| 摘要 |
PURPOSE:To attain connection of a terminal device to a telephone line without giving effect on an insulation test by providing a switch disconnecting electrically the terminal device from the telephone line when the test is judged as the insulation test. CONSTITUTION:When the insulation test is performed at a voltage of 200V positive at a terminal 1a, a high-voltage analog switch 2 keeps the interrupting state. When the insulation test is performed at a voltage of 35V positive at the terminal 1a, the switch 2 is conducted but a low-voltage analog switch 4 is not conducted and terminals 1a, 1b are terminated by a high resistance. When the insulation test is performed at 48V, the switches 2, 4 are conducted and the terminal device 5 is in the operating state, but since the call signal from a central device is not detected by a terminal device 5 within a prescribed time, a release signal 6 is generated and the switch 2 is brought into the interrupting state. |
