You can connect more than 3 7'' slave monitors and 2 doorbells. All can see, hear, speak to the outdoor camera and unlock the door. Power Supply: There is an external power supply (12V2A) in the housing for the base-mounted indoor display. If you don't want to use an external power supply, you can extend the cord to your DIN rail power supply or home power center. You can set the detection area of the doorbell camera. When motion detection is triggered, the device will start recording or taking photos even if no one presses the doorbell. Optional Ringtones: 12 types of ringtones are built into the indoor monitor. You can set different volume levels for different times of the day and you can adjust the ringing duration as needed. Automatic Recording: When a visitor presses the doorbell or triggers motion detection, the doorbell camera will automatically record a video or take a photo and save it on an additional card to protect your home. (SD card needs to be purchased separately)