Go to yahoo groups and do a search for ftuner project you will find schematics on how to build your own interface. There seems alot of people selling more rather than developing. You can build it cheaper. The chips to use are as folow: ELM320, ELM322, ELM323 or ELM327 dependant on application. (Some years back Jeff had worked on a project in Yahoo groups and then commercial versions came about from this) The original Schematics and project detail from 2001 was located on his site complete with schematics and layout for a OBDII to RS232 converter:
http://www.planetfall.com/~jeff/obdii/
However keep in mind any of the ELM____ microchips are simple program chips which convert RS232 to your application be it VPW, PWM etc.. Motorola makes several flavors to choose from such as MC32199, MC33290 and MC33390. These chips convert K-line ISO9141 into RS232 or J1850 (VPW) into RS232 and in addition to all of this there are some companies which give sample quanities of thier chipsets (never hurts to ask)
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If you are wondering which protocol your vehicle uses I have included the following for 1996 and later vehicles:
If you look at the OBDII connector you will commonly notice that not all pins are populated. Use the following information to compare.
J1850 VPW--The connector should have metallic contacts in pins 2, 4, 5, and 16, but not 10.
ISO 9141-2--The connector should have metallic contacts in pins 4, 5, 7, 15, and 16.
J1850 PWM--The connector should have metallic contacts in pins 2, 4, 5, 10, and 16.
By the way (no pun intended by this) But you don't have to use Elm scans software or better said yet "You are not limited to Elm Scan" eithier. There are several pieces of freeware software out there that you can use dependant of your application (Vehicle Make etc)
Hope this helps