| 摘要 |
A master unit is connected to wire lines (3) and a radiophone is connected through radio lines to the master unit. The unit comprises two switches, the first (26) for estabilishing a DC loop of the lines and the second (30) for opening and closing the D.C. loop. A controller (12) drives the second switch to send a dial impulse to the lines (3) under a condition that the first switch is closed. The first switch comprises contacts of a line relay (29) that is turned ON in a speech mode. The second switch comprises a transistor. The master unit further comprises an isolator (31) through which the controller (12) drives the second switch. The isolator (31) comprises a photocoupler.
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