APPARENT FAILURE OF THE CENTRAL AIR DATA COMPUTER CAUSES MD80 FLIGHT CREW TO DECLARE AN EMERGENCY AND DIVERT TO A NEARBY AIRPORT.

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WHILE ENRTE; I LOST MOST OF MY PRIMARY FLT INSTS. AIRSPD 'OFF' FLAG IN VIEW; PLUS AIRSPD APPROX 80 KTS FASTER THAN STANDBY AND FO'S AIRSPD INDICATOR. TAS INPUT FAILURE AND AIRSPD INPUT FAILURE ON THE GFMS. VERT SPD INDICATOR INPUT ERROR MESSAGES AS WELL AS IVSI NEEDLE NOT DISPLAYED. CAPT'S ALTIMETER WINDING DOWN AND SETTLED 11000 FT LOWER THAN STANDBY AND FO'S ALTIMETER. ATC QUERY AS TO OUR ALT -- XPONDER #1 INDICATED TO ATC; FL190 WHEN WE WERE JUST DSNDING DOWN THROUGH FL300. SWITCHED ATC XPONDERS AND SECOND ONE NOW INDICATED ANOTHER LOWER YET ALT; CANNOT REMEMBER THE EXACT ALT. BOTH AUTOTHROTTLES AND AUTOPLT DISCONNECTED AND WOULD NOT RE-ENGAGE. THRUST RATING PANEL; NO MODE ILLUMINATED. OVERSPD CLACKER SOUNDING BECAUSE CAPT'S AIRSPEED NEEDLE WAS INACCURATE COMPARED TO STANDBY AND FO'S AIRSPD INDICATOR. OVERSPD WARNINGS OCCURRING CONTINUOUSLY AS WE CONFIGURED THE SLATS AND FLAPS FOR LNDG. THE FMA WAS BLANK; AS WAS THE FLT GUIDANCE PANEL. EPR BUGS WERE NOT WORKING. SINCE WE WERE IN VMC AND RIGHT OVER ZZZ; I ELECTED TO DECLARE AN EMER AND GAVE THE FLT CTLS TO THE FO. I HAD NO PRIMARY INSTRUMENTATION TO FOLLOW. WE RECEIVED THE WX AT ZZZ AND LANDED ON RWY 36R WITHOUT INCIDENT. I TAXIED THE ACFT TO THE GATE AND WE RECEIVED ANOTHER ACFT TO CONTINUE THE FLT. THIS ACFT HAD A HISTORY WITH A PREVIOUS MECHANICAL DISCREPANCY OF THE CAPT'S VSI AND A NEW CENTRAL AIR DATA COMPUTER INSTALLED TO REPAIR THE PROB.