To link up AppleTalk via an InteCom IBX switch, use the LANmark Ethernet on the
IBX. LANmark is a featured product of InteCom, the IBX folks. The LANmark
Ethernet is a slower (1Mhz vs 10 Mhz) version of an ordinary Ethernet. Then use
a Kinetics PastPath Bridge to connect AppleTalk to the Ethernet. Both Kinetics
and InteCom should be able to supply you and RPI with more detailed
information.
Also, you can less expensively but not as elegantly use half-bridges such as
those from Hayes and connect via Synch or Asynch Data Option Boards.