The 40540 Data Interface allows a Fireberd mainframe to test communications systems that meet the Bell System T1 (DS1) 1.544 Mb/s Digital Channel Service Specifications, CCITT 1.544 Mb/s Recommendations, and AT&T Publication 54016 specifications for Extended Superframe. T1 signals represent serial digital data. Binary data is represented using a differential bipolar (3-level) return-to-zero (RZ) signal. Either alternate mark inversion (AMI) or bipolar with 8 zero substition (B8ZS) coding can be used. The data interface operates with either framed data, required by Digital Access Crossconnect Switches (DACS) and channel banks, or with unframed data used in unswitched networks. This data interface can terminate, bridge, or loop a line. It also allows error analysis based on either bit errors or bipolar violations (BPVs). This interface is capable of transmitting either loop-up or loop-down codes and is also capable of operating in an Extended Superframe (ESF) environment.