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Subject: ANNOUNCE: Rockbox emulates M-Bus car CD changer (Alpine and OEM)

ANNOUNCE: Rockbox emulates M-Bus car CD changer (Alpine and OEM)

From: [IDC]Dragon <>
Date: Thu, 20 Nov 2003 13:01:58 +0100 (MET)

Hello for a longer post,
(quote wisely ;-)

I'm proceeding to what I actually bought my Archos for:
To be world's largest CD changer in my car, controlled by the radio.

I have an older Alpine radio, which uses their "M-Bus" for the communication
between radio and CD changer. For details, see my page:

Alpine also did this as OEM for other brands (Honda, Acura, Volvo, BMW,
etc.) Nowadays Alpine uses a different protocol, called Ai-Net, not subject of
this. (As well as all other protocols, please don't ask me for such!)

Hookup to Archos works by connecting the headphone output including the
remote pin (you need a 4-ring 3.5 mm plug for that) to the changer jack of the
radio. M-Bus radios have a DIN-style circular jack with 8 pins (7 in a ~270
degree circle, one in the center). A standard 5-pin DIN plug is OK for us, since
we don't use the other (power) pins.

I have found an Alpine M-Bus pinout here:
As OEM, they shuffled the pins around a bit, better check first if it's not
genuine Alpine. The bus pin is pulled high to 12 volts with a ~2kOhm
resistor, pulses driven low. Because it's open collector, this is not harmful to the

And this is my ASCII art of the 4-pin headphone plug: (available in Germany
from Reichelt for 1.95 EUR)
 / \
 \_/ left -> Alpine pin 5
 |_| right -> Alpine pin 4
 |_| remote -> Alpine pin 1
_|_|_ ground -> Alpine pin 2
| |
| |

The remote pin can be programmed bidirectional, that's the reason this
works. Very luckily the M-Bus uses a single wire communication and the two radios
I tried are happy with the 3.3 Volt level the Arcos can deliver. So the
connection is a simple cable! For all protocols requiring more lines, an external
controller would be necessary. Archos FMs don't have the remote pin
connected, but the one I looked into was internally prepared for it, a matter of
closing a bridge, see:

I have made a plugin which now is in shape for a first release. It emulates
a changer such that the audio signal can be fed into the radio. The plugin
contains all the M-Bus send and receive code, plus a "demo" emulation which
follows all radio commands and keeps the communication alive. So you can change
disks and tracks, play, pause, stop, FF/FR, etc., but this is a fake. You'll
have to prepare your playback and volume beforehand, start the plugin when
everything's running.

My files are here, rocks for Recorder and Player, plus source:

I haven't added it to cvs since the use is probably too exotic. Plus, a
plugin can't control the mp3 playback nor query play state information. This
could be added, but then you still would need to start that plugin (unless we can
get something like autorun a plugin or better a .cfg which calls a plugin)
and can't use the box in the normal way.

My current plan is to integrate the code into Rockbox (after 2.1 release)
for a special build. So you'd have to decide what the remote pin will be used
for: Archos remote or M-Bus.

Happy cable soldering!

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Received on 2003-11-20

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