Volvo HU-803 and the MELBUS protocol

I've made two CD-changer emulator / AUX cables for the Volvo HU car audio systems. The build is pretty much as described in Gizmosnack's blog.

The code I initially used was an older version of visualapproach's Arduino implementation with a single hardcoded text changed to point to this page.

Turns out that HU-803 doesn't support SAT (later versions like HU-650 and HU-850 do) so it doesn't show any text at all, anyway. An MD-changer does support track information but no one has published its protocol. Several people have reverse engineered it separately (probably with a device and a logic analyzer) and gone on to sell their own proprietary devices. Sigh.

So I have a couple of things I might hack on and write about:

• The MD-CHGR protocol. Probably very difficult without hardware supporting the protocol.
• A reimplementation on better hardware (say, an ARM board like the Blue Pill or ESP32 for bluetooth support), maybe in Rust.
• The minor things I did differently that made the second cable i made better than the first one.

But for now, this page is mostly a placeholder. It has to exist because I've written the URL in places. :D