CAPT OF AN MLG ACR ACFT UNDERSHOT DSCNT XING ALT RESTRICTION DUE TO MAKING AN FMC PROGRAM ERROR.

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I WAS PIC AND PF DURING A SCHEDULED AIRLINE FLT ON JUL/TUE/93. THIS FLT WAS OPERATING FROM CVG TO ORF AND WAS THE FINAL SEGMENT OF A 5 LEG DAY THAT BEGAN WITH A XA21 SIGN-IN. WE WERE APPROX 70 NM W OF MONTEBELLO VOR (MOL) AT FL330 WHEN THE CTLR GAVE A CLRNC TO CROSS 25 W OF MOL AT FL270. FO ACKNOWLEDGED THE CLRNC; AND I ENTERED THE NEW RESTRICTION IN THE FMS. ALTHOUGH I UNDERSTOOD THE XING RESTRICTION; I INCORRECTLY PROGRAMMED THE WAYPOINT TO BE 25 E OF MOL AT FL270. AT ABOUT 35 NM W; THE CTLR ASKED IF WE WERE GOING TO BE ABLE TO MAKE THE XING RESTRICTION; 25 W OF MOL AT FL270. AS FO RESPONDED; I DEPLOYED FULL SPD BRAKES; CHKED TCASII FOR CONFLICTING TFC; AND BEGAN A DSCNT AND A R TURN OFF COURSE. FO ADVISED ATC OF TURN AND DSCNT AND CONTINUED TO MONITOR TCASII FOR CONFLICTS. DSCNT WAS COMPLETED BY 22 NM W OF MOL AND A L TURN BACK TO COURSE WAS INITIATED PASSING FL280. AT NO TIME WERE ANY TFC CONFLICTS DETECTED BY ATC OR TCASII. THE FLT CONTINUED TO DEST UNEVENTFULLY. THIS SIT WAS CAUSED BY IMPROPER FMS PROGRAMMING. IN MITIGATION; THE ERROR WAS GREATLY INFLUENCED BY CREW FATIGUE SINCE WE HAD THEN BEEN ON DUTY SINCE XA22; APPROX 8 HRS. THIS EVENT COULD HAVE BEEN AVOIDED BY HAVING THE PNF XCHK ENTRIES MADE BY PF. SUCH XCHK WILL BE RIGIDLY INCLUDED IN ALL MY FUTURE OPS.