Patent GB-644794-A (GB)
Title: Selector and switching system
IPC: H04Q3/00
Filed: 1947-04-23
Abstract: 644,794. Automatic exchange systems. GRUNDIN, I. V., and ERLANDSSON, E. G. V, April 23, 1947, No. 10879. [Class 40 (iv)] An electric selector comprises a chain of relays operated successively, each relay remaining operated due to remanence until a de-energizing current is applied to it. In a simple embodiment, Fig. 2, a station A can establish connection with any one of five stations B1-5 connected to an automatic satellite. The station A controls relays R26, R27 to send a series of impulses of alternating polarity over rectifiers 29, 30 to energize R21-25 in succession, each such relay -remaining operated due to remanence until the succeeding relay pulls up and applies reverse current to release it. According to the number of pulses, so one of the relays R21-25 remains energized and connects the related station B1-5 to the trunk and, when R28 is operated, to the station A. In the system shown in Figs. 3-5, a number of subscribers connected to a switching centre share a pair of trunks Tr. 1, 2 to the exchange, the trunks being provided at each end with relay selectors for interconnecting particular subscriber with a corresponding line at the exchange. The subscribers' lines are normally all connected across one of the trunks through rectifiers 117, 118 and 127, 128 . . . in parallel with R114 through rectifier 122 to provide a common calling circuit which is switched over to the other trunk when the first is appropriated by a call in either direction. Calls to the exchange. Normally relays R405, 114 are energized over a circuit through the windings of R402, but the current is insufficient to anergize R402. When a subscriber, say S2, closes his loop he draws extra current over trunk Tr.1 and through R405, 402, the latter pulling up to operate R410 slowly. R410 causes R406, 404 to energize and release alternately and continuously, whereby R404 sends impulses of alternating polarity over trunk Tr.1. The current reversals pass through rectifier 123 to control a relay chain R100-110 at the switching centre, each relay holding due to remanence, when the exciting. current ceases, and releasing when the next relay in the chain operates. The first reversal operates the preparatory relay R100 and the first restoration operates R112, which releases R114, which transfers the calling circuit to trunk Tr.2. The second reversal and restoration, respectively energizes R101 and releases R100. The third reversal and restoration energizes R102 and releases R101. Relay R412 in the exchange operates at the first impulse and relays R420- 430 of a selector in the exchange operate and release in like manner to follow the progress of the chain R100-110. Operation of R102 opens the calling circuit to release R402 and the release of R101 connects the subscriber S2 individually to the trunk Tr.1, so maintaining current in the trunk to hold up R405, 406, whereby R404 is prevented from operating. R410, 406 are caused to release to energize R407, 409 to connect the trunk through the contacts of the chain R420-430, with a line corresponding to the calling subscriber and leading to the exchange proper, which is then called. R409 is released by current through a release winding 455. If the exchange is automatic, the subscriber now dials, R402 interrupting the current in the line in step with the impulses. Calls from the exchange. All a-wires of the lines are connected to a common calling circuit 458, which is connected to R411 or R511, say the former, according to whether R512 or R412 is operated, indicating the next trunk to be used. When a call from the exchange is received the line is connected to both A.C. and D.C. sources to operate R411 and R410, whereupon R404, 406 operate and release alternately in the manner already described to operate the chains R100-110 and R420-430 until that relay of the latter chain corresponding to the line from the exchange operates. R401 then pulls up to transmit ringing current to the called subscriber by connecting a winding 465 of R402 to ringing generators SU, ISU. The A.C. ringing current operates R113 in the switching centre to disconnect the circuits to the selector and ring the subscriber's bell. When the subscriber answers, R401 releases to stop the ringing and operates current feed relay R402, which energizes R407 and connects the upper winding of R409 to the exchange to indicate that the call has been answered. If both trunks are now busy, the common calling circuit is connected to the transformer 458. Interruption of a call in favour of another call. A call is interrupted in favour of a longdistance call by a reversal of current in the a-wire of the junction and the upper winding 455 of R409, which causes R431 to operate its " x contacts momentarily to energize a selfinterrupter R435 which rapidly steps the cams 461-463 and therefore the contacts of R433, R434 to normal position, whereupon the selfinterrupter starts again in the manner described below to provide time limitation for the new call. If the subscriber connected to the switching- centre hangs up when the local call is thus interrupted, R402, 405, 407 release but R411 is held so that when R409 operates momentarily, R401 operates and the subscriber again receives ringing current. If the subscriber hangs up after R409 has released, application from the exchange of A.C. signals reoperates R409. Return of switching elements after a call. A clearing signal from the subscriber at the switching centre interrupts current in the subscriber's line and the trunk to release R402, R407 slowly. When the distant subscriber clears down, the polarity of line lead b is changed and R411 releases to operate R410. R406, 404 then operate and relapse successively to step R420-430 and R100-110 in the manner previously described, after which R112 falls back to restore all relays in the repeating set except R405. R114 operates and R111 releases and the apparatus is again normal. Supervision and time-limiting device. A supervisory device R431-6 restores the exchange to normal after a certain time if no dialling impulses arrive, whereas if impulses arrive the device restores the exchange after a longer time. When R407 operates for a call to the exchange, it energizes thermal relay R436 to operate R431, which responds in two steps, the first step after a few seconds closing the " x '' contacts to release R436 and operate R410, 404, 406 to restore the apparatus to normal. For normal calls, R432 operates to cause R436 and R431 to operate and release alternatelv to cause a slow intermittent energization of R435 which advances the cams 461-463 step-by-step. After a time the contacts of R434 operate to apply a weak buzzer tone and the next release of R436 operates R410 to restore the apparatus to normal. Details. If the selectors of one of the trunks have to pass a position corresponding to a subscriber connected to the other trunk, the selectors are not halted because the winding of R413 is short-circuited and cannot operate. To prevent A.C. signalling disturbing the selecting relays, R113 operates to isolate them, a choke coil 121 and resistance 124 being provided in case R113 should act slowly. If one of the selector relays R101-110 be released due to a fault after a call is established, R112 also releases to restore the apparatus to normal. If R112 releases due to a fault, the call is not interrupted but when completed causes stepping of the chain R100-110, whereby R112 can operate again to return parts to normal. A momentary short-circuiting of the trunk does not operate R410 because of the slowing effect of condenser 451 and resistance 450.