| 摘要 |
PURPOSE:To integrate a polarity inversion detection circuit without spoiling the function of a trunk line termination circuit by inserting a photocoupler to at least one circuit of transistors (TRs) constituting a current suction circuit to detect the polarity inversion of a talking current. CONSTITUTION:When the trunk line L1 is positive at the time of dialing, a base current is fed to a TR Q1 via a resistor Rb1 and a light emitting diode of a photocoupler PC1 from the trunk line L1 and a high-level voltage signal is obtained across a resistor R3. When the polarity is inverted by the answer of an opposite party, the voltage of the trunk line L2 is positive, a base currentis fed to a TR Q2 from the trunk line L2 via a resistor Rb2 and a light emitting diode if a photocoupler PC2 and a high-level voltage signal is obtained across the resistor R4. When the talking is finished and the polarity inversion is caused again, the photocoupler PC1 is operated and a high-level voltage is obtained across the resistor R3. Thus, the polarity inversion of the trunk lines L1, L2 is detected.
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