AI-C-123L-A: Single master set Set includes a door station (AI-C-D) and a master stations (AI-C-ML/A). Additional masters may be added. Each master requires a separate power supply, either the SKK-620 or 4 "C" batteries. The door station calls the master(s) by pressing a call button that initiates a chime. Communication is is Push to talk/release to listen at the master and hands free at the door station. Communication times out after being idle for twenty seconds.