The ISU 512 is designed to operate in a dual port mode for videoconferencing at 112/128 Kbps. This allows end-to-end compatibility when communicating with a video system utilizing two Switched 56 DSUs or a dual port ISDN terminal adapter. For this application, 56/64 Kbps is transmitted over the V.35 interface and the RS-530 interface. A special RS-366 Y cable provides the two RS-366 interfaces for this application. For convenience in communicating with multiple video sites, the ISU 512 can be configured to transparently switch between the dual port mode at 112/128 Kbps and the single port mode of 384 Kbps without user intervention and reconfiguration of the unit.