_RNG TIOCM_DSR DSR (data set ready)

A simple C program can now be written to set DTR. However, when I measured the voltage on the DTR pin with a multimeter, it would start out low (RS232 signaling levels are ±12V):

and briefly spike high when executing TIOCMBIS:

but would fall back to low after the program exits (at least, on my system). To cope with this behavior, I wrote set_dtr.c to hold DTR while it is running; it pauses indefinitely, then it can be killed to return DTR to low:

// set_dtr.c: hold DTR (pin 4) of RS-232 port high #include <stdio.h> #include <stdlib.h> #include <sys/ioctl.h> #include <fcntl.h> #include <unistd.h> int main ( int argc , char ** argv ) { char * port_path = "/dev/ttyS0" ; int fd = open ( port_path , O_RDWR | O_NOCTTY ); if ( fd < 0 ) { perror ( "open" ); exit ( EXIT_FAILURE ); } int status = TIOCM_DTR ; int request = TIOCMBIS ; // set if ( ioctl ( fd , request , & status ) < 0 ) { perror ( "ioctl" ); exit ( EXIT_FAILURE ); } printf ( "DTR set, press ^C to quit

" ); pause (); printf ( "exiting

" ); return 0 ; }

Compile with clang set_dtr.c -o set_dtr .

To more conveniently allow starting/stopping the printer, wrote a systemd service, saved in /etc/systemd/system/power3dprinter.service :

[Unit] Description=Power on the MPSM 3D Printer through DTR signal of RS232 serial port [Service] ExecStart=/home/admin/rs232/set_dtr [Install] WantedBy=multi-user.target

Now it is possible to raise the DTR signal high using sudo systemctl start power3dprinter.service , and return it to low with sudo systemctl stop power3dprinter.service .

As for the circuit, an NPN BJT transistor driver will do. Whipped this up in CircuitLab:

I used a random transistor I had on hand, the 2SC2688 “designed for use in Color TV chroma circuits”, probably where I salvaged it (datasheet (maybe from Pioneer SD-P453S Rear-Projection (RPTV) teardown: inside an 80s vintage big screen TV?). The h FE , DC current gain, measured about 50. R1 was calculated to saturate the transistor, allowing the relay to turn on. D1 is a flyback diode to protect from inductive voltage spikes as the relay coil deenergizes.

C2688’s V BO0 , maximum emitter to base voltage, is 5.0V. This poses a problem, since RS232 is ±12V. Stack Overflow: Protecting NPN transistor from negative base-emitter voltage? suggests a solution: a clamping diode from ground to base. This is D2 in the circuit above. I actually wired it up without this diode at first, and it worked, but little did I know I may have been zenering the transistor (as in zener diode), interesting discussion: sci.electronics.design: Zenering a big transistor:

I’m using a TIP31C (pnp in to-220 pac) as a temp sensor (diode connected) There’s a couple of depletion fets in series as current limiters. (LND150) The b-e junction starts to zener at ~30 V (only two tested so far).

see also: Stack Overflow: At which negative Vce voltage will an NPN transistor be damaged?.

To double as a standby power indicator, I used an LED for D2, so when DTR is low, it lights up. When DTR goes high, it turns off. Will the high +12V voltage be a problem? I’m not sure, but it hasn’t yet.

The final problem: how to power this thing? The relay requires 12V. Would be nice if could power the relay off of the RS232’s 12V signals, but serial ports won’t provide enough current (~60 mA for the relay coil), not much more than necessary to say power a serial mouse. Considered powering over USB, but USB (without USB-PD), only provides 5V; could use the buck converter from XL6019 boost/buck DC-DC converter unboxing, but would be too bulky. Instead, I salvaged a Molex connector from an old hard drive enclosure:

The disk drive Molex connectors provide +12V on the yellow wire, perfect. I ran the 12V wire out through the Kensington Security Slot, and covered it in heat shrink tubing (Aliexpress: 580PCS 8 Sizes 5 Multi Color Polyolefin 2:1 Heat Shrink Tubing Tube SleeveTube Assortment Sleeve Wrap Wire Kit tubes Kits, shrank with Youyue 858D hot air gun first look):

The 12V wire joins the serial connector cable. Only GND and DTR are used currently, but since I reused a CAT 5 cable with 6 wires, I hooked up a few other signals for future expansion. Coloring code for my future reference:

Wire color Function Use black ground ground yellow 12V power the 12V relay blue DTR “GPIO” to control the relay red TX green RX white RTS

Notably, RTS (Request to Send) is another RS232 signal easily usable as GPIO, through TIOCM_RTS in Linux tty_ioctl, and TX/RX are for transmit/receive which could be used if serial communication is to be implemented later, for some reason. Nonetheless for now, only three wires are used to control the AC-switching relay from the PC: ground, 12V, and DTR.

OctoPrint plugin

The printer can now be turned on and off through the set_dtr program or systemd , but it would be nice to control it from the OctoPrint web interface.

I searched for power plugins, and found:

Most of these plugins are for specific IoT hardware. Is there anything to let me control power through shell commands, sudo systemctl start power3dprinter.service and sudo systemctl stop power3dprinter.service ?

OctoPrint-PSUControl looks promising:

Switching Method PSUControl can switch on your power supply using either: … System Command Executes a system command on the OctoPrint server. Note: Commands on *nix systems will be executed within the sh interpreter.

however, the Python compatibility string is “>=2.7,<3”, meaning it is not currently compatible with Python 3. And I installed Python 3.8.0. Why use anything older? PEP 373 – Python 2.7 Release Schedule explains when Python 2 will be EOL’d:

The End Of Life date (EOL, sunset date) for Python 2.7 has been moved five years into the future, to 2020.

The future is now—the clock is ticking, literally next week Python 2 will be sunset, at the time of this writing 3 days from now according to Pythonclock.org. Not that it will suddenly stop working, but I wouldn’t want to rely on unsupported software.

Fortunately, the OctoPrint Plugin Tutorial explains how to get up and running with a custom plugin in no time. I followed the steps to create OctoPrint-Helloworld , and replaced the octoprint_helloworld/__init__.py with:

# coding=utf-8 from __future__ import absolute_import import octoprint.plugin import subprocess import json class HelloWorldPlugin ( octoprint . plugin . StartupPlugin , octoprint . plugin . TemplatePlugin , octoprint . plugin . AssetPlugin , octoprint . plugin . SimpleApiPlugin ): ##~~ AssetPlugin mixin def get_assets ( self ): return dict ( js = [ "js/helloworld.js" ], css = [ "css/helloworld.css" ], ) ##~~ SimpleApiPlugin mixin def get_api_commands ( self ): return dict ( turnOn = [], turnOff = []) def on_api_command ( self , command , data ): try : if command == 'turnOn' : rc = subprocess . check_output ( "/home/admin/on_3dprinter.sh" ) response = { "status" : str ( rc )} elif command == 'turnOff' : rc = subprocess . check_output ( "/home/admin/off_3dprinter.sh" ) response = { "status" : str ( rc )} else : response = { "error" : "bad command" } except Exception as e : response = { "error" : str ( e )} return json . dumps ( response ) __plugin_name__ = "Hello World" __plugin_implementation__ = HelloWorldPlugin ()

The JavaScript, in octoprint_helloworld/static/js/helloworld.js , only has to implement methods to call turnOn and turnOff:

$ ( function () { function HelloworldViewModel ( parameters ) { var self = this ; // https://github.com/jneilliii/OctoPrint-TPLinkSmartplug/blob/master/octoprint_helloworld/static/js/helloworld.js self . sendTurnOn = function ( data ) { $ . ajax ({ url : API_BASEURL + " plugin/helloworld " , type : " POST " , dataType : " json " , data : JSON . stringify ({ command : " turnOn " , data : {}, }), contentType : " application/json; charset=UTF-8 " }). done ( function ( data ){ console . log ( ' Turn on command completed. ' ); console . log ( data ); }); }; self . sendTurnOff = function () { $ . ajax ({ url : API_BASEURL + " plugin/helloworld " , type : " POST " , dataType : " json " , data : JSON . stringify ({ command : " turnOff " , data : {}, }), contentType : " application/json; charset=UTF-8 " }). done ( function ( data ){ console . log ( ' Turn off command completed. ' ); console . log ( data ); }); }; window . sendTurnOn = self . sendTurnOn ; window . sendTurnOff = self . sendTurnOff ; } OCTOPRINT_VIEWMODELS . push ({ construct : HelloworldViewModel , dependencies : [], elements : [], }); });

Finally, octoprint_helloworld/templates/helloworld_navbar.jinja2 provides the HTML template to insert into the OctoPrint navigation bar:

<nobr> <a href= "#" onclick= "sendTurnOn(); return false" > 🔌 </a> | <a href= "#" onclick= "sendTurnOff(); return false" > ❌ </a> </nobr>

Would have liked to use the Unicode power symbols new in Unicode 9.0, but they are not yet supported in my operating system; if they are in yours these symbols should not appear as question marks or empty boxes:

Power ⏻ ⏻ Toggle Power ⏼ ⏼ Power On ⏽ ⏽ Power Off ⭘ ⭘ Sleep Mode ⏾ ⏾

Anyways, with this change the new buttons show up in the UI:

They may not be properly aligned, and placed dangerously close to the settings button, but they function properly. Now I can click 🔌 to turn on the printer, and ❌ to turn it off, easy as that.

It would still be a good idea to use PSUControl once it updates, as this quick-and-dirty plugin doesn’t implement any additional features such as auto-connecting after turning on, turning off after a print, warning before turning off during a print, and so on.

systemd user woes

A final note before we move on, here is the contents of /home/admin/on_3dprinter.sh :

sudo /usr/bin/systemctl start power3dprinter.service

and /home/admin/off_3dprinter.sh :

sudo /usr/bin/systemctl stop power3dprinter.service

Recall how the power3dprinter.service is installed under /etc/systemd/system . Would have liked to install it as a user service under /etc/systemd/user , and start with systemctl --user , so OctoPrint wouldn’t have to escalate from the admin user to root. However, when executed from OctoPrint the user command failed with an error about DBUS_SESSION_BUS_ADDRESS , see similar problems reported at:

First attempt to solve this was to set the value to what I observed over SSH:

export DBUS_SESSION_BUS_ADDRESS = "unix:path=/run/user/1000/bus"

This worked… until I logged out of SSH, and interrupted my print job! Whoops.

An approved solution is to use dbus-launch, like so:

export $( dbus-launch )

This sort of works, in that the service is started, but then is immediately stopped once the shell returns. I’m not sure why, or the best way to fix it. In any case, running as a system service instead works around the issue, and I can now remotely control the power of my printer without interruption.

USB webcam

In the first iteration, I used a Raspberry Pi Zero with a camera. Printed a case: One Piece Raspberry Pi Zero + Camera Case (with GPIO), but the camera viewport was too high; cut out a hole lower. Even with the case, using the Pi Zero only for a camera felt like a waste.

Therefore, I switched to a standard USB webcam, with a better quality 1080p resolution. Like most webcams it is supported under by the USB Video Class (UVC) driver with mjpg_streamer , making it easy to adapt into OctoPrint. All I would have to do to make it into a workable solution is build a camera mount.

Camera mount

There are many mounts for MPSM on Thingiverse, I liked this one:

MPSM_C270_Mount by TheKrush

It attaches to the front of the bed, so it moves as the bed moves, giving a consistent view of the print for timelapsing. This mount almost works with my tripod camera. I printed:

Model Est filament (m) Est filament (g) Actual time Estimated cost MPSM_C270_Mount 5.06 16 3h 13min 25¢ Tripod_Arm 1.39 4 49min 6¢ Tripod_Top 1.69 5 40min 8¢

The bed mount and arm fit, but there is a problem with the “Tripod_Top”. Even though it is called “tripod”, it doesn’t have a standard Tripod screw thread of 1/4-20 UNC. The creator says “I reused the old plastic bolt/screw that came with the C270 to attach the hinge”, but I only have a standard tripod mount screw hole.

Found this tripod screw model:

Printed with 0.22m of filament, ~0g, in 13 minutes for an estimated cost of 0.4¢:

Carefully able to thread the tripod screw into the camera base:

This should work. It screwed into up where I marked the thread with a sharpie, and this is how the parts would be assembled:

All we’ll have to do is combine the 1/4-20” UNC tripod screw model with the Tripod_Top model. Imported both STLs into Tinkercad, and positioned the screw over the top hole:

then exported to STL. The 1/4-20” UNC tripod screw sticks out of the top where the Tripod_Top hole was.

The first print attempt failed at the screw:

even with supports. Reprinted with a large raft:

This time it succeded (1hr 32min, 3.02m, 8g, 16¢). Carefully screwed the camera onto tripod mount:

The final task was to assemble the mount. The designer used metric fasteners, which I didn’t have, so I ordered:

A few days later the box arrived:

I used:

4x M3-12 (didn’t come with 10mm) + 4x M3 nuts to attach base, self-taps into the plastic, tighten with allen wrench

2x M3-20 + 2x M3 nuts to attach arms

first, tapping the screw through the plastic:

then attaching to the bed:

.

The hex nut fits snugly in the holes in the arm:

Although I had to warm up one of the holes with water to shape it into place. The mount is attached:

With everything said and done, the OctoPrint GUI now shows the complete view of the bed through the webcam:

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