| 摘要 |
PURPOSE:To allow telephone sets connected in parallel to execute a call in accordance with a signal level, by detecting the signal level supplied in common to communication equipments connected in parallel and discriminating whether the simultaneous communication can be executed or not, and cutting off a signal transfer of one of the communication equipments connected in parallel, and allowing the other communication equipment to be communicatable, when the simultaneous communication is impossible. CONSTITUTION:A voltage level of a reference voltage Vref is set to a voltage level by which an electronic telephone set 2 can be operated. When a relation of a detecting signal Va and a reference voltage Vref becomes Va<Vref, a control signal Vb obtained from a voltage comparator 22 becomes a high level, a hook switch S1 is operated by driving a driving circuit 23, and switching from a contact (b) to a contact (a) is executed. As a result, a signal transfer path of a black telephone set 3 is cut off, and the impedance is reset to the impedance 200OMEGA of the electronic telephone set 2. Accordingly, a terminal voltage of the electronic telephone set 2 rises to 4V, and a call can be executed. Therefore, even if the telephone sets of a low impedance are connected in parallel, a call can be executed by the other telephone set.
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