I found out that the data is encoded using some modulation to ensure the data is valid and that the Appletalk driver itself (in software) talks directly to the port, not via any other serial method. I was confused to find out that schematics for creating a DIY phonenet box didn't include *anything* with DTR/DCD lines, but that's because the mac is more or less bit-banging the serial port itself rather than relying on the standard spec. I found a book called Inside Appletalk that i'm trying to procure now for more information. If anybody has this book PM me.

http://www.mactech.com/articles/mactech/Vol.02/02.04/ATalkConnections/index.html