PURPOSE: To detect an A line earth signal from a subscriber's terminal side by making a semiconductor switch whose one end is connected to the second terminal of an A line earth detecting part and other end is connected to a B line an operating state, and connecting a negative battery to an A line side through the A line earth detecting part.
CONSTITUTION: When a semiconductor pnpn switch 20 is in the non-operating state, one of either an 'B' telephone set 70 or a 'A' telephone set 90 of two common telephone sets is hooked off, the loop detecting circuit of a common subscriber circuit 1 detects the call of the subscriber's terminal. The subscriber circuit 1 switches relay contacts 50 and 52 for the calling of the telephone set keeping normal switches 12 and 18 in the operating state. And, the semiconductor switch 20 connected to the B line of a subscriber's line 100 is made the operating state, and an A line earth detecting circuit 40 is connected to the A line of the subscriber's line 100. When the detecting circuit 40 receives a current I as an earth detecting signal, the earth detecting of the A line is outputted. When a switchboard receives an earth detecting output from the detecting circuit 40, the switchboard confirms the 'A' telephone set 90 has called.
| JP2933013 | VOICE COMPRESSION CODEC POOL SYSTEM |
| JPH0253396 | SUBSCRIBER EXCHANGE |
EGAWA YUICHI
ISHIZAWA AKIRA
NIPPON TELEGRAPH & TELEPHONE