Telephone exchange USHD 2/10

This telephone exchange was suited for small industrial or administrative facilities. Ten phones and two other exchanges can be connected. The exchange can handle two connections at the same time, either between its own phones or between a phone and another exchange (e. g. a state-owned telephone line). It contains a relay oscillator generating the 25 Hz ringing frequency, a transistor generator of the 450 Hz announcement tone and a relay generator of the clock impulses that control the switching of the announcement tone. The user selection is controlled by the impulses coming from the rotary dial of the phone. These impulses control a stepping relay and by chaining the outputs of the stepping relays, the connection with the requested user is provided.

The automatic telephone exchange technology originates from the beginning of the 20th century and was used until the 1970s, when it was superseded by asynchronous exchanges with crossbar switches. Since the 1980s, electronic exchanges were used, analog at first and later digital. Some of the electro-mechanic exchanges have been in operation until the 21st century. An electro-mechanic exchange for a whole city would be as large as an entire building and was extremely complicated and expensive. It required continuous maintenance and supervision.