ine select on the RTU-4200, selects No. 1 or No. 2 transponder, placing the unselected transponder in standby mode. The active transponder (ATC1 or ATC2), is shown in cyan and in large letters on the ATC main display page. Document Number 21394-FMS-01 Revision A Page MD-1 SDNY_GM_02757701 SUBJEC