This project was a continuation of the Automobile MP3 Player project. After getting the MP3 player working with the toggle switches, the next obvious step was to try to get the player tied in with the radio controls, specifically, the controls on the steering wheel. I thought it would be great to control functions such as scrolling thorough the songs, and traversing directories with these buttons and eliminating the bank of switches on the console that I added for the MP3 player.

    While investigating how the steering wheel buttons were connected to the radio, I discovered the world of K-Bus. The K-Bus is a standard bus protocol for automobile electronics. In other words, it is a way for devices within a car to communicate with each other. On the BMW, there are several devices on this bus. These include the radio, the steering wheel radio button module, the CD changer, the light control module, etc. I was initially only really interested in how I could read the state of the steering wheel buttons, but I got a little caught up in the technology.

    I read an article on www.openbmw.org that described some of the details of how the K-Bus (or I-bus as it is called in the case of the BMW bus). This got me started on the road to decoding the bus.

    I started by researching electronics that would help me in building my K-Bus extractor. The device I selected was the Melexis TH3122 "K-Bus Transciever with Integrated Voltage Regulator". This part is as close to a perfect solution as you can get. It connects to the the K-Bus, Ground and the cars 12 volts and outputs 5 volts to power other components.  The only drawback for the home hobbyist is that it only comes in a surface mount SOIC16 package. I was able to get a "SOIC to DIP" adaptor (don't remember the source right now).

    The following is a schematic of the circuit that I put together to translate K-Bus to Serial RS-232. It is simple and most of the parts are fairly common (Digikey is a great source for electronics, but at the time of this writing, they did not carry the TH3122, although they do carry other Melexis parts). I acquired this part as a sample from the company.

Click in the box to get a .bmp image of the schematic.

Of course, you must use it at your own risk. I make no guarantees, and take no responsibility if you screw up your car!

    The pictures below show the assembled version of the K-Bus to Serial converter. I assembled it on a standard prototype board and soldered all connections. I suppose this could be done on a PC board from ExpressPCB (I used for the PIC Demo Board), but I was only making one. I tied into the K-Bus where the connection for the mobile phone is. (Under the center console). There was an article on the web about installing a mobile phone that gave the pin-outs for the connector.

XXX Add the following pictures: Connector end, circuit board in vehicle.

Assembled Circuit

Another View

    The next obvious step was to start collecting data from the bus. The first attempt involved using the standard Windows HyperTerminal program. This was useful in verifying that I could collect data, but it doesn't do a very good job with data that is outside the standard ASCII character set. I had some strange results, but it was very useful initially.

    Then I decided to write my own program that would capture data to a file for analysis later. I went through some attempts to use the time between messages to distinguish message from each other. This was somewhat successful and would have been enough to extract the information that I needed. But as I often do, I got obsessed with a particular project and decided to write a generic program to capture, timetag, and filter messages on the bus.  The following screen shot shows the results of some data I captured on the bus.

The program will read back the stored data using a filter as shown below. The green means that a message with that attribute will be displayed, red means it will not.

I don't know what all the codes mean yet, I only know what codes showed up when recorded data. I got some info from a web article that I read but I don't remember where I got it from.