TTSC4 Dual Dapol Servo Semaphore Signal DCC ControllerDual Dapol Servo Semaphore Signal DCC Controller. The SC4 is specifically designed to power and control the new Dapol Servo controlled signals which have three wire inputs for each signal. An SC4 can control one, two or a twin head (e. g. Bracket) servo signal and each can easily be assigned any DCC accessory address & run the two arms independently or together in Home Distant pairs. It also incorporates a regulated power supply to provide a stable