Power Source Transformer or adaptor ( AC 110V - 240V ) Assurance_temperature -40~+ 50oC Max_Wiring Distance Within the distance of 50meters, from the door station to main indoor phone,take use of RVV5X0.4 multi-core copper wire. Ring Bell Ding Dong Wiring 4 wires in polarity. Extension One door station supports four monitors,includign one main monitor and three extra monitors. Door Release function Signal Unlocking Specification Features: 1) double dingdongchime2) 1 door station can connect to 4 handsets at most3) Power supply: adapter4) Flush mount for door station.