Duo Dock: Modem Port Not Available After Mac OS 8.5 Installation

Only the Printer Port option appears in the AppleTalk and Modem control panels, after installing Mac OS 8.5 or higher, when a PowerBook Duo is in the PowerBook Duo Dock.
Apple is aware of this limitation.

Workaround
Perform a clean installation of Mac OS 8.1 or earlier on the PowerBook Duo hard disk or the PowerBook Duo Dock hard disk.
Published Date: Feb 18, 2012