ACR X TCASII RA HAD LTSS FROM ATX Y. SYS ERROR.

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ACR X B737 DEPARTED STL. I CLBED HIM TO 6000 FT ON INITIAL CONTACT. AN ATX Y BA31 DEPARTED SHORTLY AFTER. I CLBED HIM TO 5000 FT. LOW ALT APREQ A SATELLITE DEP ABOUT 13 MI S OF STL WBOUND. I WAS KEEPING JETS AT 6000 FT W OF THE PROP DEP'S ACFT ALSO CLBING PROPS TO 6000 FT FOR TFC (NOT MY COMMON PRACTICE). ACR X LATER CALLED A PREVIOUS DEP IN SIGHT AND LEVEL AT 6000 FT. INSTEAD OF KEEPING HIM WIDE AS PLANNED I TURNED HIM S TO JOIN A DEP RTE. ATX Y I LATER TURNED TO A 160 DEG HDG AND CLBED TO 6000 FT. ACR X SAID HE HAD TFC OFF HIS NOSE AND A MI. HE ASKED WHAT IT WAS AND WHAT ALT. I ADVISED A BA31 AT 6000 FT. I SOON CLBED ACR X TO 15000 FT AND TURNED ATX X FURTHER ON COURSE. THERE WERE NUMEROUS VFR DATA TAGS ON THE SCOPE MAKING IT DIFFICULT TO DISCERN TARGETS. I USE THE SMALL DATA BLOCKS TO HELP ELIMINATE CLUTTER. THERE ARE 4 SIZES OF DATA BLOCKS AVAILABLE; BUT ONLY 2 SIZES CAN BE USED. THE SUPVR MONITORING THE AREA COULD NOT READ THE DATA BECAUSE OF SIZE. SUPPLEMENTAL INFORMATION FROM ACN 265286: DEP CTL ISSUED A CLB TO 6000 FT MSL. I RESPONDED BY GIVING CALL SIGN AND 6000 FT TFC AT 10 O'CLOCK 3 MI IN SIGHT A DC-9. DEP THEN CLRED US TO MAINTAIN VISUAL; TURN L HDG 180 DEGS AND INTERCEPT LINDBERG 2 DEP; LITTLE ROCK TRANSITION. ONCE ESTABLISHED ON THE DEP; I NOTICED AN ATX Y 11 O'CLOCK LOW AND CONVERGING. I WATCHED THIS ACFT CLB TO OUR ALT VISUALLY AND WITH TCASII. I THEN QUERIED DEP AND WAS TOLD TO STANDBY. WE SLOWED SLIGHTLY SO AS NOT TO OVERRUN TFC. DEP ASKED WHAT WE WANTED AND I REPLIED IN QUESTION ABOUT OUR 12 O'CLOCK 1 MI TFC. DEP'S RESPONSE WAS HE'S AT 6000 FT. I REPLIED YES SO ARE WE. IMMEDIATELY THE TFC TURNED L AND WAS NO LONGER A FACTOR. A CALL TO DEP ON THE PHONE UNVEILED THAT HE SIMPLY FORGOT THAT WE WERE AT 6000 FT MSL.