Patent GB-1380091-A (GB)
Title: Network connectable telephone switching and intercommunication equipment
IPC: H04M9/00
Filed: 1972-05-30
Abstract: 1380091 Automatic exchange systems L'- ELECTROMQUE APPLIQUEE 30 May 1972 [28 May 1971] 25273/72 Heading H4K A PABX comprises a local switching network that is used to interconnect extensions in response to key actuation thereat and an external switching network that is used for extending outgoing calls to external lines from privileged extensions and for extending the external lines initially to an operator's position. Both networks consist of horizontal links that are connectible to vertical multiples one for each extension PT1-PT35, via cross-points BPR external and BSA local such that effectively for an internal call the network consists of two stages. The external network's links each terminate on a control circuit RR through which the link is extendable to a unique external line LR via a switching relay so that the external network effectively consists of only one stage. Each cross-point BPR and BSA comprises logic circuitry that serves to control the speech switches in various manners dependent on the functional condition of a call, e.g. call transfer, paging &c. The speech switches themselves are thyristors which are gated on by the discharge currents supplied by capacitors. The operator PT1 can gain direct connection to the external lines by actuation of appropriate keys with consequential control of the corresponding cross-points but the extensions have only one external time key whose operation (except in special circumstances indicated below), causes a chain circuit type allotted to seize a next free line whereafter the relevant cross-point is operated. The links used on an internal call are accorded a fixed order of priority and the first free one is always selected irrespective of the type of caller, i.e. operator or extension. It may be noted that whenever an external line is involved in a connection any internal call that may be required during that connection, in particular an interim call, utilizes the already seized link of the external network and does not utilize the local switching network. The usual facilities of interim call, call transfer, break-in call, paging, outgoing calls barred and night service are provided. The various lamps mentioned below are preferably incorporated in the keys to which they pertain. Local call.-A caller actuates one of his extensions keys TAP (Fig. 2), thereby causing a pair of relays in a marker MA (Fig. 1, and Fig. 8, not shown), to be brought up due to a particular combination of rectified A.C. so as to mark a sub control circuit DCM (Fig. 1), pertaining to the wanted party. In DCM, transistor gates are opened to find the first free internal connection link circuit CC 1  in a chain of such circuits. On finding, the DCM of the caller is alerted over a sub control bus system so that it may operate a transistor gate in a cross-point circuit BSA pertaining to the caller in the chosen connecting link circuit. The operation of the gate is effective to busy this link circuit in the chain thereof so that a further such circuit may be selected for the call. When the calling key is released a thyristor cross-point is enabled in the caller's circuit BSA. In each circuit DCM, a thyristor is triggered so as to convey a busy marking to a free/busy marker Mo wherein corresponding relays come-up to selectively mark with rectified A.C. conductors AL1-AL18 whereby two lamps LAP corresponding to the two busy, i.e. the calling and called, station are lit in every station. In addition in the called station's circuit DCM a relay supplies intermittent A.C. to the called station over a lead CN so as to energize a buzzer, while the caller is supplied with ringing tone from the control circuit R1 pertaining to the particular link circuit CC. When the called party replies ringing (current and tone) are discontinued and the thyristor cross-point in this circuit BSA is enabled so as to complete the speech path via the chosen link circuit CC. If called station does not reply the caller may actuate a new key whereby his cross-point BSA releases so causing the link control circuit R1 to release the called side cross-point BSA and thus via his DCM and Mo to extinguish his busy lamp in all the stations. The position in so far as the rest of the circuitry is concerned is now just as it was when the caller first called. Called line busy.-A caller may be unaware of this due to a fault such as a bus lamp not lit but the effect when a key is actuated is to lock the caller's sub control circuit DCM causing the signal from the marker MA to actuate a pair of transistors in the called DCM so as to apply an inhibit to the sub control bus system. External calls, line seizure.-Depression of the external line key TOIR again permits the caller's DCM to obtain exclusive use of the marker MA which applies a mark to a selecting lead MR passing through all the sub control circuits RR pertaining to the exchange line links CCR whereby the first free circuit (starting at RR1), encountered is selected. The relay used for breaking the selection chain in the selected link control circuit in operating also triggers a thyristor in the caller's cross-point circuit BPR (Fig. 6, not shown), provided that a transistor therein is enabled-this signifying that the caller is not subject to a calls-barred condition. The caller's DCM circuit is then released from the bus extending to the marker MA so that further calls may be initiated. The external line is looped by an inductor pending the release of the caller's key whereupon a through speech path is made by the actuation of the cross-point thyristors in his cross-point circuit BPR. The free/busy marker Mo is notified, over the column bus wire OC and the sub control bus system wire DCM, of the seizure of the external line and a corresponding busy lamp is consequentially illuminated in the operators set PTI. Dialling operation of a dial or a key set causes interruption of contacts CNc thus pulsing wire CN. In the caller's cross-point circuit BPR a transistor/diode/Zener diode circuit responds so as to repeat the pulses over a common signal wire that links the circuits BPR to the cross-point row control circuit RR1. In the latter a transistor driver operates a relay OR whose contacts repeat the impulses over the external line loop. A further relay (TN) holds off an incoming call reception circuit at least during the transmission of an impulse train. The relay (OR) can be actuated to break the external loop by a manually operable cut-off push button (BCO) in series with the wire CN in each station. Interim call.-If the caller wishes to reach a local extension he actuates the key pertaining thereto whereby the extension is connected up and rung in the same manner as for a local call except that a cross-point circuit BPR on the already siezed link CCR is utilized rather than an internal cross-point circuit BSA. When the caller restores his key he receives ringing current which is also extended over the external line. When a called party lifts his handset a relay operates in the link control circuit RR which breaks the connection between the external line (although this remains held), and the speech link wires AA<SP>1</SP>B over which the calling and called extensions now converse. Interim call, no reply from called party.-The caller either actuates another key TAP whereby a new extension is called, in an exactly similar manner to the above, or he operates key TOIR whereby his connection with the external line is restored. Call transfer.-The called extension actuates his key TOIR thus releasing the original caller and connecting himself to the exchange line. If the called party quickly replaces his handset the presence of a capacitor and Shockley diode arrangement in the link control act RR (Fig. 4), permits the external line to be switched through to the operator's positions. (A speech connection is not fully made until the operator signifies her acceptance of the call which is denoted by a flashing rather than a steady line lamp.) If the called party is engaged but his lamp is out due to a fault condition, the line is also switched through automatically to the operator's position again using the Shockley diode circuit. The operator herself has a key for enabling this circuit when for example she wishes to handle a second call while keeping the first outgoing line on hold. Incoming call.-A.C. ringing current brings up a relay in the sub control circuit RR on which the line terminates. A slow-to-release relay follows in order to supply a continuous ringing signal to the operator and also to light an appropriate call button (Fig. 3, not shown), in the operator's set as well as to busy the circuit RR against seizure by an outgoing call. The operator closes the key corresponding to the illuminated lamp so as to actuate the thyristor cross-point in the link circuit corresponding to the calling line and to remove the ringing condition. As soon as she releases her key a full D.C. path is made and the appropriate busy lamps light. Operator break-in.-She depresses the key pertaining to the wanted line or extension thereby operating the relevant one of her crosspoints so as to provide an A.C. coupling (through capacitors), to the already seized link via the speech pair Li running through the sub control circuits DCM (also Fig. 7, not shown). A ring current may be superimposed on the connection so as to warn the parties of the intrusion. Paging.-This arises as a result of a call to the operator who attempts to reach a wanted third party by operating the relevant key. If the party does not reply, the operator actuates the delay release key appertaining to the calling external line thereby causing her control circuit DCM to enable MO to relinquish control of the calling lamp of the required extension in favour of a flicker circuit (Fig. 10, not shown), in DCM. This causes the lamp to flicker in all extension sets thereby indicating a paging condition. Re-operation of the delay