data systems

changing voltage on a wire is multiplex

data is a different beast

it is essentially computer language 010101110000101010

heres a site for some reading

www.canhack.org

http://en.wikipedia.org/wiki/OBD-II#OBD-II

http://www.interfacebus.com/Automotive_SAE_J1850_Bus.html

http://en.wikipedia.org/wiki/Controller_Area_Network

A data system is simply a way to transmit more then one signal on a wire - there are a ton of different ways to acomplish it.

There is voltage monitoring - It is possible to use one input pin on a microcontroller and have 1024 different possibilities (on a 10 bit system).  On a 5 volt system this gives a resolution of ~0.005 vdc.  As you can see, you can send all kinds of info in to the microcontroller based on this resolution.  The "down side" here is if you try to send more then one signal (like from two switches) at a time you either confuse the processor, or you have to break down the voltage in to known parts and determine what all signals are present.

Then there are switching matricies - commonly found in Nissan/Infiniti vehicles in order for the BCM to communciate to the accessories on the steering wheel (lights, turn signals, fog lights, etc).  The Nissan system uses 5 outputs and 5 inputs (10 total wires) and ends up with 25 total possibilities.  They use 5 possibilities for diagnostics leaving them with the ability to send 20 signals down 10 wires with no possibility of "confusing" what switch is on at any given point.  This is the same theory behind how computer keyboards work, and is a very popular way to control a large amount of IO points with a minimal amount of IO.

Then there are serial communication lines.  These can be done in a variety of different ways, and CAN and LIN networks would be examples.  You basically send data in a series of predetermined ons and offs (or 1s and 0s) between two intelligent devices.  The data encoded in the message is unlimited and can be virtually anything in the car.  Most manufacturers stick to standards that dicate things like how a message is started, how conflicts are resolved (if two modules try to "talk" at the same time), the format for the message, and other perameters that are required to make an error free, reliable system.  CAN networks are cool because you can theoretically cut one of the two wires and the bus will continue to send info - just at a much slower rate.  If you have some free time read up on CAN networks, they are fairly ingenious in their implementation.

howie when are you gonna let me install an alarm with ya, , i see now why you're cynical sometimes, don't worry they'll invent something for the magic key soon, or maybe you can, lol, i really have to see a mercedes with this magic key, i've never seen it, but then again how often do you see a merc. on the road in my parts, lol