| 摘要 |
PURPOSE:To detect a fact that a telephone set has been used, when a MODEM is being connected to a telephone line and to detach automatically the MODEM from the telephone line, by setting the threshold values corresponding to the respective connected states of the MODEM and the telephone set. CONSTITUTION:In a MODEM connecting circuit for connecting or detaching by an opening/closing means 5 to or from telephone lines 2, 2 to which a telephone set 1 has been connected, the first threshold voltage which is lower than a terminal voltage of the telephone lines 2, 2 at the time when the telephone set 1 is not being used and a MODEM 3 is not being connected, and higher than the terminal voltage at the time when the telephone set 1 is being used and the MODEM 3 is not being connected to the telephone line is set by a Zener diode 14. The second threshold value which is lower than the terminal voltage at the time when the telephone set 1 is being used and the MODEM 3 has been connected to the telephone line, and higher than the terminal voltage at the time when the telephone set 1 is being used and the MODEM 3 is being connected is set by a Zener diode 24. When the MODEM 3 is being connected to the telephone line, if the telephone set 1 is used, it is detected that the terminal voltage becomes lower than the second threshold value, and the MODEM is detached automatically.
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