Someone else posted this on another thread: http://www.scantool.net. It looks like you can buy a cable for only $25 and download the software for free. I am not sure if you need additional parts as I have not read in detail. Has anyone actually tried one of these?
From my reading, it looks like you need some sort of chip to decode the signal from the ecu.
The scantool kit seems to be the best deal. It comes with the cables for only a few bucks more over the multiplex. But, you do have to pay extra if you want a case for it.
The multiplex one comes in a case, but you need to get at least a serial modem cable. And, if you want to stealth it, you may need the odb cable to relocate the bulky adapter.
I've read that the chip doesn't cost much. I've also found some schematics on various sites. But I'm not sure if I'm ready to tackle such a project. I would hate to fry the ECU. Maybe a prefab is the way to go.
If you only got free usb ports, use a usb->serial adapter.
Serial is a lot easyer to program, i looked at some USB stuff just to control some relays and would never go thru such a process to send a few volts
there are manufacturers of USB components (that can with a little bit of soldering become relay controls) that have all the work for you done in modules.
but in the end serial adapters and parallel port adapters are always cheaper
car computer rev 5: 8" lilliput and usual suspects
I have the scantool as well, but it was way cheaper when I got it I think.
I only wish the software was better. v2 is supposed to becoming out b4 the next ice age.
If the code was in VB, I would tackle it.
http://www.obddiagnostics.com/ Made by Beverly Roadman, and has it's own software that has an option for virtual gauges (one of the things I want to have displaying real-time) AND it's supported by freediag
$88 shipped includes the cable. It also supports the 3 major protocols.