| 摘要 |
PURPOSE:To obtain a small-sized call signal circuit packaged to high density for a telephone switchboard, by ANDing a delay output and a comparison output, and then detecting a DC current to be superposed upon a call signal current. CONSTITUTION:This system is equipped with a comparing circuit CMP, a delay circuit 24, and a gate 25. The comparing circuit CMP compares a call signal current sent out of a call signal sending circuit 1 with a prescribed reference value, and then outputs a logical value 1 when it is decided that the call signal current is greater than the reference value or logical value 0 when not. The delay circuit delays its comparison output by a half as long as the period of the call signal current, and the gate 25 generates the output of AND between the delay output RTD and comparison output RTP. On the basis of the output of the gate 25, a DC current to be superposed upon the call signal current is detected. |
