As far as I know, no it is not proprietary. I picked up an Actron reader from Amazon.com not too long ago. For my base '98 986 it connects to the purple port on the left under side of the drivers side foot well with no problems or adapters.
I reviewed the page below before getting my scanner.
http://mike.focke.googlepages.com/acheckenginelight%28cel%29
Hope this helps point you in the right direction.
Peter