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.
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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 said ringing control relay to release said line relay during a predetermined interval of time when ringing current is not applied to said make contact.
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.
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.
1. In a switching system having a plurality of telephone sets, line circuits, and trunks, means including a time division bus for selectively interconnecting said line circuits with said trunks, and means for defining active and silent 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 exhibited at a trunk, 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
3. In a switching system according to
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
5. In a switching system according to
6. In a switching system according to
8. A line circuit according to
10. The combination according to
12. A telephone switching system according to
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This invention relates to time division telephone switching systems and more particularly to the implementation of key telephone service in time division private branch exchanges.
Time division PBXs conventionally employ solid state crosspoints which are operated for a fraction of a second called a time slot and which provide for the transmission of voice samples without, however, the ability to provide a d.c. or metallic conduction path between the subscriber's station port circuit and the trunk port circuit. The sleeve lead conventionally found in space division switching systems is also not normally present. This normally causes no undue problems when the switching network is simply handling ordinary telephone sets. However, when a key telephone set having a hold button and one or more illuminated line pick-up keys is served a problem arises which has been discussed in R. M. Averill, Jr. U.S. Pat. No. 3,420,961 issued Jan. 7, 1969. Briefly, when a call involving such a set is placed on hold by the key set user a holding bridge is inserted across the tip and ring conductors. The holding bridge is physically located in a key telephone unit that is interposed between the telephone set and the line circuit appearance in the switching network. A typical key telephone unit of the so-called 400-series type is shown in R. E. Barbato et al U.S. Pat. No. 3,436,488 issued Apr. 1, 1969.
The operating of the hold button at the key telephone set causes an A relay in the 400-series equipment to release and thereby insert the winding of a loop-monitoring L-relay in series with the holding bridge. When the station user removes the hold condition by operating the pick-up key for the line, the A relay is reoperated, the L-relay is released, and the holding bridge is removed. Other contacts of the A relay control other relays that steer the key button lamp illumination potential between a source of steady current when the line is picked up to a source of wink lamp illumination current when the line is placed on hold.
When such a holding bridge arrangement is employed in a conventional step by step or crossbar PBX and the call is abandoned by the party at the remote end of the trunk, the on-hook state of the trunk is reflected by a change in the d.c. state of the line circuit and the holding bridge is removed and the lamp illumination state changed to reflect the true state of affairs. When such a holding brige arrangement is employed with a time division PBX however the holding bridge is not released and the lamp illumination continues at the wink rate giving the station user the erroneous impression that the remote party is still being held.
Heretofore the problem has either been ignored or has required the use of the rather complicated special scan rate distinguishing circuitry disclosed in the above mentioned R. M. Averill, Jr. patent. In that patent, the line circuit was required to be provided with equipment to distinguish between the scanning rates present for held lines and the interruption of scanning by the processor when it determined that the call had been abandoned. It would be advantageous to provide a somewhat less complex arrangement at the line circuit for dealing with the changes occurring on the trunk side of the PBX network and for reflecting those changes at the station side.
It would also be advantageous to provide a means for releasing the hold bridge at a line circuit when the call is abandoned at the remote end of the trunk, or by the other party in an intercom call.
The foregoing and other objects of my invention are achieved in one illustrative embodiment in which the subscriber's line circuit is equipped with a ringing control relay that is normally operated by the central processor to apply ringing to the tip and ring leads of the subscriber's station. The ringing relay is advantageously provided with a back contact that normally isolates the ringing current from the line circuit transformer and thus from the time division hybrid.
In accordance with my invention, when the central processor detects that the call has been abandoned by the party at the remote end of the trunk circuit, either on the abandonment of a held condition or otherwise (since the difference cannot be distinguished), it reoperates the ringing control relay in the line circuit. The ringing control relay, however, is operated at a time when the ringing generator would normally not be connected to the line circuit, i.e., in the so-called silent interval of ringing. The back contact of the ringing relay, when operated under the control of the processor during the silent interval of ringing, interrupts the d.c. talking current to the subscriber's loop. The interruption in this current to the subscriber's loop causes the holding bridge to be released and lamp illumination to be switched from winking to steady just as if the holding bridge arrangement had been used with a line circuit appearing in a step by step or crossbar PBX. Moreover the break in the d.c current occasioned by the reoperation of the ringing control relay and the release of the holding bridge by the key equipment circuits permits the line relay in the line circuit to report the line state as idle to the line circuit scanpoint where it is sensed by the scanner just as if it were an ordinary on-hook line served by the time division PBX.
The silent interval of ringing may be utilized in either of two fashions according to whether the ringing phase is assigned to a line by the processor from a register storing an indication of currently available idle ringing phases or whether ringing phase is predetermined by association of the line circuits in a group with a particular ringing phase bus. In either case ringing is frequently supplied to a group of line circuits through a common ring-trip and disconnect circuit. This invention also applies when a single common ringing voltage bus serves the entire system.
In the preferred embodiment ringing is assigned on the basis of currently available ringing phases, up to four lines in the group served by the common ring-trip and disconnect circuit may be rung, each of the lines receiving its active one second of ringing at a time. If less than four lines in the group are assigned to active ringing, there are idle ringing phases available. During any of these idle ringing phases, a line requiring call abandon correction may have its line circuit ringing relay operated just as if it were to receive active interval ringing but at the same time a disconnect relay is operated in the common ring-trip and disconnect circuit so that no ringing current is actually applied. This disconnect relay may be operated at this time because it corresponds to a portion of the silent interval for any of the other stations in the group because they are assigned other ringing phases. It is thus an aspect of this embodiment that another of the line circuits in the same group may be receiving normal ringing phase signals during the active phase of ringing while a particular line circuit in the group may have its ringing relay and the disconnect relay in the group ring-trip and disconnect circuit operated during the silent interval of that ringing phase to accomplish hold-abandon correction.
The foregoing and other objects and features of my invention may be better understood from the following description when read together with the drawing in which:
FIG. 1 is a block diagram of the groups of key telephone line circuits and the processor of a time division PBX arranged in accordance with my invention;
FIG. 2 shows the arrangement of a conventional key telephone set and prior art 400-type key telephone unit in association with the contacts of the line circuit ringing relay and other details of the line circuit appearing in an illustrative time division switching private branch exchange;
FIG. 3 shows the details of a group ring-trip and disconnect circuit;
FIG. 4 shows the line port shift register ringing relay control flip-flop and interconnections to the data address buses of the time division switching system; and
FIG. 5 shows how FIGS. 2 through 4 are to be arranged.
Referring now to FIG. 1 there are shown the relevant portions of a time division private branch exchange arranged in accordance with one illustrative embodiment of my invention. Other aspects of the operation of the system shown are the subject of copending patent applications filed of even date herewith by D. J. H. Knollman Ser. Nos. 521,648 and 521,649 and D. G. Hill et al, Ser. No. 521,651, it is it connected to the ring lead R of the ringing telephone set. This causes no inconvenience however inasmuch as the ringing is immediately interrupted.
As previously described in connection with FIG. 1, circuit 300-A includes a ringing disconnect relay RD-1 having break contacts RD-1 in circuit with the ringing supply bus RSG-1. Relay RD-1 is operative by the flip-flop including cross-connected NAND gates RDFS and RDFC. Relay RD-1 is operated by the application of a low signal to lead RDS-1* by the processor and is cleared by the application of the low signal to the lead RDC-1*.
While I have thus shown an arrangement for dealing with the hold abandoned condition in a private branch exchange having a time division switching network controlled by a processor 900 having wired logic registers and gates arranged as shown in FIG. 1, it is to be understood that the connection of the line address registers to the gates may also be made under the direction of stored program control without departing from the spirit and scope of my invention.
In particular, my invention can be used in other types of systems, not making use of a centralized ring-trip detecting circuit, by causing the common ringing current generator to have a short interval of zero voltage, during which the ring relay can be activated on all circuits requiring hold abandon correction, thereby causing hold bridges in those lines to be removed. Further and other variations will become apparent to those of skill in the art.
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| Executed on | Assignor | Assignee | Conveyance | Frame | Reel | Doc |
| Sep 15 1978 | Bell Telephone Laboratories, Incorporated | (assignment on the face of the patent) | / |
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