Patent GB-1229406-A (GB)
IPC: H04Q3/54
Filed: 1967-04-12
Abstract: 1,229,406. Automatic exchange systems. GENERAL ELECTRIC CO. Ltd. 16 April, 1968 [12 April, 1967], No. 16883/67. Heading H4K. In a PBX having a three-stage reed-relay [switching network wherein there is only one link between switches of adjacent stages and some of the outlets of each C-switch are different in type, a path between a calling line connected to an A-switch and one of the outlets of a particular type of any C-switch is selected by marking all the free AB links from the caller's A switch, marking all the free BC links from all the C switches having the required type of free outlet, scanning the AB and BC links so as to select a C-switch which is accessible from the calling line, then scanning the AB and BC links so as to select a B switch which is accessible both from the calling A and the selected C switch and finally scanning the outlets of the selected C- switch so as to select a free outlet of the required type. The different types of outlet comprise: calling-side junctor inlet; called-side junctor outlet; outgoing-junctor inlet; incoming-junctor outlet; and special service junctor inlet. Logic is provided to ensure that, before any scanning occurs, at least one free path to a free outlet exists. Calls to be set-up by or supervised by an operator are routed to her board via one of the special service junctors and another switching network. Fuller details of these junctors are disclosed in Specification 1,218,344. In response to a calling line, line relay LR (Fig. 3) comes up and causes capacitor 28 to discharge through an associated set of cores in a Dimond ring type of coder 31 so as to identify the caller in a pre-allotted register 34. The latter brings up LMK in the calling line circuit and hence relay AB in the so-called A-B access circuit individually associated with the A switch to which the caller is attached. The register also brings up over lead 42 a relay T (Fig. 5) in each " extension supervisory circuit " (local junctor) irrespective of whether the junctors are free or busy and in addition pulses lead 51 so as to reset all the bi-stable circuits 46-50 in common control 17. In all those junctors which are free (contact BS2 open), negative battery on leads 53 opens gate 56 (lead 55 is marked from the reset output 75 of bistable 46) whereby + 50 V. is applied to the ten H (hold) wires of the ten links terminating on the C-switch shown. On those links which are busy, the + 50 V. is effectively suppressed by earth potential. In the A-B access circuit associated with the caller's A switch, + 50 V. is extended from battery 43 to the ten H wires of the ten links emanating from the A switchalthough on busy links this + 50 V. is again suppressed by earth. Consequently, provided there is at least one free route from the caller to a local junctor, one of the detector circuits 61 which are individually associated with the B switches will respond and set bi-stable circuit 46 (via gates 67, 71, 73). If there are no free routes, the common control times-out by bringing up relay NLA and thus releasing the register (over lead 106). However with bi-stable 46 set, ring counter 76 is started so as to sequentially interrogate the BC access circuits. The first such circuit having a marked lead 53 viz. the first C switch having a free junctor, causes its gate 56 to open (in coincidence with a pulse on its address lead 55) and hence, via a relevant detector circuit 61 and gates 67, 71, 83, a bi-stable 48 is set so as to stop the counter. Consequently gate 112 in the chosen BC-access circuit is opened to provide an operating earth for relays CB, 1BC and 2BC. With a C-switch now selected, ring counters 84, 85 are started in order to select a B switch by successively opening the gates 67 connected to the detector circuits 61. The first one that produces an output viz. the first B switch having access to the selected C switch, sets bi-stable 47 via gates 71, 83, 86 and thus stops the counters. One relay from each of the two sets of relays S-W and X-Z therefore comes up to provide the identity of the selected B switch. The setting of bi-stable 47 starts ring counter 91 (via gate 88, bi-stable 49, gate 89) so as to interrogate the leads 53 of the selected BC access circuit in order to find the first free local junctor associated with this circuit. Selection of a junctor is denoted by the setting of bi-stable 50 and thus the operation of MKL and the appropriate one of N, M, L. Consequently + 50 volts is applied to the relevant C stage crosspoint's hold wire 107 via contacts L1, T2 and " earth " is applied via a 30-volt breakdown Zener diode 110 and relay contact chains S1 X1, 2BC5 to the junction of the crosspoint relay's coil and a make contact C4 thereof. An operating potential is thereby extended to relay B which is marked from 20-volt breakdown Zener diode 109 and finally to crosspoint relay A which is marked by Zener diode (10 volt) 108. Switch-through of the speech path brings up relay A in the junctor followed by BS which applies holding earth to the hold wire 107. The speech wires 95 (Fig. 5) are connected via a register connector to the allotted register 34 which reverts dial tone. If the first dialled digit denotes the requirement of an outgoing junctor or of an operator's position the register pulses the lead 96 corresponding to the junctor-type required (similar to lead 42 for local junctors) and thereafter a similar path-finding operation to that described above is initiated. It is not described how the existing connection is broken down. An incoming call involves a known junctor and of course a particular line circuit these being marked from the register by the operation of relays ML and LMK respectively. ML brings up 1BC, 2BC in the relevant BC access circuit and marks over lead 58 the free BC links having access to the junctor's C switch. Relay CB is brought up when bi-stable 115 is set directly from the register. The selection of a B switch and switch-through of the path is then similar to that described above.