Patent GB-1416546-A (GB)
Title: Communciation systems
IPC: H04M3/42
Filed: 1972-11-22
Abstract: 1416546 Automatic exchange systems WESTERN ELECTRIC CO Inc 22 Nov 1972 [24 Nov 1971] 53923/72 Heading H4K In a PBX which utilizes the switching network of its parent exchange for setting-up its own internal and external calls, an incoming call is routed to an operator who then seizes a 3 port conference link at the parent exchange in order to set up a three-way connection between herself and the calling and wanted parties. The lines between the PBX and the parent exchange are of ordinary type since signalling is effected by voice communication or by simple impulse dialling or two-VF key-dialling thus eliminating the need for data trunks as are used in remotely controlled attendant or centrex PBXs where a dedicated switching network is provided at a subscriber's premises. General (Fig. 1).-The parent exchange is of standard stored programme controlled type utilizing a relay or cross-bar switching network. On the line link side of the network are terminated ordinary subscribers lines 190, 191, and PBX lines 161-165. Of the latter, 161, 163 are used for extending an incoming caller to a PBX attendant while 164, 165 provide access to the extension stations of the PBX. The conference circuits have (at least) three appearances on the trunk link network. They each consist (Fig. 10, not shown) of coupling transformers and capacitors which are matched to those of the other circuits with the aid of a balancing network and also of three relays which permit line splitting and supervision to be effected. Incoming call, connection to operator.-The central control seizes the first free line of the PBX group 161-163 and applies ringing thereto. This is detected by a photo-diode and coupled photo-sensitive resistor 803, 804 (c.f., Fig. 8) located in the so-called " loop circuit " at the PBX end of the selected line. Consequently a ringing detector flip-flop 857 is set so as to permit a loop lamp 817 in the attendants' positions to be flashed on and off at a rate of 60 impulses per minute (i.p.m.) by an interrupted power source connected to lead R. An operator responds by depressing the lamp-associated non-locking loop key 818 thereby enabling a pulse generator 713 in an equipment common to all the operators. The generator after a 10 msec. pause produces a 30 msec. reset pulse, a 70 msec. pause, a 20 msec. hold pulse and then a 20 msec. set pulse on the leads 716, 715 and 714 respectively. These pulses have different effects at different stages of call setting-up. In responding to ringing by depressing key 818, the net effect is that a seize flip-flop 851 is set, the ring flip-flop 857 is reset to cut-off ringing, a 480 i.p.m. flashing source FL is connected to the loop lamp, a hold flipflop 852 is set, transistor pair 857, 858 is energized to connect the answering operator's set 741 to the line pair 801, 802 over which ringing arrived, and to turn-on transistor 864 and lamp/resistor combination 863, 869 so as to turn-on transistors 860, 861 which thereby briefly short-circuit the full wave rectifier bridge 872 and thus convey an answer signal to the central office. A capacitor 873 ensures that transistor 860 soon turns off and thus terminates the answering flash. A resistor 862 in series with the bridge rectifier provides a holding loop to the central office. The latter responds to the flash by terminating ringing. The calling party informs the operator of his requirements. Incoming call, manual completion by operator.- Having ascertained the wanted party's 4 digit number, the operator depresses her loop key again. The hold flip-flop 852 is subjected to pulses such that it acts as two uncoupled gates whereby an answer flip-flop 869 is set (to no immediate effect) and a break-timer 718 is started. The latter produces a 500 msec. pulse that inhibits the recet pulse output of generator 713 and via lead PBK is effective to turn-off transistor 864, lamp 863 and thus transistor 861. Thereby, the loop is broken for 500 msec. thus indicating to the central office that a digit receiver should be connected to the operator's line 801, 802 while the original connection thereto of the calling party should be remade to one port of a free conference circuit and a path between the operator's line and another port should be reserved. The caller's line is split at the conference circuit in one of two different manners depending on whether he is an own or distant exchange subscriber. The receiver returns dial tone, then accepts four dialled or VF keyed digits. During dialling an off-normal flip-flop 880 is set but due to the presence of capacitor 894 this has no immediate effect. The processor is aware of the operator's class of service as from the time a digit receiver was allocated to her line 801, 802 so that reception of just 4 digits is sufficient for the operator/ digit-receiver path to be cleared down and the reserved operator/conference-circuit path to be set up. A called-party/conference-circuit path is reserved and ringing is applied to his line (i.e. to one of the lines 164-165 shown in Fig. 1). The ringing is also applied via a network path to the as yet unused port of the conference circuit so as to be audible to the operator but not to the caller thanks to the line split. When the called party replies the ringing is removed, the reserved path set up and a break supervision signal is sent to the PBX operator's line. The break signal is reflected as a potential (not current) change through 18,000 ohm balanced resistor pair 883, 884 such as to discharge capacitors 880, 882 through transistor 879 and thereby reset flip-flops 880 and 869. The operator depresses her key once more whereby the break-timer 718 is again enabled to transmit a 500 msec. break signal over the line. The central office responds by removing the split in the conference circuit so as to permit a triangular call. The operator may release at any time from the call (following transmission of the wanted extension number that is). She does this by closing her release key 707 or by depressing another loop key in response to a call via another loop circuit. In either case the pulse generator 713 is restarted and the seize flip-flop 851 is reset. When replying to another call the loop lamp 817 is changed to a wink rate (120 i.p.m. with long on and short off portions) until such time as the called party to the considered call replies. The aforementioned break signal is then able to reset all the remaining flip-flops and extinguish the loop lamp. The central processor recognizes the release of the operator and sets up a direct calling/called network path so as to release the conference circuit for further use. Incoming call, semi-automatic completion (Figs. 3-6, not shown).-In this embodiment after the operator has seized a central office digit receiver she simply depresses a non- locking key (in a matrix of such keys) pertaining to the wanted extension-provided a lamp within the key is extinguished denoting freedom of the extension. A subscribers line circuit is shown in Fig. 2, the leads terminating on components 204, 210 being extended to a lamp in Fig. 3 (not shown). The operation of such a circuit is described in Specification 1,416,547. The key identity is entered via a diode translator into a register having seven stages per digit place. The register also includes a crossconnection field on which is permanently registered the first two digits of every subscriber to the 100 line PBX considered here. Insertion of the identity enables a sequence which sets'a direct selection flip-flop 853 in the loop circuit, enables a transistor pair 8101, 8102 so as to connect the incoming line 801, 802 to a MF generator and then sequentially enables the latter in accordance with the two fixed digits and the two variable digits stored in the register so as to transmit the wanted extension's identity to the central office. When sending is complete the sequences marks lead DBK in Fig. 8, thereby resetting flip-flop 853, sending a 500 msec. break signal to the central office by turning off transistor 864 and lamp 863 in a manner described hereabove, and disconnecting the line 801, 802 from the MF generator at terminal DT, DR (i.e. transistors 8101, 8102). If the operator is answering another call the lamp in the considered loop circuit will now commence to wink whereas if she has remained with the loop circuit the lamp will flash at 480 i.p.m. When the called party replies the resultant break signal is effective to clear the loop circuit. Outgoing and intra PBX calls are not des cribed except in so far as an operator originated call produces a loop lamp signalling condition called blink which occurs at a 120 i.p.m. rate with short on and long off periods.