The PA-8T-X21 synchronous serial port adapter provides up to eight synchronous serial interfaces for the Cisco routers. PA-8T-X21 network interfaces provide a direct connection between the high-speed bus in the router and external networks. Each PA-8T-X21 interface provides full-duplex (FDX) operation at T1 (1.544 Mbps) and E1 (2.048 Mbps) speeds.All eight PA-8T-X21 interfaces connect to external networks through a single port that has a 200-pin, D-shell receptacle. You must use an X.21 compact serial cable to connect PA-8T-X21 interfaces to an external data service unit (DSU) or channel service unit (CSU). The compact serial cable attached to the single receptacle determines the mode (DCE or DTE) for all eight interfaces.