B737 CREW HAD ALT UNDERSHOOT.

Narrative

WE WERE GIVEN A CLRNC TO DSND FROM FL290 TO FL240 30 NM S OF FLO VOR. THE FO; WHO WAS THE PF; ENTERED THE DSCNT FIX INTO THE FMC. I MONITORED CAREFULLY AS HE ENTERED THE INFO BECAUSE IT CAN BE CONFUSING TO PLACE AN ALT AT A FIX THAT IS NOT INITIALLY ON THE FMC RTE AND MUST THEREFORE BE BUILT ON THE FMC RTE PAGE. ALSO; BECAUSE SOME OF OUR COMPANY ACFT HAVE DIFFERENT SOFTWARE DATA UPDATES; PERFORMING CERTAIN FUNCTIONS; EG; CREATING FIXES; REQUIRES DIFFERENT METHODS (A DEFINITE SHORTFALL). THE FO BUILT THE ALT XING RESTR 30 NM N OF FLO INSTEAD OF 30 NM S AND I DID NOT RECOGNIZE THE MISTAKE. CTR NOTICED WE HAD NOT STARTED A DSCNT AND ALERTED US. WE REALIZED OUR ERROR AND BEGAN AN IMMEDIATE DSCNT; BUT FAILED TO MAKE FL240 AT THE ASSIGNED POINT. NO CONFLICT WITH OTHER ACFT OCCURRED AND THE CTR CTLR DID NOT EXPRESS CONCERN. APPARENTLY THE ALT RESTR HAS MORE TO DO WITH AIRSPACE JURISDICTION THAN TFC. THIS SIT EMPHASIZES THAT AS HELPFUL AS AUTOMATION CAN BE; A PLT MUST ALWAYS BE WARY. AN OVER RELIANCE ON COMPUTERS WITHOUT A LITTLE BIT OF SKEPTICISM CAN LEAD TO COMPLACENCY.