Sy/Max
Definitely look at Niobrara (NR&D) for this type of product. They probably know more about Sy/Max communications than any one at Schneider Electric.
You can get NR&D convertors that mount in Symax racks (EPE-5, MEB-RT - see the link from 93lt1) as well as devices for Quantum racks (QUCM) and Momentum systems (MUCM).
The Sy/Max port is RS422 4 wire.