| 摘要 |
The method allows a switching signal transmitted by a subscriber (TN) by pressing his earth key to be identified in a central station in a telephone switching system with electronic speech path through-connection. For this purpose, a supervisory station (SU) is assigned to each subscriber station which responds not only to the normal loop current changes but also to a current change caused by the pressing of the earth key in the subscriber line loop. When a current change is identified, the impedance of the subscriber device (TN, TA, TS) concerned is measured and, if an impedance is detected which corresponds to the impedance value of a subscriber device when the earth key is pressed, the occurrence of an earth key actuation is recognised. In a telephone switching system in which the operation of subscriber stations with DC signalling by means of an earth key per se is not provided, it is made possible in this way for subscriber stations of this type to be connected and operated even subsequently at an acceptable cost. <IMAGE>
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