ACR Y CLB TO OCCUPIED ALT HAD LTSS FROM ACR X TCASII RA EVASIVE ACTION TAKEN. SYS ERROR.

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ACR X HAD REACHED 5 DME S OF THE PRIMARY AND I ISSUED A L TURN TO 020 DEGS AND HE COMPLIED. I THEN TOLD ACR Y TO CLB TO 5000 FT. HERE IS WHERE THE INCIDENT BEGAN. Y AND X WERE NOW ON CONVERGING COURSES WITH Y CLBING TO 5000 FT AND X LEVEL AT 5000 FT. CONTINUING TO WORK TFC; I CLRED THE C-172 TO 3000 FT; THE DH-8 WAS RECLRED TO 15000 FT AS THE PLT WAS NOT SURE OF HIS ASSIGNED ALT; THE PA-28 WAS FINALLY CLRED TO A LOWER ALT; AND THE BE-35 RPTED CONTINUING HIS DSCNT BELOW CLASS B. I THEN ISSUED TFC TO X -- 11 O'CLOCK; 3 MI EBOUND AT 4000 FT; AN SW3. I ALSO ISSUED TFC TO Y -- 2 O'CLOCK; 3 MI; NBOUND; A B-737. I HAD MADE 1 MORE XMISSION TURNING THE C-172 ON COURSE WHEN Y REQUESTED CONFIRMATION OF ASSIGNED ALT 5000 FT. I THEN NOTICED Y WAS CLBING OUT OF 4200 FT. I IMMEDIATELY TOLD Y; 'NEGATIVE; MAINTAIN 4000.' I THEN TOLD X TO CLB TO 6000 FT. X ACKNOWLEDGED LEAVING 5600 FOR 6000 FT. I COULD HEAR HIS TCASII GIVING X A CLB RA. THE 2 ACFT WERE NOW INCREASING SEPARATION AS THEIR TARGETS MERGED. I CONTINUED TO WORK TFC WITHOUT FURTHER INCIDENT UNTIL I WAS RELIEVED FROM THE POS A COUPLE OF MINS LATER. LOOKING BACK AT THE INCIDENT 2 THINGS COME TO MIND. FIRST; I WAS SURE THAT I HAD ASSIGNED Y 4000 FT; NOT 5000 FT. I WAS PLANNING A CROSSOVER WITH X. I KNEW X WAS AT 5000 FT; THEREFORE; ANY ACFT XING UNDERNEATH WOULD HAVE TO BE AT OR BELOW 4000 FT. ALL THIS TIME; I WAS THINKING 4000 FT WHEN I HAD ACTUALLY ISSUED 5000 FT TO Y. I THINK THAT PART OF EFFECTIVE LISTENING IS LISTENING TO WHAT YOU ARE SAYING. IT'S NOT ENOUGH TO MAKE SURE PLTS GIVE A CORRECT READBACK. YOU HAVE TO LISTEN TO YOURSELF AND MAKE SURE YOU'RE SAYING WHAT YOU WANT TO SAY. SECOND; TCASII WORKS! I THINK IT WAS THE TCASII WHICH PROMPTED HIS CONFIRMING THE ASSIGNED ALT. CERTAINLY; BY THE TIME I ISSUED A CLB; HE WAS ALREADY RESPONDING TO AN RA. I BELIEVE TCASII PREVENTED A VERY BAD SIT FROM BECOMING THE WORST POSSIBLE SIT.