A B737-300 IN CRUISE FL330 DECLARED AN EMER AND DIVERTED DUE TO LOSS OF CAPTS ENTIRE EFIS DISPLAYS AND FO'S DISPLAYS BLANKING EVERY 2 SECS CAUSED BY SYMBOL GENERATOR AND DIGITAL ANALOG ADAPTER FAILURE.

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HAD JUST LEVELED AT FL330 WHEN MY OUTBOARD; INBOARD; AND UPPER DISPLAY UNITS WENT BLANK. AT THE SAME TIME THE FO'S OUTOARD; INBOARD AND LOWER DISPLAY STARTED BLANKING ABOUT EVERY 2 SECS. AT ABOUT THE SAME TIME BOTH ENGS HAD AN ELECTRONIC ENGINE CONTROL FAULT. I TRIED SWITCHING MY DISPLAYS TO THE FO'S WITH NO SUCCESS; AND AFTER TALKING WITH DISPATCH; DECIDED TO RETURN TO CHICAGO. WE ACCOMPLISHED THE CHKLIST FOR THE ELECTRONIC ENGINE CONTROL PROB AND TRIED TO TROUBLESHOOT THE MULTIPLE DISPLAY FAILURE AS BEST WE COULD SINCE THERE IS NOT A CHKLIST FOR THIS PROB. AT THIS TIME MY OUTBOARD AND INBOARD DISPLAYS AS WELL AS THE UPPER DISPLAY WERE BLANK. I WAS ABLE TO GET SOME FLT INFO FROM THE HUD; BUT FOUND THE ALTIMETER TO BE OFF OVER 400 FT IN THE DESCENT SINCE I COULD NOT CHANGE ITS SETTING FROM 29.92. THE FO'S INBOARD OUTBOARD DISPLAYS WERE WORKING; BUT CONTINUED TO BLANK EVERY 2 OR 3 SECS. QUESTIONS FROM THE CENTER CONCERNING OUR NAV TO A FIX CONVINCED US THAT THE NAV DISPLAYS ON THE FO'S SIDE MIGHT NOT BE ACCURATE. AT THIS POINT WE DECLARED AN EMER AND REQUESTED A RETURN TO ORD. I DECIDED ON ORD SINCE THERE WAS A BROKEN CLOUD COVER AT DTW AND ORD WAS CLR WITH GOOD VIS AND LIGHT WINDS FROM THE E. THE ONLY SPECIAL HANDLING WE REQUESTED WAS FOR RADAR VECTORS TO ORD AND RWY 9R WITH A TURN ON AT LEAST A COUPLE OF MI FROM THE MARKER. THE REASON FOR REQUESTING RWY 9R WAS THAT IT FAVORED THE WIND AND THE RWY WAS LONG ENOUGH TO ALLOW A LOW REVERSE THRUST SETTING SINCE THE ENGS WERE BEING OPERATED WITH THE ELECTRONIC ENGINE CONTROLS OFF. LNDG AND TAXI TO THE GATE WERE NORMAL. AFTER SWITCHING PWR TO THE APU AT THE GATE AND SHUTTING DOWN THE ENGS WE STILL HAD THE SAME DISPLAY PROBS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED AT FL330 THE ENTIRE CAPT'S ELECTRONIC FLT INST SYS WENT BLANK WITH NO SWITCHING FROM THE FO'S #2 SYS. THE RPTR SAID THE PROB WAS COMPOUNDED BY THE FO'S DISPLAYS BLANKING CONTINOUSLY EVERY 2 SECS WITH THE NAV DATA SLOWLY DEGRADING. THE RPTR SAID ATC HAD TO GIVE HDG CORRECTIONS FOR THE RETURN TO ORD. THE RPTR STATED WHEN ON THE GND THE PROBS WERE STILL PRESENT FOR MNTNC TO OBSERVE AND THE CORRECTIVE ACTION TOOK 3 DAYS AND WAS PROVED WITH A TEST FLT. THE RPTR SAID THE CAPT'S SYMBOL GENERATOR AND DIGITAL ANALOG ADAPTER HAD FAILED. THE RPTR SAID THE FO BLANKING AND SWITCHING FAILURE WAS CAUSED BY A DATA BUS COAXIAL CABLE CONNECTOR FOUND LOOSE.