NEW PROC FOR PDC AT ORD COMPLICATED BY MD80 CREW WHEN FO REVERSES LAST 2 NUMBERS ON HIS XPONDER. ATC THINKS THEY ARE GOING TO ANOTHER DEST AND GIVES THEM A WESTERLY RADAR VECTOR FOR AN EBOUND FLT.

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DURING COCKPIT SETUP; I INADVERTENTLY REVERSED LAST 2 NUMBERS OF SQUAWK. CAPT ALSO MISSED THE REVERSAL ON BEFORE TKOF CHK. CLRED TO TAXI TO RWY 22L VIA TXWY D. SECOND OF 2 ACFT IN RWY 27L PAD WAS JUST ENTERING PAD AS WE APCHED PAD. ABOUT HALFWAY ALONG THE PAD; WE CHKED FINAL; SAW AN ATR ON SHORT FINAL TO RWY 27L AND TOLD TWR TO CANCEL OUR TKOF CLRNC. TWR COMPLIED AND AFTER A FEW SECONDS ASKED WHERE WE WERE. WE WERE JUST TURNING CORNER AT END OF TXWY D TO APCH RWY 22L. SO INFORMED TWR AND AFTER ATR LANDED; TWR AGAIN TOLD FLY HDG 270 DEGS; CLRED TKOF RWY 22L WHICH I READ BACK. AFTER TKOF THERE WAS AN UNUSUALLY LONG; FOR ORD; DELAY SWITCHING US TO DEP. CAPT ASKED TWR IF THEY HAD US. NEW VOICE SAID TURN SOUTHERLY; 180 DEGS I BELIEVE; HURRY TO 5000 FT. CAPT AND I BOTH THOUGHT 270 DEGS FROM RWY 22L FOR EBOUND FLT A BIT UNUSUAL; SO WERE WATCHING TCASII CLOSELY. TWR SAID CHK XPONDER. CAPT SAW THE ERROR; AND CORRECTED IT. WE SAW NO CONFLICT EITHER VISUALLY OR ON TCASII. WE WERE GIVEN FURTHER TURNS E AND CONTINUED WITHOUT INCIDENT. NEW PROC AT ORD SAID DON'T READ PDC INFO TO CLRNC; JUST TELL METERING IF CLRNC RECEIVED. I DID SO; GIVING THE SQUAWK WHICH I READ FROM XPONDER READOUT WINDOW. IN FUTURE; I AM GOING TO CHK BOTH PAPER AND XPONDER TO BACKUP OTHER PLT AND CONFIRM NO ERROR.