From Ross-Tech Wiki

3.4 Can I build my own Interface Adapter?
The current version of VAG-COM (versions 500.x and newer) are not compatible with 3rd party interfaces at all, even in shareware mode. To use the latest version of VAG-COM, you need to use a genuine KEY or HEX Interface purchased directly from Ross-Tech or from one of our distributors.
There are a number of websites with plans and schematics that claim to work with older versions of VAG-COM. However, we will not provide links to them, nor do we sell connectors or parts. We do not have the time or inclination to help people debug their home-built interface adapters via e-mail or over the telephone. If you choose to build your own adapter, that's fine -- we have no problem with that. Just don't expect any assistance from us until you have VAG-COM talking to your car in shareware mode. We have reports that some 3rd party Interface designs do not work correctly with some controllers, Once you've got it working, you can register the software for $99 and get full support for the software. Of course if you buy one of our complete hardware/software kits, we will provide start-up support as well as free updates for at least one year from date of purchase.