Patent GB-260048-A (GB)
IPC: H04Q3/00
Abstract: 260,048. Western Electric Co., Ltd., (Bell Telephone Laboratories Inc.). July 23, 1925. Semi-automatic exchange systems.-In a system in which toll connections are set up automatically by operators, all impulses, signals and control currents between exchanges consist of single alternating currents in the voice range interrupted at different frequencies, so that satisfactory operation is ensured irrespective of the nature of the toll lines, which may include for example repeater equipments, high-frequency channels, and composite or phantom lines. In the system described, dialled impulses are transmitted between exchanges at frequency 1000 and the various signals consist of alternating current of frequency 1000 (for forward signalling) or 700 (for backward signalling) interrupted at a frequency 20 or 40. The impulses, and in most cases the signals, are received by thermionic amplifying and rectifying valves and the receiving arrangements for the signals include relays tuned to the interruption frequencies. The toll operator dials a 3-digit number identifying the wanted exchange, which may be accessible through one or more intermediate exchanges. At each exchange the 3-digit number is recorded on a register-controller and simultaneously operates a series of Strowger-type toll group selectors to find an idle trunk to the next exchange, the register-controller then transferring its entire record to the next exchange and releasing itself. At the wanted exchange the third toll selector finds a special potential on the terminals to which it is set, which results in a signal being sent bach to the toll operator and in the transference of the incoming toll line from the toll first selector to a local first selector. The toll operator then dials the number of the wanted subscriber to complete the connection over the local selectors of the wanted exchange. The operator controls the ringing of the subscriber and the release of the connection, signals being sent back to her when the connection is completed, when the subscriber answers, and when he disconnects himself. Since the whole number of the wanted exchange is transferred from each exchange to the,next during the progress of a call, it is possible, merely by connecting different groups of trunks in the same level of a toll selector, to provide for the extension of the call over a number of alternative routes through different intermediate exchanges. A toll selector which has been set on a divided level chooses, if possible, a trunk corresponding to the preferred route, but if all such trunks are busy it goes on to find a trunk corresponding to another route. The toll equipment at one exchange is shown in Figs. 1-5. that at other exchanges being identical. The circuit shown in Fig. 3, which is accessible through hunting switches 16, Fig. 1, from jacks 15 and through hunting switches 710, Fig. 2, from incoming toll trunks 525, terminates in a toll first selector 505, Fig. 5, but can be disconnected therefrom by a relay 220 and connected through a hunting switch 609, Fig. 6, to a local first selector 611. A hunting switch 410, Fig. 4, connects the circuit shown in Fig. 3 to an idle register-controller 411 of a set that are common to the toll operators. First exchange ; selection of idle toll group selector and idle register-controller. Insertion of the plug 14 energizes relays 22, 25 but not the marginal relay 21. If the wiper 27 is on a grounded terminal, relay 26 is energized and the switch 16 advances to find an idle toll circuit, Fig. 3, whereupon relay 26 relapses and energizes relays 24, 30, and relay 210, Fig. 3, is energized over the sleeve circuit. Relay 210 energizes relays 310, 212, 213, 218. If the wiper 418, Fig. 4, is on a grounded terminal, relay 215 is energized and the switch 410 advances to find an idle register-controller, whereupon relay 215 relapses and energizes relays 417, 422. The tip and ring of the plug 14 are now connected to relay 429 in parallel with relay 510, Fig. 5. The operator shifts the key 19 to the right to energize relays 23, 38, 37. Relay 39 is energized in the sleeve circuit in place of relay 22 and lights the lamp 40, and relay 37 bridges the tip and ring of the plug through a back contact of relay 46, so as to energize relays 429, 510, which energize relays 419, 435, 440, 512. Relay 221 is energized in series with relay 510 and energizes relay 222. Operator dials number of wanted exchange; setting of register-controller. The dial 18 controls relay 46 so that the impulses are repeated to relays 429, Fig. 4, and 510, Fig. 5. During the interruptions which constitute the repeated impulses, alternating current of frequency 1000 is connected to line from a source 54, but at this stage without effect. Relay 429 repeats the first digit to the stepping magnet 436 of the first register 412, which is a dial transmitter arranged to be set and released electromagnetically. Relay 435, being quick to release but slow to operate, relapses at the first impulse and energizes relay 443, which locks itself. Relay 442 is energized as soon as the register leaves normal. At the termination of the digit, relay 435 becomes energized again and energizes relay 444, which locks itself and transfers the impulse circuit to the next register 413, which is set by the second digit, and so on. Operation of toll group selectors at first exchange. Meanwhile the dialled digits have also been repeated by relay 510, Fig. 5, to control the group selectors 505, 506, 507 which are of known Strowger type. Transfer of recorded number to register controller at second exchange. When an idle trunk to a second exchange is found, such as the trunk 525 shown incoming in Fig. 2, since this trunk is metallically open the relay 221 relapses and energizes relay 238, which connects to the trunk alternating current 1000 x 40 which is disconnected again after a moment by the relapse of relay 226, which energizes relay 224 to de-energize relay 238. At the incoming end of the trunk the signal 1000 x 40 controls relays 711 (which may be of the type described in Specification 224,940), 712, 713, 714 to energize relay 715, and on the cessation of the signal the relay 716 becomes locked up in series with relay 715 and energizes relay 720, which starts the switch 710 to find an idle toll group selector, whereupon the relapse of relay 723 energizes relay 721. This relay locks itself in series with relay 725 and energizes relay 227, Fig. 3, which acts like the relay 210 at the first exchange to energize the relay 310 and start the switch 410. When this switch finds an idle register-controller the relay 417 energizes relay 219, which energizes relay 228 and in conjunction with relay 230 sends back to the first exchange a short A.C. signal 700 x 20. This signal is received by amplifying and detecting valves 319, 320, relay 315 and a tuned relay 321, which short-circuits relay 322 to energize relay 231 and magnet 460. Relay 231 locks itself and energizes relay 232, which supplies holding-ground to the third-wire circuit of the selectors 505, 506, 507. Magnet 460 releases the register 412, and when this returns to normal the relays 440, 443, 444, 442 are unlocked and the release circuit transferred to magnet 468 of the second register, and so on. Each register during its return controls 464, which repeats the impulses in the form of alternating current of frequency 1000 to the second exchange, where they are received by the valves 319, 320 and relay 315, which controls relay 228 to repeat them to the selectors 505, 506, 507 and to the register-controller, Fig. 4. Release of register-controller at first exchange. When all the registers have returned to normal the relapse of relay 459 unlocks relay 231, which receives insufficient current through relay 470, and relay 216 is energized to disconnect the register-controller, which results also in the release of relay 219. Final exchange; release of register-controller and toll group selectors; selection of idle local group selector. The register-controller at the second exchange transfers its record to a third exchange in the same way and then releases itself. It will be assumed that in the present instance the third exchange is the exchange to which the wanted subscriber belongs, in which case the selector 507 at that exchange brings its third wiper on to a grounded terminal 526 to energize relay 217. Relays 218, 417, 422 are thus de-energized to disconnect the register-controller. The relapse of relay 429 energizes relav 431, which locks its left winding in series with magnet 460 to release the register 412, after which the registers 413, 414 are released in turn. Relay 419 relapses shortly after relay 429, but the register controller is now marked busy by relay 431, which is kept energized over its left winding until all the registers have returned to normal, whereupon it relapses, the current received through relay 470 being insufficient. Relay 232 is not energized as at previous exchanges, so that no holding- ground is forwarded to the selectors 505, 506, 507 and these release. The relay 217 also energizes relays 239 and 606, Fig. 6. Relays 608, 605, 607, 604 become energized and relay 606 is released. Relay 604 starts the switch 609 to find an idle local group selector 611, whereupon relays 616, 622, 614, 625 are energized and 616,622 are immediately de-energized. As soon as the selector 611 has seized an idle register-controller, relay 601 grounds the a wire to energize relay 627 which energizes relay 235 over circuit 681 to send back an A.C. signal 700 x 40 to the first exchange, where it operates a tuned relay 325 to short-circuit relay 326, which connects ground through a resistance to circuit 312, 36, 35 to energize relay 39, which lights the lamp 40 before the operator. Operator dials number of wanted subscriber. At this stage it is the A.C. impulses of frequency 1000 f