Patent GB-788592-A (GB)
Title: Improvements in or relating to selector switches and to automatic telecommunication systems using such selector switches
Inventor: BRAY FREDERICK HARRY
IPC: H03K17/52
Filed: 1953-09-02
Abstract: 788,592. Automatic exchange systems. STANDARD TELEPHONES & CABLES, Ltd. Aug. 27, 1954 [Sept. 2, 1953], No. 24261/53. Class 40 (4). In a selector switch for use in an automatic exchange there is provided for each possible inlet-outlet connection a three-electrode gasdischarge device, having two anodes and one cathode, for each wire of the talking circuit, each device having one anode connected to an inlet wire and the other to an outlet wire, and when a connection is to be extended from an inlet to an outlet, a first marking. potential is applied to the cathodes of the two devices concerned, and a further marking potential to their outlet anodes, whereupon a low-impedance two-way path is established from anode to anode and is maintained by the potentials on the cathodes. In the exchange described a similar switch operating on a one-wire basis is employed as a register-connector. General description, Figs. 1, 2. Each selector consists of multi-gap units, each having a common cathode, a commoned set of one hundred anodes constituting the inlet, and one hundred individual anodes constituting the outlets. Each unit may be one hundred individual 3-electrode tubes suitably connected, a number of multi-gap tubes, or a single tube having all the necessary electrodes. The system is controlled by pulses supplied by the generator 1 (Fig. 1), the pulse outputs from which are indicated in Figs. 27, 29 and 30. When a subscriber 2 initiates a call, a pulse individual to that subscriber is connected to the relevant outlet of all line finders, such as 3, serving his 100-line group. The pulse generator 1 also supplies individual marking pulses to all line finders; each such pulse is of duration T (Fig. 27), and is applied to the commoned cathodes of the line finder concerned. When coincidence occurs between a line finder mark. ing pulse and a calling subscriber's line pulse, discharge occurs between the two anodes of the pair of anodes concerned and the common cathode. Thus, the line finder is seized and the subscriber is connected by the line finder via a core circuit 6 to a first group selector 7, the line finder and the calling line being busied. A pulse PG is allocated to each group of first selectors and each selector within a group is identified by a pulse PGL (Fig. 27), so that when a first selector is seized the combination of PG and PGL pulses individual thereto causes a marking condition to be applied to the common anode of the register hunter 8 of the cord circuit, which consists of a multiple diode. As only one selector can hunt for a register at any one time multiple seizure of registers is impossible. Registers 9 are connected to the individual electrodes of the hunters, which electrodes are all scanned during a single period T under control of pulses RH from the generator 1. If a register .is free, coincidence of the RH pulse and the hunter marking condition fires the relevant gap, whereby the first selector is connected to a register 9 and the latter is busied. Dial tone generated in the first selector is reverted and the caller dials the wanted number, which is repeated by the first selector to the register. The system is assumed to be a four digit non-director exchange, non-local calls being indicated by the first digit " 1 ". Local call. When the register 9 has received the last digit, it connects itself to the call distributer 11, common to the whole exchange, as soon as 11 is available, and then commences to control selection. Each selection operation occupies a time period T, and as soon as the call distributer 11 has enabled the register to start controlling a selection, it disconnects itself therefrom and associates itself with another waiting register which willthen be enabled to control a selection during the next period T. Thus at each selector stage only one register can be engaged in controlling a selection operation at any time; but at the same time as one register is controlling selection in the first selection stage, another can be doing the same in the second selection stage and so on, the time T for each selection being 30 milliseconds. After the register 9 has received a start signal from the call distributer 11, a pulse indicative of the value of the first stored digit is sent from the register 9 to the first selection stage control circuit 13 common to all first selectors. This pulse indicating the group to be hunted is sent over lead 14 to mark the outlets of that group in all first selectors. Now the first selector 7 which has been seized has its commoned cathodes marked, and if a free outlet exists, coincidence between the pulse on lead 14 and the marking causes the relevant gaps to be fired, thus extending the connection to the second selection stage. Circuit 13 is released and is available for use by another register during the next period T. The second selector 15 is operated in the same manner, the second selection occurring in the period T following-that in which the first selection took place. If all outlets from a selector are busy, a special gap in that selector is fired and busy tone is reverted therethrough. Associated with all the final selectors, one of which (25) has. now been seized, is a single final selection stage: control circuit 17 which, when it becomes available, receives two pulses from the register over leads 18 and 19, indicative of the tens and units digits respectively. The reception of these two pulses causes the sending of a pulse on the relevant one of a set 21 of one hundred bus-bars which extends to all subscribers' lines having the corresponding tens and units digits. In the final selector 25 seized, coincidence of the pulse from the bus-bar and the marking. on the commoned cathodes causes the gaps relevant to the called subscriber to be fired unless he is busy. If the line is busy, the pulse from the bus-bar is ineffective, and a following pulse fires the busy gap in the final selector. If the dialled number corresponds to a dead line, it is arranged that the number unobtainable gap in the final selector be fired and N.U. tone is reverted instead of busy tone. After sending the tens and units digits pulses, the register is released. Outgoing call. The first selector is controlled in the same manner as for a local call and the connection is switched to the outgoing junction. The register released and subsequent digits dialled are repeated by the first selector over the junction. The call distributer gives priority to waiting registers handling outgoing calls and the selection of a free junction is performed during the inter-digital pause following the first dialled digit. If all outgoing junctions are busy, the busy gap is fired and busy tone is reverted. Detailed description. A typical circuit for the gas-gap or static selector used is shown in Fig. 3, A1 and C being the common anode and cathode respectively and A2 the individual anodes. Such selectors are transformer coupled and provide balanced input and output speech connections. The 200 volts connected between the anodes and the cathodes is insufficient to cause breakdown, which occurs only when the cathode voltage is reduced by a further 50 volts or so and the anode voltage is increased by the same amount. When the tubes ionize, MR1 and MR2 conduct and present very low resistance compared with R1 and R2 so that any further application of control voltages will be ineffective, viz. the selector is busied. If R3 and R4 are equal, currents I flow in the balanced circuit, and if R3 is then increased, the current in R3 decreases and that in R4 increases. Similarly, if R3 is decreased, the reverse happens. Increasing R5 decreases the currents in R3 and R4 equally, but variation of R3 or R4 does not appreciably vary the current in R5. Initiation of call. Closure of the loop of the calling line brings up L (Fig. 4), which connects a positive pulse (PL), individual to the calling line within its 100-line group, to the anodes of that line in the group of line finders serving the 100-line group, MR3 being blocked by this pulse. Line finder marking pulses (PF) are applied in turn to each finder (Fig. 5) and, if the finder is free, coincidence of the line pulse PL and the finder pulse PF causes the relevant gaps in the A, B, and M tubes to be fired. Rectifiers MR3 (Fig. 4), and MR4, MR5 (Fig. 5) conduct, rendering further pulses PL and PF ineffective and thus busying the calling line and the finder. The operation of L (Fig. 4), also inserts R9 into the anode circuits of the A and B tubes, whereupon the current in the anode winding of relay A (Fig. 5), increases. Since the current in the centrode winding of relay A remains substantially constant, A changes over its contacts from " S " to " M," thus momentarily depressing the cathode potential of LF (Fig. 7), via MR6 and C1. Tube LF fires and the positive voltage produced at its cathdo causes tube S (Fig. 6) to fire when pulses PG and PGL of the line finder next occur, thus effectively locking out all other tubes S so that only one line finder can be operated at any one time. The positive cathode potential of the fired tube S is applied via R13 and R14 to the common anodes of the register hunters RHS1 (Fig. 5) and RHS2 (Fig. 6), which are multidiodes. Pulses 13 to 112 and 141 to 240 are the PGL pulses allocated to selectors in a group of one hundred, so each selector has two individual pulses in the full cycle of 256. Negative pulses RH are connected in pulse positions 114 to 133 and 242 to 5 to the 20 outlets respectively of the register hunters, whereupon if S is fired an RH pulse on the cathode of a free register causes the relevant gaps in the two hunters to fire, thus establishing connection to that free register..MR7 and MR8 conduct and busy the hunter and the register. In the register, the resulting rise in potential across R20 (Fig. 9) fires TA, thus dousing TB, and C4 is charged. With T11 (Fig. 16), normally fired this results in the