YHK-Cat-Thermal-Printer

This is yet another project with a Cat/Rabbit thermal printer. Other GitHub sources are also accessible, however none of them were successful for me because they all used the Cat-Printer with BLE protocol.

Unfortunately, my cat-printer uses a different firmware version that is based on the Classic Bluetooth protocol rather than the GATT based protocol. YHK-XXXX was broadcast by my printer. The last four characters of the printer's MAC address are XXXX.

The Android and iOS app named WalkPrint is compatible with my cat printer. Although the app is worthless, some features need logging in.

My starting point: I was motivated from This blogpost and planned to have my own printer. I did spent some time working on the bitbank2/Thermal_Printer project, but I soon found that since my printer is different so no other code will run on it.

You can also read the full product review here

Other reference projects repositories

• bitbank2/Thermal_Printer
• WerWolv/PythonCatPrinter
• amber-sixel/PythonCatPrinter
• the6p4c/catteprinter
• JJJollyjim/PyCatte
• xssfox

Some real work of RE 🚀️

In order to obtain certain internals, I have started my own reverse engineering.

To examine the packet exchange between the phone and the printer, I decompiled the Android app, grabbed the BT snoop log from my phone, and then opened the log file in Wireshark. And indeed, the BLE/GATT-based system was not the cause. Decompiled code for Android supports that.

Action Replay 😄

Therefore, everything is straightforward. I attempted to send the same commands and data packets from my dependable Raspberry Pi Zero W through RFCOMM on the terminal based on the WirteShark logs. I finally succeeded in printing the identical image that was on my phone after a few failed attempts.

So things are simple. Based on the Wireshark logs, I tried to send the same commands/data payloads from my trusty Raspberry pi Zero W via RFCOMM on terminal. After few trial, I was able to print the same image as it was from my phone.

The Final Result 👀️

To make this functional, the next task was to produce the data payload from my script. I went back and pulled three routines from the decompiled code to capture the BITMAP, transform it to 1 Bit pictures, and append some bytes as file headers. This step was more difficult because I was only able to obtain the function name. I then tried writing the similar routines in some other Python code, and it succeeded.

How to use the script? 🎉️

• Scan the MAC address of your printer using Bluetoothctl
• Run scan on if printer found run pair xx:xx:xx:xx:xx:xx ADDR and trust xx:xx:xx:xx:xx:xx
• Exit bluetoothctl
• Run sdptool add --channel=N SP, where "N" is the channel, remember this as you will need this in the script. I have selected 2 in my case.
• Run sudo rfcomm bind N xx:xx:xx:xx:xx:xx, N = channel = port
• Run cat-printer.py

GitHub Repo: https://github.com/abhigkar/YHK-Cat-Thermal-Printer