SMA UNABLE TO XMIT SQUAWKS EMER CODE INSTEAD OF RADIO FAILURE.

Narrative

AS I APCHED MSD CLASS D AIRSPACE; I LISTENED TO ATIS; THEN ATTEMPTED TO CONTACT TWR. I RECEIVED NO ANSWER. I REPEATED SEVERAL TIMES; CIRCLING TO REMAIN OUTSIDE OF CLASS D. I TRIED ADJUSTING RADIOS; RECHKED ALL CONNECTIONS; TRIED A HAND HELD MIKE; CHKED RADIO PANEL SWITCH SETTINGS; AND STILL RECEIVED NO ANSWER TO MY XMISSIONS. I WAS EXPECTED TO LAND AT MSO TO PICK UP A PAX; SO DECIDED TO USE MY XPONDER TO NOTIFY TWR OF RADIO FAILURE; AND WAIT FOR A LIGHT SIGNAL TO LAND. I KNEW WHICH RWY WAS IN USE AND HAD OTHER TFC IN SIGHT. THERE WAS NO DANGER OF COLLISION AT ANY TIME. UNFORTUNATELY; MY MEMORY CONVINCED ME THAT RADIO FAILURE CODE WAS 7700 INSTEAD OF 7600; AND I WAS SO SURE THAT I DIDN'T CHK MY CHKLIST TO VERIFY. I HEARD TWR TRYING TO CONTACT ME; AND BLINKED MY NAV LIGHTS AS REQUESTED BY TWR. I WAS CLRED TO LAND AND GIVEN A GREEN LIGHT (LATER FOUND OUT TWR DID NOT SEE MY LIGHT RESPONSE; SO DIDN'T KNOW I COULD RECEIVE). I WAS AT FIRST SURPRISED TO SEE CRASH/RESCUE VEHICLES COMING TO MEET ME ON THE TXWY; THEN REALIZED I WAS SQUAWKING EMER. RED FACE. I CALLED TWR ON PHONE TO APOLOGIZE; AND GAVE PLT INFO TO CRASH/FIRE PERSONNEL. I WILL NOT AGAIN FORGET RADIO FAILURE SQUAWK CODE.