The Heat Is On

Thermal printers have a lot going for them. They are fast, durable, and cheap. And perhaps most importantly, they don’t guzzle outrageously expensive ink that costs more than the printer itself before long. Aside from all of these advantages, they are just plain cool. I mean, selectively heating up the special paper to produce images is just way more interesting than spitting out ink.

I’m not the only one who feels this way — thermal printers are quite popular in the hardware hacking community. Mel from ThirtyThreeDown is also all in on these devices, so when he came across a cheap model (Core Innovation’s CTP-500) at Walmart, he had no choice but to buy it. However, as you might expect from cheap, no-name electronics found at Walmart, the user experience was severely lacking.

The biggest problem with this little printer is that, out of the box, it can only be controlled via a phone app. What?!? What if you want to print from your laptop or desktop computer — you know, where you actually create documents that you care about printing? Nope, Core Innovation doesn’t see any value in that.

Unsurprisingly, that is exactly what Mel wanted to use this device for, so it was time for some old-fashioned hacking. After cracking the case open, labeled DM and DP pads from the USB interface were quickly found. While these could be a great way to interface with the printer, Mel wasn’t ready to start soldering things to an unknown system. You’ve got to have it for at least a few days before you break it, right?

For this reason, a safer alternative was explored. The printer is designed to communicate with a phone app via Bluetooth. So Mel fired up Wireshark and started sniffing packets. Fortunately, the protocol proved to be pretty straightforward. After some time spent staring at raw data packets, the code was cracked.

With knowledge of the protocol, software was developed to wirelessly control the printer. It is a very odd system in which all prints, even text, are converted to bitmaps and sent to the machine one line at a time. But with all of that complexity abstracted by the software, it is possible to print text or images with the press of a button. And now, that $20 purchase made by Mel is looking a whole lot better.