Automatic engaged condition recognition method esp. for telecommunication line - using comparator with variable reference provided by controller which is responsive to position of hook switch, to compare supply voltage on exchange line with reference voltage from control unit

摘要

The method is based on detection of loop closure by comparing (KP) the supply voltage on the exchange line (AL) with a reference voltage from a control unit (ST), via stepping resistors (RR). The voltage for comparison is derived from a bridge rectifier (GL) and a resistive divider (R1/R2). The telephone line is recognised as engaged when the line voltage falls below the reference level. An incoming call causes the control unit to supply a slightly lower reference voltage when the subscriber's hook switch is closed. An outgoing call instigates a slightly lower reference when the hook switch is open.