Line circuit for key telephone systems

Details Line circuit for key telephone systems Line circuit for key telephone systems

H - Electricity

04

M

379/33, 379/67

H04M 3/42 (2006.01) H04M 1/72 (2006.01) H04M 3/22 (2006.01)

Patent

CA 1073135

ABSTRACT OF THE DISCLOSURE A line circuit for a key telephone system using two relays, a ring relay (B) and a combined off-hook and hold relay (C), and solid state devices including a first and a second optical coupler. On a ring signal from the exchange, the first optical coupler is activated to cause the energization of the ring relay. On a station of the line going off-hook (busy), the conventional A lead ground operates the C relay and causes de-energization of the relay. When a station of the line goes into hold, the circuit must differentiate this condition from a hang-up condition. This differentiation is performed by sensing the line voltage as low (continued loop closure at the start of hold) or high (open loop at the stations of the line). The C relay is held during this determination so that the circuit may enter the hold condition when indicated. On a high voltage condition, indicating a hang-up condition, the second coupler switches on to prevent the B relay from operating, and thereby preventing the circuit from entering the hold condition.

273360

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