| 摘要 |
PURPOSE:To require no zero adjustment before a measurement by inserting an arithmetic circuit between a voltage measuring circuit and a display device, opening a relay to hold a data before the measurement in the arithmetic circuit and closing the relay to input the data after the measurement. CONSTITUTION:An operation is all done based on a command of an MPU61 constituting an arithmetic circuit 6. Initially, by opening relays 3, 4, a voltage measuring circuit 5 is separated from a subscriber's speaking line L. Since a voltage dividing circuit 51 is connected to the same earth as a (b) line B of the speaking line L, a voltage to this earth is outputted as a data of the voltage measuring circuit 5. The MPU61 of the arithmetic circuit 6 holds this data as a reference voltage. Then, by closing the relays 3, 4, the voltage measuring circuit 5 is connected to the speaking line L. An output of the voltage dividing circuit 51 is digital converted by an A/D converter 52 to input the data to the MPU61. The MPU outputs a difference between the previously held data before the measurement and the newly inputted data after the measurement as a line voltage to a display device 7. |
