The OnStar System is becoming more and more common as GM installs it by default in many of their vehicles. At the heart of this system is a GPS receiver, manufactured by Motorola, called the Oncore. Once you make a serial connection to this Oncore GPS, you can recieve real-time positioning information on your Laptop or PDA - In either Motorola's proprietary (but well documented) format, or in standard NMEA format.
In order to do this, you will need the following items:After tearing apart my OnStar, I found the actual GPS board inside to be a Motorola Oncore GTPlus. I decided that the easiest way to interface with this would be to connect directly to this board, rather than trying to use the OnStar wiring. The main reason I decided to go this route was due to the lack of infomation about Onstar's wiring and protocols, and the wealth of information about the Motorola Oncore pin-outs and protocol. I also wanted to be able to talk directly to the GPS regardless of the current status of the OnStar system, and have no idea if that is possible through the OnStar's wiring.
In order to connect a serial port directly to the Oncore you will need an RS232 to TTL level converter. These are available for sale on the web, as well as schematics to build your own. I decided to use a pre-built converter, the RLC1 from Digital Nemesis. I chose this one because it was built into the DB9 shell, so it would require minimal wiring. I figured this was also one less thing for me to screw up. I cut the Molex connector off the end so I could solder it directly to the board, and soldered the RTS (Request to Send) and CTS (Clear to Send) wires together since these aren't used by the oncore.
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