Station loop control arrangement for telephone switching system

Abstract

An arrangement for controlling the d.c. state of a subscriber's loop which has a port appearance in a time division switching network is disclosed. Because such a network provides no metallic path between the subscriber's line port and the trunk circuit, the abandonment of the call by the trunk circuit must be detected by a trunk scanner rather than by the line circuit directly and conventionally, a processor operation being required to interpret the scanner information. Heretofore a complex subscriber line circuit would have been required to respond to the processor operation so as to inform the station user that the call has been remotely abandoned. The present arrangement allows the processor to reoperate the ringing relay in the subscriber's line circuit during an interval when the group of line circuits including that of the affected subscriber would not normally receive active ringing. The ringing relay is equipped with back contacts that disconnect the subscriber's telephone set from the remainder of his loop circuit allowing the holding bridge in the subscriber's loop to be released and allowing the line scanner to inform the processor that the line is "on-hook". If the subscriber maintains the set off-hook when the ringing relay is released, the processor may properly return dial tone and respond to subsequent call signaling from the subscriber set.

What is claimed is: 
     
       1. In a switching system having a plurality of telephone sets, line circuits, and trunks, means .Iadd.including a time division bus .Iaddend.for selectively interconnecting said line circuits with said trunks, and means for defining active .Iadd.and silent .Iaddend.intervals of ringing for a group of said line circuits, the combination comprising a ringing relay in each of said line circuits normally operable during a defined active one of said intervals for applying ringing current to its respective line,   means for detecting an abandoned call condition .Iadd.exhibited at a trunk.Iaddend., and   means responsive to said detecting means and controlled by said defining means during the silent interval of said ringing in said group of line circuits for operating the ringing relay in the one of said line circuits connected by said time division bus to said trunk detected as exhibiting said abandoned call condition.   
     
     
       2. In a switching system according to claim 1, the combination wherein said means for defining said active intervals of ringing for said group includes a source of ringing control signals having predetermined phases, means for allocating said predetermined phases to lines of said group requiring ringing, and wherein said means responsive to said detecting means includes means for allocating a previously unallocated one of said ringing phases. 
     
     
       3. In a switching system according to claim 2, the combination further comprising a source of continuous ringing normally connected through an operated ringing relay in any of said line circuits to apply ringing to the associated one of said telephone sets, and   means controlled by said means for allocating said previously unallocated one of said ringing phases for disconnecting said continuous ringing source during said last-mentioned one of said phases.   
     
     
       4. In a switching system according to claim 3, the combination wherein there is interposed between one of said telephone sets and its associated one of said line circuits a holding bridge, and wherein said ringing relay includes a back contact operable to disconnect said holding bridge from said line circuit. 
     
     
       5. In a switching system according to claim 4, wherein said line circuit includes a line relay, wherein said holding bridge is normally inserted to maintain said line relay operated when said telephone set has applied a holding condition to said line circuit, and wherein said back contact of said ringing relay when operated disconnects said line relay from said holding bridge. 
     
     
       6. In a switching system according to claim 5 the combination wherein said holding bridge includes a relay insertable in circuit with said line relay and normally operated during the continuance of a holding condition and wherein said back contact of said ringing relay is operable to release said holding bridge relay. 
     
     
       7. A line circuit including in combination, a line relay, a pair of tip and ring conductors for connecting said line relay to a telephone set, a ringing control relay having a set of transfer contacts including a make contact for normally applying ringing current to one of said pair of tip and ring conductors and a back contact for disconnecting said ringing current from said line relay, and means for operating .Iadd.said ringing control relay to release .Iaddend.said line relay during a predetermined interval of time when ringing current is not applied to said make contact. 
     
     
       8. A line circuit according to claim 7 wherein said means for operating said line relay includes flip-flop means settable to operate said ringing control relay and resettable to trip ringing. 
     
     
       9. In combination for serving a group of telephone sets, a source of superimposed ringing current; a ringing supply bus; switching means for normally connecting said source to said bus; a plurality of line circuits each including a pair of tip and ring conductors connectable to a respective telephone set; a line relay; a ringing control relay having a set of transfer contacts including a make contact operable to connect said ringing supply bus to one of said pair of tip and ring conductors and a back contact for disconnecting said line relay from said one of said conductors; and means for simultaneously operating said switching means to disconnect said source from said bus and for operating said ringing control relay of one of said plurality of line circuits.   
     
     
       10. The combination according to claim 9 further including holding bridge means interposed between one of said pair of tip and ring conductors and said line relay in one of said plurality of line circuits, said holding bridge means normally maintaining said line relay operated, and means including said ringing control relay back contact when operated for releasing said holding bridge and said line relay. 
     
     
       11. A telephone switching system having a plurality of line circuits each including a line relay, holding bridge means connectable to one of said line circuits to maintain the associated one of said line relays operated, a source of ringing voltage, means for defining a number of active intervals of ringing for a predetermined number of said line circuits, means for allocating said active intervals of ringing to connect said source to certain of said line circuits, means for detecting an abandoned call condition for said one of said line circuits, and means controlled by said detecting means for allocating an unallocated one of said active ringing intervals to said last-mentioned line circuit. 
     
     
       12. A telephone switching system according to claim 11 further comprising switching means controlled by said detecting means for disconnecting said source of ringing voltage from all of said line circuits during said unallocated one of said ringing intervals.