I received an off-line response to the "[AD] microOBD 200 article" post:

"I find the concept of publishing an article on a project to be interesting. 
The problem is that I never got my hands on an on board diagnostic system. 
It is all greek and latin to me."

Wikipedia has an article on OBD:

http://en.wikipedia.org/wiki/On-board_diagnostics

You can think of OBD as a gateway to the vehicle's computer system. Most 
cars have a network of electronic control modules (ECUs), each one of which 
performs a specific function such as controlling the engine, transmission, 
ABS, airbags, power windows, locks, seats, et cetera.

With the introduction of OBD-2 in 1994, the physical, data, and application 
layers were standardized and made available to third party manufacturers. A 
lot of this information is now available on the internet, and there is a 
number of OBD to UART converters on the market.

The most common use of OBD is diagnostics, any decent scan tool nowadays can 
pull trouble codes and display vehicle parameters (some cars support 
hundreds of parameters). Another use involves controlling actuators, for 
example you could send a message to start the vehicle, or control the volume 
of the radio, or even control the throttle position.

While some of the information may be had by intercepting messages that 
modules send to each other (for instance, instrument dashboard could be 
periodically requesting fuel level status), most of the time you have to 
specifically request it. For example, if you wanted to know the RPM, this is 
what a typical exchange would look like:

    01 0C    // Mode 1, parameter ID 0C = RPM
    41 0C 01 23    // Response to Mode 1, PID 0C

The last two bytes are the RPM, encoded as 1/4 RPM per bit. You can find 
more generic PIDs here:

http://en.wikipedia.org/wiki/OBD-II_PIDs

I would be happy to try to answer any OBD related questions that you may 
have.

Best regards,

Vitaliy Maksimov
ScanTool.net LLC
http://www.scantool.net 

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