| 摘要 |
PURPOSE:To monitor a reception signal by a telephone set by applying a DC voltage to the telephone set during data communication to hook off the telephone set and to supply a DC current and superimposing a part of a reception signal of a MODEM to the DC current via a series circuit comprising an operational amplifier and a capacitor. CONSTITUTION:In a network controller 30 where a MODEM 2 or a telephone set 4 is connected to a telephone line 5 switchingly, when the telephone line 5 is connected to the MODEM 2, a DC voltage V is applied to the telephone set 4 and the DC current is supplied when the telephone set 4 is hooked off. Moreover, a monitor circuit 40 superimposing a part of the reception signal of the MODEM 2 onto the DC circuit via a series circuit comprising an operational amplifier 21 and a capacitor 22 is provided. Thus, the reception signal is monitored by the telephone set 4.
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