Patent GB-708450-A (GB)
Title: Improvements in or relating to connecting devices for automatic telephone exchanges for emitting subscriber calling signals
IPC: H04Q3/00
Filed: 1951-10-05
Abstract: 708,450. Automatic exchange systems. TELEFONAKTIEBOLAGET L. M. ERICSSON. Oct. 5, 1951 [Oct. 6, 1950], No. 23286/51. Class 40 (4) A connecting device for ringing a called subscriber's line, the calling and called subscriber's line being connected over switching devices by connection of the line branches to two conductors in a link, is characterized in that energization of a line relay at the initiation of a call breaks the connection between the switching device and that line branch corresponding to one of said two conductors and connects the line relay to that line branch over a holding contact. Connection to register.-The invention is illustrated by a call from subscriber A1 to subscriber A2 in an exchange comprising a number of cross-bar switches' having common select magnets S1-S10. Initiation of a call by subscriber A1 operates call relay LR1, which closes circuit in parallel through all the windings of select magnets S2-S10 associated with free links SNR. Only one of the magnets, say S10, can operate after which relay G3 in the call allotter N pulls up slowly to energize hold magnet BM1 which together with S10 closes cross-point contacts a1-d1. Contact d1 energizes R7 in the selected link SNR to operate one of relays R5, R6, say the latter, corresponding to a free register REG, in series with RR1 in the register. Circuit is then made from contacts 12 and c' over the subscriber's loop and wire t4 to impulse relay RR2, which energizes RR3 to return dialling tone from Su2.  Simultaneous calls.-Should two calls take place simultaneously from subscribers A1 and A2, both hold magnets BM1, BM2 in parallel are connected in series with resistance r1 so that neither magnet operates. Relay G2 then pulls up slowly to release the select magnet whereupon G3 falls back followed by G2. During the. release of G2, select magnet S1 operates and shunts out resistance r1, whereupon BM1 and BM2 both pull up and close the contacts corresponding to the select magnet S1. Busy tone is then returned to both subscribers over transformer T. Link or register busy.-If there is no free link or no free register available, G3 pulls up without operation of any of select magnets S2-S10 and G2, S1 operate as just described to supply busy tone.  Selection of called line.-After connection to the register the subscriber dials two digits. The first impulse releases RR2 to energize RR4 which holds until the end of the impulse train. The first train operates magnet KM to set wipers K1-K5. This train also energizes the upper winding of two-step relay RR5 to close contact 1155, the end of the train energizing the second winding. The second train operates magnet GM to set wiper K11 and also energizes the lower winding of two-step relay RR6 which closes contact 1161. After the train, RR4 restores and RR6 energizes fully so that a test circuit is connected from relay RR8 over contact 1160 and the selectors to the hold magnet BM2 of the called line. If the line is busy RRS pulls up. If the line is free RR6 energizes the relay P1 corresponding to the register, in a lock-out chain P1-P2. PI operates RR7 and re-operates the select magnet 810 from contact 1011, relay G1 also pulling up to prevent connection to a calling line. RR7 then causes operation of hold magnet BM2 over the selectors K1-5 and K11, BM2 closing cross-point contacts a2-d2. Ringing called line.-Relay R1 in link SNR then pulls up to release R7 slowly, R4 and current feeding relay R2 energizing to hold BM1, BM2 and R1 over contact 45. Generator RG supplies ringing current through the upper windings of R3 and R4 and condenser C10 to the called subscriber while buzzer tone from Su3 on the lower winding of R4 is returned to the calling subscriber over the upper winding of R4 and condensers C10, C11. When the called subscriber answers R3 pulls up, R2 falls back quickly and R4 slowly, R3 holding up over contact 44 until R2 re-energizes from contact 43 over the subscribers' lines in parallel. The talking circuit comprises the subscriber's lines, the related level of the cross-bar switch and the selected vertical leads a, b. Called line busy.-As described above if the called line is, busy, RR8 energizes operating relay G2 over wire v, so that BM1 pulls up and R7 restores slowly. RR8 also releases S10 so that S1 operates as previously described to return busy tone. Release of R7 releases the register, homes the selectors and returns the apparatus to normal.