Telephone subscriber's appts. with series extensions - has measuring device for signalling that exchange line is engaged

摘要

The system has a measuring device located in the telephone line before the first accessory or extra unit. The measuring device signals to the chain of extension devices that the exchange line is engaged. The measuring device may respond to a voltage and signals that the exchange line is engaged by dropping below a given voltage threshold. The advantage lies in the simple manner in which line engaged is signalled.