Nelko P21 Label Printer

A Python command-line tool for printing text and images to a Nelko P21 Bluetooth thermal label printer over bluetooth.

Prerequisites

Ensure the current user is in the dialout group

This allows access to /dev/rfcomm0 without root:

sudo usermod -aG dialout "$USER"

Log out and back in for the group change to take effect.

Pair the powered-on P21 device

bluetoothctl
# In the bluetoothctl shell:
scan on
pair <DEVICE_MAC>

Output:

❯ bluetoothctl
Waiting to connect to bluetoothd...
[bluetooth]# scan on
[NEW] Device 15:B5:EF:46:08:B6 P21
[P21]# pair 15:B5:EF:46:08:B6
Attempting to pair with 15:B5:EF:46:08:B6
[CHG] Device 15:B5:EF:46:08:B6 Connected: yes
[P21]# [CHG] Device 15:B5:EF:46:08:B6 Bonded: yes
[P21]# [CHG] Device 15:B5:EF:46:08:B6 Paired: yes
[P21]# Pairing successful

Bind the device as an RFCOMM serial port

sudo rfcomm bind /dev/rfcomm0 <DEVICE_MAC> 1

You should now see /dev/rfcomm0.

Example Usage

Print a text label

./p21.py --text "100Ω" # Single line
./p21.py --text $'100\n(Ω)' # Multi line

Print an image

./p21.py --image test-template.png

Preview only (no printing)

Display the generated label instead of printing it so you can sanity-check layout and waste less tape:

./p21.py --text "100Ω" --preview-only
./p21.py --image test-template.png --preview-only

Acknowledgements

This script was made possible thanks to the excellent protocol reverse-engineering work achieved in merlinschumacher/nelko-p21-capture.