CL65 ACFT IN CRUISE HAD CAUTION MESSAGE THAT AIRSPD AND ALT READOUTS WERE UNRELIABLE. AS PLT'S AIRSPD READOUT INCREASED; RPTR CAPT COMMENCED A CLB TO PREVENT AN OVERSPD. WHEN DSNDING BACK TO ALT TCASII GAVE AN RA ON ACFT 1000 FT BELOW AND APPROX 5 MILES. SEPARATION WAS LOST DUE TO ALT DEVIATION.

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IN CRUISE AT FL310 AT MACH .80 EFIS COMPARATOR MONITOR CAUTION MESSAGE APPEARED; FOLLOWED BY MACH TRIM CAUTION. THIS CAUSED THE PRIMARY AIRSPD AND ALT READOUTS TO FLAG AND BECOME UNRELIABLE. PLT'S AIRSPD READOUT BEGAN TO INCREASE AND OVERSPD WARNING SOUNDED CONTINUOUSLY. THE ALT READOUTS VARIED AND THEN DISAPPEARED FROM VIEW. AIR TRAFFIC CTL WAS INFORMED THAT WE HAD LOST SOME OF OUR PRIMARY INSTRUMENTS AND DUE TO THE OVERSPD POSSIBILITY OUR ALT MAY VARY. INITIALLY A CLB WAS MADE AND PWR REDUCED TO ENSURE THE ACFT WAS NOT BEING OVERSPED. TRANSITION WAS MADE TO THE STANDBY INSTRUMENTS AND A DSCNT INITIATED TO RETURN TO FL310. AS WE NEARED FL310 ON THE STANDBY INSTRUMENTS; TCASII TRAFFIC MESSAGE SOUNDED FOLLOWED BY AN RA COMMANDING A CLB. THE CLB WAS BEGUN TO COMPLY WITH THE TCASII RA. WE OBSERVED A TARGET AT 1000 FT BELOW OUR ALT AT FIVE MI RANGE ON THE TCASII SCREEN. ZAU GAVE US A PHONE NUMBER TO CALL AND INFORMED US THAT MINIMUM SEPARATION HAD BEEN VIOLATED. DURING THIS TIME THE QUICK REFERENCE HANDBOOK WAS CONSULTED AND CHECKLIST PROCS TO REGAIN INSTRUMENTS BEGUN. IT WAS DETERMINED THAT AN AIR DATA COMPUTER HAD FAILED. AFTER SELECTING THE ALTERNATE AIR DATA COMPUTER THE AIRSPD AND ALT TAPES REAPPEARED BUT THE OVERSPD CLACKER CONTINUED TO SOUND SO WE CONSIDERED THE PRIMARY INSTRUMENTS UNRELIABLE AND CONTINUED ON STANDBY INSTRUMENTS. WE WERE GIVEN A DSCNT AND MADE AN UNEVENTFUL LNDG IN ATW. A MAINT WRITE-UP WAS ENTERED IN THE LOGBOOK AND THE SYS INSPECTED IN ATW. NO DEFECTS WERE FOUND IN CHECKING THE AIR DATA SYSTEM AND THE ACFT WAS FLOWN BACK TO CVG AND CONTINUED IN SERVICE. THIS ACFT HAS A HIGHLY AUTOMATED COCKPIT AND IT IS VERY USER FRIENDLY AS LONG AS THE AUTOMATION WORKS. IT WAS VERY DIFFICULT TO TRANSITION TO THE STANDBY INSTRUMENTS BECAUSE OF THEIR SIZE; AND THE SPD OF THE ACFT AT THE MOMENT OF THE MALFUNCTION MADE IT NECESSARY TO FLY VERY CAREFULLY TO AVOID LARGE DEVIATIONS IN AIRSPD OR ALT. THE SMALL SIZE OF THE ATTITUDE INDICATORS ALSO CONTRIBUTES TO THE DIFFICULTY OF MAINTAINING AIRSPD AND ALT. IF ANYTHING THIS INCIDENT POINTED OUT TO ME THE NECESSITY TO TRAIN WITH THE AUTOMATION REMOVED SO IF THE ELECTRONICS FAIL THE AIRCREW CAN CONTINUE THE FLT WITHOUT SERIOUS CONSEQUENCES. A CONTRIBUTING FACTOR TO THE LOSS OF SEPARATION WAS THE FACT THAT THE PRIMARY INSTRUMENTS HAVE A MACH TRANSDUCER TO COMPENSATE FOR ERROR IN ALT AND AIRSPD BUT THE STANDBY INSTRUMENTS DO NOT. THE STANDBY ALTIMETER ALWAYS VARIES FROM THE PRIMARY BY AT LEAST 400 -500 FT AND SOMETIMES MORE AT HIGH ALT. BY FOLLOWING THE STANDBY ALTIMETER WE WERE ALMOST CERTAINLY GOING TO BE AT AN ALT THAT WOULD RESULT IN REDUCED SEPARATION.