| 摘要 |
PURPOSE:To evade a circuit cut off even when an operator operates a change- over switch erroneously, by providing a hook-off detection circuit, and not performing a switching to a call mode when a telephone set is set in a hook-on state. CONSTITUTION:In an exchange circuit modem 2 having a change-over function for a data transmission mode/call mode, and a call function, a series circuit consisting of a telephone set 6 and a varistor 8 is provided, and it is connected in parallel with light emitting elements of positive direction photo-isolator 11, and of negative direction photo-isolator 12, and a communication line is connected to the series circuit in the call mode. In the data transmission mode, a switching is performed so as to connect the modem to the series circuit in parallel, and to connect it to the communication line, and the hook-on/off of the telephone set 6 can be decided according to whether either of both photo- isolators of the light receiving element is turned on. In this way, the operator can be prevented from operating the change-over switch although the telephone set is in the on-hook state.
|
