A Server Printer

Code for the Raspberry Pi and the Adafruit Mini Thermal Receipt Printer to connect to my A Server project, and print out whatever is on it.

Written in Python, for Python 3. Makes use of a slightly modified version of Adafruit's provided Python Library. See Adafruit_Thermal.py for licensing (hint: MIT) and more info.

Getting Started

Hardware Dependencies

• Rasbperry Pi installed with Raspbian and internet/wifi connection (I use a Pi Zero W)
• Adafruit Mini Thermal Receipt Printer
• 1x LED
• 2x Momentary Push Switches

Software Dependencies

To run as I wrote it (for retrieving from A Server) you will need

• Python 3
• RPi.GPIO
• Python Requests
• An instance of A Server running up on AWS lambda

And optionally:

• Emoji2Text - A library I wrote to replace emoji with their text based version. Without it, will just print a few question marks when you come to emoji.

Of course, it should be pretty easy to repurpose the software to communicate with something else. Just re-implement the AServerConnection object in communications to connect to whatever endpoint you want using whatever library you want.

Hardware Setup

• Printer Input to Pi Tx
• Printer Ground to Pi Ground
• Printer Output disconnected (I wasn't able to get this working, if connected, needs a resistor)
• "Print" button to Pi pin 23 (BCM) and ground
• "Clear Server" button to Pi pin 24 (BCM) and ground
• LED + to Pi pin 25 (BCM) and - to ground

Pins for the buttons and LED can be changed in .config.json (see Software Setup).

Software Setup

Firstly, setup the connection to the printer to allow us to communicate over serial. Open up terminal and type:

sudo raspi-config

A menu will come up, select Interfacing Options, then Serial. For the first question "Would you like a login shell to be accessible over serial?" select <No>. For the second question "Would you like the serial port hardware to be enabled?" select <Yes>.

A screen will pop up, it should say "The serial login shell is disabled" and "The serial interface is enabled". If it doesn't something was set up wrong, try running the config again. When finished, exit and reboot.

Then copy .config.example.json to .config.json. Change variables to your local setup. You can add any extra config defined in DEFAULT_CONFIG of main.py.

You'll then need to set the app to run whenever the pi turns online. I've provided a short script to help you with this. Call:

sudo ./setup-runtime

It'll create a new script in /etc/init.d/ and subscribes it to startup.

After this point, if you restart the pi:

sudo reboot

Wait a bit, and then you should see the status led flashing. Once it gives three long pulses and switches off, it's ready to print.

Runtime

Press the "print" button (default pin 23) to connect to the server and print. Press the "clear server" button (default pin 24) to wipe whatever is on the server. The button on the thermal printer feeds the paper a few extra rows.

Text being printed is formatted accordingly:

• attempted conversion of emoji to a text representation (e.g. 😎 to [smiling face with sunglasses]);
• safe conversion of a few common unicode replacements (e.g. quotation marks);
• removal of other unicode; and
• wrapping text to fit within the 32 character spacing

Troubleshooting

• The printer prints without being told to:

  Run the config:

    sudo raspi-config
  

  Go to 5 Interfacing Options -> P6 Serial. Disable login shell over serial but keep serial port hardware enabled. It should display a message:

    The serial login shell is disabled
    The serial interface is enabled
  

  Then edit the file /boot/cmdline.txt

    sudo nano /boot/cmdline.txt
  

  And remove any console=serial0,xxxxxx style setting. Do not remove console=tty1.

  Restart the Raspberry Pi.

  Printing should now behave as expected.

• Things aren't starting up and I don't know what's going wrong!

  Take a look at the logs in ~/log/printer.log. That might help.

• Emoji aren't printing as text even though I installed emoji2text

  Because of how startup scripts are called, python is running under sudo. So a workaround that I've found is to install emoji2text using sudo pip...

    sudo pip3 install emoji2text
  

  I'm not 100% happy with this solution, but I have been running out time so this is the quickest workaround I've got...

Future Work

I'm not sure I'm going to do much more on this. As with any project, there are a heap of things I'd have done differently if I had my time again. I've actually been tossing up about rebuilding it some time with something like an ESP8266 instead of a pi, which would of course mean this entire library needs to be thrown out. That said, if anyone has suggestions or contributions, please let me know. If you've used any of what I've written for some of your own work, I'd love to hear about them.