Patent GB-741545-A (GB)
Title: Improvements in or relating to automatic telephone exchange systems
Inventor: MURRAY LAWRENCE JOHN
IPC: H04Q3/00
Filed: 1952-01-26
Abstract: 741,545. Automatic exchange systems. AUTOMATIC TELEPHONE & ELECTRIC CO., Ltd. Jan. 15, 1953 [Jan. 26, 1952], No. 2191/52. Class 40 (4). In a system employing cross-bar switches for completing connections between subscribers' lines in which common equipment is provided for controlling the operation of the cross-bar switch magnets, eachof the switch magnets is individually associated with a gas discharge tube controlled from the common equipment, the arrangement being such that the common equipment is enabled to release upon the striking of the appropriate gas discharge tubes and before the operation of the switch magnets. Each switching stage consists of primary and secondary cross-bar switches connected by link circuits and a connection is set up through a stage by marking all available paths extending from a primary switch through the secondary switches, selecting one of these available paths, and by causing the operation of the switch magnets appropriate to that path due to current flow through the associated gas discharge tubes; the marking of the paths, the selection of the one path, and the operation of the magnets being effected under the control of the common equipment common to the whole or part of the stage. General operation, Fig. 1. A subscriber's line passes through the MDF and IDF in the normal manner to the exchange equipment SLC individual to its line circuit. The switches of the combined line-finder and final selector LF/FS are controlled by a line-finder/final selector common control circuit LFCC. Outgoing calls are extended to a group selector GS through a trunk relay set TRS, which also gives access to a register REG via a register connector RC, and the register is associated with a common control circuit GSCC for the group selection stage by a connecting circuit CCC. Incoming and outgoing relay sets IRS and ORS respectively connect the group selector GS to junctions EJ to other exchanges. On the initiation of a call, the appropriate line-finder/ final selector common control circuit is seized, sets up a free path through a line-finder/final selector unit electronically, and is, immediately released. This operation is completed in 2-3 milliseconds by striking cold cathode discharge tubes associated with individual select and bridge magnets of cross-bar switches, the tubes being maintained conducting to operate the switch magnets after the common control circuit has been released. A trunk relay set is seized by the line-finder/final selector unit and takes a register into use. The trunk relay set repeats the dialled impulses as V.F. impulses to the register, which, when dialling is completed, seizes the group selector common control circuit. The group selector, if free, is then seized by the common control circuit via the register. The register transfers the stored digits to the group selector common control circuit which, if the call is a local one, tests the line finder/final selector common control circuit of the appropriate group of lines. If this circuit tests free, the identity of the wanted line.is transferred thereto and it marks that line. This marking causes the line to simulate a calling line and free paths to the incoming trunks of the line finder/final selector are chosen by the LF/FS common control circuit. A free trunk which corresponds to one marked by the group selector is selected and the call is set up over this path. The group selector common control circuit and the register are released as soon as the call has been set up electronically in the group selector and line finder/final selector. During the call, no cold cathode tubes remain conducting and the cross-bar switches are held by energization of the bridge magnets over the P-wire through their own mechanical contacts. Trunking in line finder/final selector, Figs. 2, 3. The subscribers are arranged in nominal 500-line groups each having 480 working lines. Each line finder/final selector (Fig. 2) comprises a primary-secondary arrangement of cross-bar switches each adapted for double-group working by the provision of double contact units at the cross-points in known manner. In each primary-secondary unit there are two primary switches PSAA and PSAB of the 14 X 10 type, and one secondary switch SSA of the 12 X 10 type. 240 subscribers' lines SL are accommodated on each primary switch, 24 being terminated on each bridge unit BR1-10. Twenty links LA, LB (one from each of the ten bridge units of each primary switch) terminate on the double contact units of the secondary switch. The contact units of each magnet in the secondary switch are internally multipled, so that although each subscriber has access to only one link via a bridge common of the primary switch, each link may be connected to any of the five outgoing or five incoming trunks AOT and AIT respectively, connected to the ten bridges of the secondary switch. The number of primary switches to which a line has access is determined by the traffic requirements. If this is to be eight, the arrangement of Fig. 3 is employed. The 500-line group of lines SLA, SLB terminate on primary switches PSAA, PSBA and so on to PSAH, PSBH and the outgoing trunks AOT to HOT and incoming trunks AIT to HIT provide a total of 40 outgoing and 40 incoming trunks for each 500-line group. With 240 lines on a single switch, the position of a line has no numerical significance in the decimal system of selection, and an intermediate distribution frame is therefore provided to convert the called number into an indication of the corresponding line circuit termination on a cross-bar switch. Detailed description-initiation, of call- Figs. 4 to 11. When a subscriber calls, the junction of R3 and R4 (Fig. 4) is raised to earth potential, rectifier MR1 becomes conductive, and positive pulses applied via C1 reach the trigger electrode of the line calling tube VK1. The pulses are generated by a 5 kc/s. multivibrator MV1 (Fig. 8) from which short pulses of about 20 Ás. (microseconds) duration are obtained by connecting a series capacitor followed by a shunt resistor in the output circuit. These 20 Ás. positive pulses are applied via a 2-way gate circuit GC1 (which normally allows their passage) to a hundreds counting tube VMK1 (Fig. 4) which is a gas-filled tube having five cathodes arranged so that the anodecathode gaps conduct sequentially on the application of negative impulses to a trigger electrode. Each cathode, corresponding to one particular hundred lines in the 500-line group, is connected to the grid of one of five thermionic tubes, such as VT1, which is normally non- conducting. The gate circuit GC1 converts the 5 kc/s. 20 Ás. positive pulses from MV1 into negative pulses for application to VMK1, and to pulse inverter PC1 which reconverts them to positive pulses for application to the grids of VT1 and so on. These last pulses are of insufficient magnitude to render VT1 conductive, but when the cathode of VMK1 associated with the tube VT1 is conducting (as it will be for 200 Ás. each ms.) VT1 conducts during the next 20 Ás. pulse. Thus the five tubes such as VT1 produce 20 Ás. pulses in their cathode leads sequentially, each tube producing one pulse every 1 ms. The tens counting tube VMK2 (Fig. 8) has ten cathodes associated with the ten tubes of the tens distributers of each hundred lines in the group, of which the first and last tubes VK2, VK3 (Fig. 4) of the first hundred lines are shown. The 1 kc/s. 20 Ás. positive pulses from the cathode of VT1 are applied to pulse inverter PC2 (Fig. 8) and the derived negative pulses step VMK2. The correspond. ing positive voltages from the cathodes of VMK2 are applied as biases (each of 1 ms. duration) to the trigger electrodes of VK2, . . . VK3 in turn in all five tens distributers. When such a biasing voltage and a 20 Ás. pulse from VT1 appear simultaneously on the trigger electrode of a tube such as VK2 (which will occur for each tube once every 10 ms.) that tube will strike. These tubes are self-extinguishing and each tube when fired delivers a sharp pulse from its cathode. The units counter VMK3 (Fig. 8) is similar to the tens counter. Negative 1 kc/s. 20 Ás. pulses from PC2 are fed into gate circuit GC2 (similar to GC1) which normally will not pass them. However, when the first cathode of VMK2 conducts, a positive biasing voltage from R12 biases the grid of a thermionic tube (not shown) in GC2 so that a single sharp 20 Ás. negative pulse passes to the trigger electrode of VMK3. Thus the cathodes of VMK3 conduct sequentially at 10 ms. intervals and a 10 ms. biasing voltage may be derived from each of them once every 100 ms. This bias is applied to the'trigger electrodes of the associated line calling tubes such as VK1. While this bias exists on these particular line calling tubes, the tens distributer tubes VK2 ... VK3 strike in turn and the pulses from their cathodes are applied to the trigger electrodes of the corresponding line calling tubes (ten tubes at a time). The tube VK1 of the calling line will fire when both the units bias and the tens pulse are received simultaneously. Firing of VK1 raises the potential across the common cathode resistor R2 (Fig. 9) thus preventing all other line calling tubes in the group from firing. The three diodes VN1, VN2, VN3 (Fig. 4) all fire. When VN1 fires the rise in potential across R20 (Fig. 9) applies a priming bias to VK4, one of ten priming tubes associated with the ten bridge magnets of the first primary switches, e.g. PSAA, of which only the first and last, VK4 and VK5, are shown. Ten similar tubes (not shown) are associated with the second primary switches, e.g. PSBA. A pulse caused by the initial surge of current when a line-calling tube, e.g. VK1, fires is applied to the trigger electrodes of all tubes VK4, ... , VK5 via a pulse delay trigger circuit PD1 (Fig. 9) so as to arrive after the priming bias has been applied. The primed tube fires and the rise in potential across th