Method and apparatus for interface communications in a tandem cross connect

A method and apparatus for effecting a digital communications in a tandem cross-connect of an intermediate station in a telephone system communications network to enable synchronization of the digital signals coupled between ports of the cross-connect. The method and apparatus provide center-tapped transformers at each port for coupling into the cross-connect, applying synchronizing signals to the center-tap of the transformer for raising and lowering the DC voltage of the transformer output cross-connect side without adversely affecting the signals being transmitted across the transformer coupled network, and processing received signals to identify and synchronize to repeating protocol patterns. In particular, the invention employs photo-optically coupling to the transformers to maintain electrical isolation of the synchronizing elements of the circuitry and employing the circuitry in particular in connection with synchronizing to the A and B signalling bits of a T1 protocol.