What was found
(CAN) AIRCRAFT RETURNED TO DEPARTURE TERMINAL DUE TO LOSS OF 6 PRIMARY FLIGHT DISPLAYS (CDU'S). SYSTEM DATA ACQUISITON CONCENTRATOR (SDCU) REPLACED. LOSS OF CDU'S WAS DUE TO MEL APPLIED THAT ALLOWED ALL CDU'S TO BE POWERED FROM NR 1 BUS. MEL WAS APPLIED BECAUSE OF DMC NR 2 PROBLEM. AIRBUS NOTIFIED OF MEL DISCREPANCY. CAIL SUGGESTED PROCEDURAL CHANGE TO SELECT ACCESS FEED PB TO ALTERNATE IS UNDER REVIEW BY AIRBUS. ALSO SUGGESTED OPERATIONAL PROCEDURAL CHANGE TO MMEL IN THE EVENT OF AC BUS NR 1 FAILURE WHEN OPERATING WITH DMC 2. PART TC: 7,514.