MD80 FLT CREW DEPARTING DFW MISSES REVISED SEGMENT ON PDC AND DEPARTS ON WRONG SID.

Narrative

UPON ARRIVAL TO ACFT NOTICED A WRITEUP ON THE FMS WHICH WAS ALSO VOIDED BY THE SAME CREW. OUR FLT PLAN SHOWED THE DALLAS EIGHT DEP RATHER THAN THE USUAL TRISS TWO DEP (RNAV); I ASSUMED BECAUSE THE SYSTEM HAD ALREADY RECEIVED THE DOWNGRADE OF THE FMS. I CALLED DISPATCH AND ASKED IF THEY WANTED TO FILE US BACK ON THE TRISS RNAV AND APPROPRIATE ARRIVAL SINCE THE FMS WAS OPERATING NORMALLY AND THEY SAID NOT NECESSARY SINCE THE ROUTING STILL PLAYED THE SAME. ON INITIAL CLBOUT; DEP ASKED WHY WE WEREN'T TURNING ON THE TRISS DEP AND WE SAID WE WERE ON THE DALLAS EIGHT 160 DEG HDG WAITING FOR THEIR VECTOR. A QUICK LOOK AT OUR PDC AND WE SAW THERE WAS A REVISION TO FLY THE TRISS TWO DEP. THE DEP CTLR SAID IT WAS NOT AN ISSUE AND VECTORED US TO TRISS INTXN. I OBVIOUSLY DID NOT RECONCILE THE FLT PLAN AND THE PDC AS I THOUGHT I HAD BEEN DOING ON EVERY FLT FOR THE PAST 18 YEARS. I NOW WILL MAKE SURE THAT I DO AND INCLUDE IT IN MY INITIAL (GENERAL FLT BRIEFING) WITH MY FO. I AM NOT SURE WHY ATC UNILATERALLY CHANGED US TO THE RNAV DEP IF THE FILED FLT PLAN INFO INDICATED THERE WAS A DOWNGRADE TO THE FMS. BOTTOM LINE; WE MISSED THE REVISION; WILL DO BETTER.