| 摘要 |
PROBLEM TO BE SOLVED: To obtain a circuit that detects a calling signal with less parts by parallelly connecting a calling signal pass filter to a switch that makes power supply fed from a subscriber line intermittent. SOLUTION: A subscriber line is connected to a power supply circuit 3 through a calling signal pass filter 1 and a station feeding control switch 2. The circuit 3 rectifies power supplied from an exchange via the subscriber line, adjusts voltage and supplies power supply to an interface 4. A loop current defined by prescribed impedance Z is caused to flow, notifying the exchange that it is in an off-hook state and makes a speech communication state by closing a switch 6 for DC closing. The arrival of a calling signal is notified to a microprocessor 5 as an interrupt. After the switch 2 is turned on after interrupt, the switch 2 is turned off to be in a waiting state when a calling signal is not detected.
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