Entryphone with internal communication network - has multiple speech stations and switches between speech and door opening states

摘要

The intercom. system has multiple speech stations for two-way communication between a visitor and the person who is to open the door. A two-line node (4) is connected via a TFE adaptor circuit (2) to an internal communication network (1). The intercom. is switched between two states: a d.c. rest state for speech and an a.c. operational state for opening the door. Indicators are provided to detect a.c. or d.c. and to register a d.c. level (9). Ringing, signalling or control buttons (11) are provided. An AC/DC converter (25) is galvanically or capacitively coupled to the intercom. (3) and connected to the internal communication network (1). ADVANTAGE - No need for special control, power, or signal lines.