| 摘要 |
PURPOSE:To enable both emergency call and the free line talking as well as to prevent the charged talking caused by operation of the special key during service interruption through a simple constituion in which three pairs of circuit switching means are provided. CONSTITUTION:The public telephone set consists of hook switches HS1-HS3, answer monitor relay X (which is operated by polarity inversion signal and released by returning the handset), button switch B for emergency call, relay Y which operates through operation of B, speech circuit N and others. And the following means are added: 1st circuit switching means CH1 which is actuated by formation of the outgoing conditions while the commercial power source is being supplied (and not actuated during the service interruption); 2nd circuit switching means CH2 which operates by receiving the polarity inversion signal transmitted from the office side when the called party gives an answer in the charged call and is released at the on-hook time; and 3rd circuit switching means CH3 which enables the signal transmission irrespective of the reception or interruption of the commercial power supply. Thus, in case CH1 has no operation when the charged call is given through CH3, the talkig current loop is opened by the action of CH2 to make the call impossible.At the same time, the polarity inversion signal flows through CH2 via CH3 to actuate CH2. |
