Bluetooth integration with M-BUS CD changer input:
There are a few people who have decoded the M-BUS protocol for old Alpine gear (for example https://github.com/picohari/atmega128_alpine-mbus-emulator). Alpine (from what I can tell) made the old volvo CR, SC, and HU radios/head units that we know and love. The CD changers (also made by Alpine, I think)that are compatible with these radios mostly use a connector like this:
Source (https://babcuvpisecek.com/diy/alpine-minidin-pinouts/)
The Volvo head units I have experience with (for example, the CR-824) use this strange connector:
But we still need a way to communicate through the M-BUS protocol that the CD changers used, so that the audio input will come through the head unit. The solution I found was from this legend in France: https://www.oldschoolalpinerepair.com/product-page/alpine-bluetooth-m-bus-emulator. He makes something similar to the widely discussed Yatour emulator (I have not tested the Yatour so no idea if it works. It might be for AI-NET or some other Alpine protocol). It emulates the communication to a CD changer so the Head unit will allow audio to pass through. This emulator has its own Bluetooth receiver and DAC to feed into the audio lines.
So, I need an adapter cable. You could absolutely just hardwire it, it is only 8 pins. I got this one from Discount Car Stereo which fit OK, but I ended up taking it apart in my wiring troubleshooting. I haven't been able to find the head unit end as a stand alone connector for DIY wiring, but I am sure it is out there. The changer end is widely available. https://www.discountcarstereo.com/vol-alp.htm
I connected it up but the transmitter wasn't getting the right power. It was confusing, the emulator wants battery and accessory (switched) 12v, and the M-BUS hot line is also about 11v! So, it looks like it has 3 power sources when you are probing the pins, some that always on and one that actually switches on the bluetooth receiver.
In a late night troubleshooting stint, I ended up cutting the adapter cable and rewiring it (removing the yellow 12v injection line entirely) and utilising an NC pin on the CD plug on the head unit, wiring that pin to a 12v rail in the head unit. I can't remember if it was battery or ignition, but it was clearly labeled on the board. When I have it all out again, I'll draw up the wiring.
All in all, it works well! You press the CD button on the head unit and the Bluetooth stream (from my phone) comes straight through. The nxt button on the CR-824 skips the songs. I think other radios with more CD controls may have even more control functionality.
At the end of the day, it will be a bit of trial and error to get your own head unit to work. Figure out what protocol it uses on the CD lines (see if you can cross reference with alpine changer models) and if a suitable emulator exists. Probe around on the head unit side, look for the 10-11v data line, and if there are battery and ignition 12v rails. It (M-BUS) is a single data line, so once you get that working, the audio pins will be immediately obvious (touching them should send signal to the speakers if the head unit is in 'CD mode'. Then, it is just a matter of finding what sort of power (switched, battery or both) the emulator/receiver wants and when.
Good luck!
