The Expansion Key Service Unit (EKSU) ( Part # 3502-01 ) mounts above the BKSU and is connected via cables that extend the voice and data signaling to and from the BKSU. An integral power supply provides enough power to support all CO lines, stations, and daughter board options.  The station connections are via a 50-pin amphenol connector. This connector is located in the EKSU. CO connections are 50-pin type and are located on the EKSU board.