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Elm Electronics is a small company that sells pre-programmed integrated circuits for a wide range of applications; around the home, electronics for your work bench and a few ICs just for fun.
They are most know for their automotive line of ICs that can interface with any vehicle that has an on-board diagnostic port, OBD1 and OBD2, and is one of 12 different protocols. The 12 different protocols are used by all domestic cars sold in the US and most foreign imports. There are very few vehicles not covered by Elms products. Soon, most car manufactures will all use the CAN (ISO 15765-4), which is supported by the Elm chip set.
There are quite a few variables, tests and trouble codes that you can capture using the OBD port of your vehicle but it all depends on the car manufacture and the protocol used.
Elm makes several different ICs based on the various protocols but has recently combined them all into one chip, the ELM327. The chip acts as an interpreter for your vehicles OBD port, it scans the port to find the protocol of your vehicle then returns the information it finds via RS232 or UART to directly connect to a micro-controller. It can even monitor the voltage of your vehicle.
Below is a demo board of the ELM327 . You'll notice that there is a lot of components required to run the chip. The circuit below includes the hardware for all 12 protocols, so if you know your only going to use the chip for one protocol then you only need the discrete parts for that particular protocol.
Elm Electronics does not provide any circuitry besides the IC, so you will have to design your own. If its not possible for you to design your own PCB or proto-board there are a plethora of devices built with the Elm automotive chip set. Search for Scan tool on Google. I'm using the ELM327 in my automotive data logger so if any would like some advice on implementing it into one of your projects I would be happy to provide some advice.