ACFT EQUIP PROB. EMER DSCNT DUE TO DEPRESSURIZATION. FAULTY CIRCUITRY IN PRESSURE CTLR PLUS TOO LONG OF DELAY FOR BACKUP CTLR TO ACTIVATE. OXYGEN MASKS DEPLOYED AND CABIN ALT WENT TO 22000 FT.

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ON APR/SAT/96 ACR FLT XX MEX-SFO WE HAD A RAPID CABIN DEPRESSURIZATION. AT THE TIME WE WERE AT FL350 1 HR FROM MEXICO CITY. WE WERE VMC ON DIRECT ROUTING TO ALTAR INTXN. WE ASKED MEXICO CITY CTL FOR THE MEA IN OUR AREA. HE WAS UNABLE TO TELL US. WE DSNDED TO THE SECTOR ALT IN VMC. AT THAT ALT WE WERE UNABLE TO TALK TO MMEX EXCEPT BY RELAY THROUGH ANOTHER ACR FLT IN OUR VICINITY. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR IDENTED ACFT AS A320. CAUSE OF DEPRESSURIZATION WAS A BAD CTLR CAUSED FROM FAULTY CIRCUITRY. ALSO; THE A320 HAD 2 PRESS CTLRS; THE ACTIVE ONE AND A BACKUP. DURING THIS INCIDENT; AS PER ACFT DESIGN; WHEN THE MAIN CTLR FAILED AND THE OUTFLOW VALVE WENT TO FULL OPEN; THE BACKUP CTLR WAS DESIGNED TO ACTIVATE AFTER 45 SECONDS; BUT WITH THE OUTFLOW VALVE FULL OPEN; 45 SECONDS WAS FAR TOO LONG OF A DELAY. SINCE THIS INCIDENT PLUS TWO OTHERS EXPERIENCED WITH THIS ACR; AIRBUS CHANGED THE CIRCUITRY SO THAT NOW IF A PRESS CTLR FAILS THERE IS ONLY A 4 SECOND DELAY BEFORE THE BACKUP TAKES OVER. THE RPTR WAS UNHAPPY WITH THE MEXICAN CTLR WORKING HIS FLT. THE CTLR WAS OF NO HELP IN PROVIDING NEEDED INFO TO THE FLC; INFO SUCH AS THE MINIMUM VECTORING ALT IN THEIR SECTOR; ETC.