schaltungsanordnung for a small fernsprechnebenstellenanlage (heimtelefonanlage) in amtsverkehr redial

摘要

A telephone system for installation in a home allows one main and four extensions to make internal calls or use the exchange line, and is controlled by a microprocessor. An outside call is made by lifting a receiver and dialling the exchange code and number to be called. This is heard by other extensions engaged on an internal call but does not interrupt their call. The microprocessor recognises the exchange code and stores the number. It switches the calling extension to the exchange busbar and connects the busbar to the exchange line. It reads out the number and sends dialling pulses on the line via a transistor pair driven by an optocoupler, which protects the microprocessor from line interference. Dialling can be repeated by dialling a one digit code. The stored number is not erased until exchange code is dialled again.