803,509. Automatic exchange systems. POSTMASTER GENERAL. Nov. 19, 1954 [March 30, 1953], No. 8794/53. Class 40 (4). Apparatus for selecting one out of a plurality of marked circuits comprises a plurality of leads each capable of carrying sets of pulse trains, each of the circuits being represented by an individual combination of one pulse train on each lead. When selection is to be made the fire;t lead is caused to carry those pulse trains associated with that lead belonging to combinations indicating marked lines and a selection is made of one of these pulse trains. The second load is then made to carry pulse trains associated with that lead of those combinations which indicate marked lines and have the selected first pulse train as a member of the combinations. A selection of one of the pulse trains on the second lead is then made and so on until a selection of a unique combination of pulse trains has been made. As described in detail, only two leads are used and the invention is used for selecting one of a number of calling lines or one of a set of called lines indicated by a marker. Thus, as will be seen in the subsequent description, the subscribers' lines are divided into groups, and when a group contains a calling line it places a marking on lead DCIL3 which together with similar leads from other groups feeds a multiplex MX3 whose output lead thus carries pulse trains indicative of all groups having calling lines. One of these trains is selected by selector SEL2, which is caused to indicate the selected group by marking leads DCIL5, DCIL6 which opens gate PG16 in that group to pass to lead PIL3 pulses indicative of the identity of all the calling lines within that group and one of these is selected by selector SEL3. Similar arrangements using selectors SEL5, SEL6 are provided for selecting one amongst a plurality of called circuits. Detailed description. Connection of calling line to register.-Each register is signalled free or busy on a D.C. lead RBL1 feeding a multiplex MX1 which produces pulses whose positions are indicative of free registers. A free register is selected by SEL1 and indicated by D.C. on a combination of two leads DCIL9, DCIL10 out of a plurality of such leads. The existence of a free register is indicated by D.C. on RFL1. Channel pulses in use for connecting any of the registers to calling subscribers and present in any of the circulating systems such as TD9, TD10 are fed to PSL7. Channel pulses already in use in the calling group, i.e. circulating in such systems as TD1, TD2 are all fed to lead PSL2 which is connected to the suppression lead of gate PSG30. This gate is fed with pulses K1 coinciding with all possible channel pulses so that its output on lead PFL1 consists of pulses free in this group. Gate PCG3 passes pulses free in both the group and register calling subscriber to lengthener PLM2 whose D.C. output indicates that a calling subscriber to register connection can be established. A calling circuit CT1 applies D.C. on H1 to multiplex MX2 to produce pulses on PL1 indicative of " off-hook " circuits. These enter gate PSG31 in which all busy pulses (i.e. pulses of subscribers engaged in connections) are removed, and whose output on PFL2 thus consists of pulses representing unconnected calling lines. These are lengthened in PLM3 whose output is gated by PLM2 out of DCG1 to lead DCIL3 indicating that a calling circuit in group 1 requires connection to a register. All such groups are connected to a selector comprising a multiplex MX3 and a pulse selector SEL2 which selects a group and indicates it on a combination of two leads such as DCIL5, DCIL6. These, through DCG2, put a mark on lead DCIL4 indicating that calling group No. 1 is receiving attention. Pulses indicating calling circuits in this group requiring attention, are passed through PG16 (opened by DCIL4) to a common selector SEL3 which selects one and indicates it by D.C. on a pair of leads such as DCIL1, DCIL2. These markings open the input gates PG1, PG2 to circulating systems TD1, TD2 in all groups. Also the selector emits on PIL4 the pulse train associated with the selected circuit (e.g. in multiplex MX2) and this is admitted to group 1 in PG19 and thence to the circulating system TD20 whence it removes this pulse from the pulses, present on PFL2, of circuits requiring attention. Circulation in TD20 is maintained by pulse outputs from MX2 or MX4 produced by hold signals on H1 or H2. DCIL4 opens PCG38 to pass suitable channel pulse trains to SEL4 which selects one and emits it on lead PIL6. This pulse is admitted to the calling group at PG4 and thence via gates PG1, PG2 to the circulating systems TD1, TD2 associated with the calling subscriber. The channel pulse on PIL6 is also admitted to the timing devices TD5, TD6 (selected by the group selector SEL2) and the timing devices TD9, TD10 (selected by the free register selector SEL1). With the modulators RM1, TM1; M3, M4 and RRM1, RTM1 all pulsed with the same channel pulse, connection is now established between the calling subscriber CT1 and the register REG1. With the hold signal on H1 now being received by the register on H4, the register reverts a hold signal on H3 which on reaching the calling subscriber's backward hold lead H2 causes the multiplex MX4 to generate pulses corresponding to the calling subscriber selected by SEL3. This train is gated out of the calling group by PG20 and when PCG4 registers a coincidence between this and that from SEL3 it emits a signal which restores the selectors SEL2, SEL3, SEL4. The connection is held by the hold signal on H3, the consequent pulses on RCGT1 being fed to lead PHL2, for maintaining the circulation in TD5, TD6, TD9, TD10, and the consequent pulses in GR1 being fed to lead PHL3 for maintaining the circulation in TD12. Subsequent setting up on connection between subscribers in different groups. Identification of calling circuit to register connection. When the register has received the dialled digits it is connected to the marker MKR1 by the connector RC. When this connection is effected the register marks lead DCL3 which causes gate PCG1 to emit the calling subscriber to register pulse train on PIL8, whence it is gated by PCG18 to lead PIL9 belonging to the calling group in which it is lengthened in PLM39 to mark lead DCL4 of the calling group. This opens PG3 to pass all pulses free in the calling group to lead PFL3. The pulses on PIL9 are also connected to all gates such as PCG16, PCG17 which compare the pulse with those in the associated timing devices TD1, TD2. The outputs are fed to pulse lengtheners PLM4, PLM7 whose D.C. outputs indicate the identity of the calling subscriber. Likewise, pulses on PIL8 are fed to gates associated with TD5, TD6 and TD9, TD10, the outputs being lengthened in PLM5, PLM6 and PLM9, PLM10 to identify the calling group and the register respectively. Choosing pulse pattern.-Whatever pattern is chosen it is always arranged that the calling subscriber-called subscriber pulse. train coincides with one or other of the pulse trains connecting the register to the called or calling subscriber. For the various methods of connection priorities are arranged as follows: (a) The pulse train already in use for connecting the register to the calling subscriber is also used for connecting the called subscriber both to the register and to the calling subscriber; (b) A common pulse train other than that at present in use between calling subscriber and the register is used for all three connections. This involves a change of pulse train for the first mentioned connection; (c) The same pulse train is used to connect the called subscriber to both the register and the calling subscriber, the existing connection between calling subscriber and register being maintained; (d) The pulse train already in use for calling subscriber to register is also used for calling subscriber to called subscriber, a further pulse train being used between called subscriber to register; (e) The same pulse train is used for calling subscriber to register and to called subscriber, this train being different from that already in use for calling subscriber to register; and a further pulse train is used for called subscriber to register. Pulse changing is required here as in (b). The marker places D.C. on one or more (in the case of PBX groups) of leads ML1 marking the called line(s). These are applied to multiplex MX5 associated with group containing the called line and if the line is not busy, output pulses therefrom pass through gate PSG5 and are lengthened in PLM11 to mark DCIL19 indicating that the group has a free called circuit. Channel pulses free in the group pass from lead PFL1 to gate PSG4, whose suppression lead is provided, over lead PSL6, with pulses busy in the register modulators on the called side and any output in pulse is lengthened to open gate PG24. Pulses on PFL1 are thereby passed via PG25, opened by PLM11, to leads PL7. The presence of pulses on these leads thus indicates that the group has a free called circuit, can obtain connection to the register, and that that channel pulses on PL7 are free in that group. If the called group is different from the calling group these pulses pass through gates PSG6 and PSG7. For possibility (a) above the PSG7 output must coincide with the existing register - to - calling circuit pulses on PIL8 and must not coincide with register called side busy pulses on PSL6. The pulse may thus pass through PCG27 and PSG9 and reach lead PL10. For possibility (b) a PSG7 pulse must coincide with one of the pulses on lead PFL3, which are free in the calling group, and must not coincide with a PSL6 pulse, or a PSL7 pulse. Such pulses pass to lead PL10 via PCG28, PSG10, FSG11 and SG1. For possibility (c) PSG7 pulses must be free in the calling group and in the register called side modulators. Such pulses pass to PL10 via PCG28, PSG10, SG2 and SG1. For possibilit