Patent GB-1404166-A (GB)
Title: Switching systems
IPC: H04Q3/54
Filed: 1972-10-13
Abstract: 1404166 Automatic exchange systems WESTERN ELECTRIC CO Inc 13 Oct 1972 [15 Oct 1971] 47340/72 Heading H4K In a call intercept arrangement, subscribers are arranged in groups and an incoming call to one member of the group can be intercepted by any other member of the group. This is achieved by a group member dialling a special intercept code at his own sub-station whilst the substation of another member of the group is ringing. The call is then automatically transferred from the other group member's substation to that of the requesting intercepting sub-station. The above arrangement is described as a modification to the PABX, of the end marked reed-relay switching network type, disclosed in Specification 1,108,701 to which Specification reference is directed for a detailed description of the normal exchange operation. Here only the call intercept feature will be described. Call intercept, Figs. 1A-1C.-It will be assumed that a call is directed to sub-station ST11, that the call is not answered and that sub-station ST10, belonging to the same group as sub-station ST11, initiates a call intercept procedure. When sub-station ST10 goes off hook, it is connected via line circuit LC10 with the trunk side appearance of a register 128 in known manner. The call intercept code is dialled, registered, and then translated by translator portion 128A of the register. A call intercept code detector 128B is thereby activated and transmits a signal over conductor CPUC to the wired logic common control 113. By means of call-back control 104, common control initiates a register call-back operation n order to identify the line circuit LC10 currently connected to the register. To this end conductor CB is marked by common control which causes the register 128 to apply negative potential to its trunk side sleeve lead appearance. A gate G10 in the line circuit LC10 turns off when it is subsequently scanned on command from common control, so that conductors SL10 and L10 are driven high. The conductor L10 extends to a station-togroup assignment field 123 and the high thereon causes a bi-stable device 109 in a call pick-up circuit 122-0 to be set, there being one such pick-up circuit for each of a defined group of subscribers. The setting of this bi-stable device indicates that a sub-station in group 0 is attempting to intercept a call directed to another sub-station in the group. The high on conductor LC10 causes common control to start trunk scanner 111 so as to scan all the trunk circuits 114 in order to determine which trunk circuits are currently in the ringing condition. When a ringing trunk circuit is scanned a signal is sent over a conductor SL- to common control which causes the scanning to be interrupted and a check to be made as to whether the ringing trunk circuit is connected to a called sub-station in the same group as the requesting sub-station, ST10, as follows. Common control receives the signal on one of the leads SL- at gate 5, which gate causes pulser 124 to suspend scanning of the trunk circuits for a predetermined interval. Common control and scanner control 111A apply a potential to conductor RTE extending to all trunk circuits which causes the scanned trunk circuit to initiate a call-back operation to identify the called line circuit associated therewith, in a similar manner to that in which the calling intercepting sub-station was identified. When the particular called line circuit is scanned its gate G- is turned off and its SL- and L- leads go high. If the called line circuit does not belong to the same group the high applied via conductor L- of the called line circuit will extend to a different call pick-up circuit from that which has a set bi-stable device. This has no effect on common control and the trunk scan is continued after the predetermined interval. If however the ringing trunk circuit is associated with called sub-station ST11, a high is extended over conductor L11 to call pick-up circuit 122-0 of which the bi-stable device 109 is set, and causes gate GSS to be activated to transmit a signal over conductor SRTS to common control. Common control then initiates the circuit actions required to release the network connection between line circuit LC11 and the corresponding trunk circuit; initiates another call-back operation from the register followed by a scanning operation to again identify the intercepting line; and causes both the trunk circuit associated with line circuit LC11 and the intercepting line circuit to mark their C- lead network appearances. The network then completes a connection between the appearances. A call pick-up timer (201, Fig. 2D, not shown) places a limit on the time devoted to an attempted call intercept operation so that common control 113 and the bi-stable devices in the pick-up circuits will be reset after a predetermined time in the event that the call intercept operation cannot be successfully accomplished, e.g. if a call intercept code is dialled at a substation assigned to a group of which none of the sub-stations are currently being called. The Specification contains a more detailed description of the call intercept operation given with reference to Figs. 2A-2D (not shown).