Subscriber monitoring in electronic telephone exchange - has circuits to block selected subscribers if power supply load limit is reached

摘要

Through a suitable interface (S) the central controller (ZST) of an electronic exchange monitors the state of the individual subscriber line terminals (TS1,..TSx). This monitoring checks whether the line impedance is high or low and establishes whether the line is not in use or whether the receiver is lifted and a low impedance switched in. The limiting resistors (R1,..R4) are controlled by electronic switches (1p,2p) operated by a relay (P) controlled by the exchange. If, for economic reasons, the exchange power supply (ZB) cannot feed all the subscribers in the low impedance state then some sort of overload protection is necessary. At times of high power supply demand the exchange controller (ZST) can order some subscribers terminals to remain in the high impedance state regardless of whether the associated instrument (Tl1,..Tlx) is indicating the desire to make a call. Similarly incoming calls are also blocked. The power demand is thereby limited.