| 摘要 |
PURPOSE:To prevent a large time lag between real on-hook and the detection of the on-hook from being generated, by connecting an on-hook detection circuit to all of the central office lines in use fixedly when a few number of central offices are used. CONSTITUTION:Discrimination whether the number of the on-hook detection circuits 201, 211,... in use is less than a prescribed number is performed, and when it is less than the prescribed number, one of the on-hook detection circuits 201, 211,... is connected fixedly to one of central office circuits 101, 111,... set at a state to be used. Meanwhile, when the number of the on-hook detection circuits in use exceeds the prescribed number, a fixedly connected state until then is interrupted transiently, and a mode is shifted to a time division mutual connection mode. Thus, it is possible to use the on-hook detection circuit connected fixedly to one central office circuit until then for the detection of another central office circuit by shifting the mode to the time division mutual connection mode, and to prevent the time lag in detection of the on-hock from being conspicuous.
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