Patent GB-623177-A (GB)
Title: Improvements relating to electric seletive systems
IPC: H04Q1/18
Filed: 1945-07-23
Abstract: 623,177. Automatic exchange systems. SIEMENS BROS. & CO., Ltd., GRINSTED, W. H., and CHRISTIAN, D. A. July 23, 1945, No. 18808. [Class 40 (iv)] In an electric selective system, particularly an automatic telephone system, the contacts of the banks of the switches are marked by alternating potentials of different frequencies and for the selection of any outlet apparatus is set in accordance with its numerical designation to connect up a circuit responsive to the frequency characterising that outlet to determine when a selecting switch is in contact with it. As shown in Fig. 1, a subscriber's line circuit comprises the usual line and cut-off relays L and K, meter SM, a preselector switch with four wipers driven by the stepping magnet DM and the transformer FT in the battery lead to the holding wire p, to the primary of which is connected alternating potential of the frequency appropriate to the subscriber's units digit. Relay L pulls up to the calling loop, earths the p wire in the final selector multiple and suppresses the A.C. on it to busy the line and the switch hunts for a free group selector, whereupon K operates, releases L and extends the speaking wires to bring up A in the control circuit associated with the selector (Fig. 2). This relay is followed by B which busies the circuit, earths the p wire to operate the preparatory relay GN in the first selector (Fig. 3), and brings up R and- CA. The first digit is registered by a switch DA which at its wipers DA1, DA2 tunes the circuits KA, FTA and KB, FTB to the frequency characteristic of the digit, L and SA coming up as the switch steps off to connect up these wipers. The release of CA and R at the end of the train connects the test relays TA, TB over the a and b wires to the test wipers and applies earth to wire l to start the selector hunting, the outlets being tested in pairs until either TA or TB operates to the characteristic frequency on the appropriate P wiper. The operated relay stops the selector, brings up R and its slave relay TTA or TTB which connects weak or strong potential to wire p to energize the corresponding switching relay GS or GZ. If no free outlet is found, positive battery from the last contact of the group brings up BT which releases the selector magnet, operates R and connects up busy tone. In normal operation the operation of R restores DA and brings up SB in place of SA to connect up the wipers of DB, on which the second digit is registered. The process is repeated, the two switches operating alternately. Final selector (Fig. 4). Testing of a final selector takes place only over the a wire. On seizure, FN pulls up to earth on the p wire and connects wires a, l and p to wiper T, magnet LM and relay FR respectively. On the release of R, earth on the l wire operates LM and the switch hunts until wiper T encounters the first contact of the wanted decade which is marked by the characteristic frequency, whereupon TA pulls up and releases LM, the subsequent operation of TTA bringing up FR which opens the test circuit and connects wire a to wiper P. The release of R after registration of the units digit restarts the switch which hunts until P finds the wanted outlet, when TA is re-operated followed by FS which switches the connection through and applies positive potential to wire l. This potential is applied to relay F in the control circuit (R up) which operates, disconnects the testing circuit and initiates ringing. On the reply of the called party TR pulls up to the called loop, trips the ringing and extends the line back to backbridge relay Y which operates and initiates metering during the release period of L. If the wanted line is engaged, BT is operated as before. PBX final selector. If the selector serves PBX lines, the same frequency is employed to characterise all the lines of a PBX group, and, if necessary, the positive battery connection at the end of a decade may be omitted to permit hunting over the whole group. Night service lines would be connected also in other positions and would be characterised by different frequencies. Outgoing junctions. These will usually be connected to the first selector and selection is similar to that of normal outlets. When a free junction is picked up, strong positive potential is extended back over wires jl (Fig. 3) and l, and on the operation of R (Fig. 2), F and J (marginal) pull up, the last relay bringing up TR, disconnecting Y and connecting in its place the looped back-bridge relay D. Any further impulsing brings up CA to close a low impedance impulsing path over which the impulses are repeated by relay A. Relay D responds to the answering condition and meters the call during the release period of L. Modified arrangements. In a modification, the control circuit is common to a group of selectors (Figs. 5 and 6, not shown) and the test relays are included in the anode circuits of thermionic valves, the control grid potentials of which are raised by rectification of the current flowing in the tuned circuits when the desired outlet is found. Other modifications include the simultaneous testing of pairs of outlets over a loop circuit (Fig. 7, not shown) instead of over separate circuits, and the use of a separate D.C. test for availability of the outlet selected (Fig. 8, not shown). A final selector of the type shown in Fig. 4 may be modified by the addition of a units marking wiper and bank in parallel with the T wiper (Fig. 9) to avoid A.C. on the test wire in the final selector multiple. On the operation of FR wiper U is connected to wire a and wiper P, and the selector hunts for the wanted outlet in bank U, the frequencies being applied to the p wire only while search is being made. Another system (Figs. 10 and 11, not shown) has the marking frequencies applied to the l wires of the selectors and employs a 200-point final selector. Prevention of seizure of circuits with defective fuses. In the circuits of Figs. 1, 3 and 4, the presence of A.C. is dependent on the fuses being intact and no separate D.C. test is necessary, but in Fig. 9 and Figs. 10 and 11 (not shown) A.C. may be present although the fuse is defective, so that the testing arrangements of Fig. 8 (not shown) are employed, the absence of D.C. potential when a D.C. test is made causing the selector concerned to move on. Arrangements for preventing seizure should either the positive or negative fuse be faulty are shown in the junction outlet from the group selector of Fig. 3. Marking defective apparatus busy. Relay L (Fig. 2) has a second winding connected in an alarm circuit. Should the fuse in the positive battery lead blow, L pulls up in series with the alarm, opens the through holding wire and earths the incoming p wire. Specifications 378,585, 584,590 and 623,184 are referred to.