PURPOSE: To omit a surge protecting circuit, to reduce the size of the titled circuit and to save the space by making the polarity of the charging potential of a telephone set with that of the potential of an a-line and a b-line and connecting these lines in a system using a subscriber's PCM device.
CONSTITUTION: A polarity monitoring circuit 13 arranged on the terminating exchange side SC1 monitors the polarity of the a-line and b-line. When the potential of the a-line and b-line is reversed to the polarity of potential applied from a call signal sending part 5 to the telephone set 7, the polarity monitoring circuit 13 detects the polarity, actuates answer relay contacts 12-1, 12-2 and exchanges the a-line and b-line to connect these lines to the telephone set 1. Thus, the polarity of the charging potential of a capacitor in the telephone set 7 is made to coincide with that of the potential of the a-line and b-line, so that a control circuit 9 is prevented from inflow of surge current.
| JPS61182355 | CALL SIGNAL CIRCUIT |
| JPS5516598 | FEEDER BRIDGE CIRCUIT |
FURUKAWA KATSUHIKO
NAGATO TAKASHI
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