Patent GB-817412-A (GB)
Title: Improvements in or relating to selection control equipment for use in an automatic telecommunication exchange
Inventor: BRAY FREDERICK HARRY
IPC: A45D8/38
Filed: 1957-02-18
Abstract: 817,412. Automatic exchange systems. STANDARD TELEPHONES & CABLES Ltd. Feb. 18, 1957, No. 5408/07. Class 40 (4). In control equipment for controlling selection by a group selector stage or a P.B.X. final selector stage there is provided for each group of outlets from the stage a cold cathode tube, such as G1 (Fig. 3), and a set of gates OG, one per outlet of the group, each of which gates is connected to its associated outlet by a lead (the top one in Fig. 3) over which the gate is disabled when the outlet is busy. When an outlet in a particular group is to be selected the tube G1 of that group is fired and primes the gates of all the outlets of that group. The thyratrons of a common set of thyratrons VP1 to VPN, equal in number to the maximum number of outlets in a group, are then fired in succession by the pulses P1 to PN and apply pulses to the gates of the first, second, ... nth outlets of all groups simultaneously, in that order. The first gate which is primed from G1 and is not disabled by its associated outlet being busy passes the pulse applied by the relevant thyratron over the lead to its outlet to effect the extension of the connection under control to that outlet. This pulse also passes via a rectifier such as MR7 to fire the tube VA, thus reducing the voltage on the second grids of the thyratrons so as to prevent further pulsing. Tube VB also conducts and provides anode supply to maintain the fired thyratron. until the connection has been extended by the selector. The control equipment is released when the long pulse applied to the grid of VB ends. Group selection stage, Fig. 1.-This consists of a number of individual switches each arranged as shown and sharing a common outlet multiple. The simple inlet of the switch is connected via transformer TF1 to the left-hand anodes of a number of pairs of two anode-one cathode tubes of the type described in Specification 734,345, [Group XL (a)], of which a single pair GDIA, GDIB are shown. The right-hand anodes of each pair of tubes are connected via a transformer such as TF2 to a different one of the outlets. On seizure of the switch relay H (not shown), operates and the cathode potential of the tubes is changed from - 200 v to - 250 v which, however, does not fire the tubes. When an outlet has been selected by the control equipment a positive pulse appears on lead P of that outlet from the associated gate of Fig. 3. The stage is such that only one connection is set up at a time and consequently only one inlet has its relay H operated. Hence the only tubes having - 250 v on their cathodes and the positive pulse on their right-hand anodes are those for connecting the chosen outlet to the calling inlet, viz. GDIA and GDIB. The righthand anode-cathode gaps of these tubes fire, the left-hand gaps are then automatically fired also and the inlet is connected to the outlet. The anode current of the right-hand gaps flows in R2 and MR1 with the results that MR1. is biased to its low resistance state to render ineffective any positive pulse which might subsequently occur at the junction of MR1 and R2, and that the potential of the junction of MR1 and R2 becomes negative, thus disabling the gate of that outlet. P.B.X. line selection.-The line circuit used, which is more fully described in Specification 811,244 is shown in Fig. 2. When the line is idle the potential at point a is near to ground potential and when the line is selected by the control equipment of Fig. 3 the positiye pulse applied to PX passes via MR2 to the left-hand anode of tube FS, which constitutes one switch of the single-wire final selection stage of an exchange such as that described in Specification 794, 812. Negative pulses are applied sequentially to the cathodes of the tubes such as FS and a tube whose anode receives the PX pulse and whose cathode also is pulsed fires and extends the wanted line via the final selection stage. Rectifier MR3 goes into its low resistance condition, thus rendering ineffective any subsequent positive pulse occurring at a, and point a goes negative, thus disabling the gate of the selected line.