Patent GB-674243-A (GB)
Title: Improvements in or relating to telephone systems
Inventor: BEALE CHARLES EDMUND
IPC: H04Q1/446
Filed: 1949-07-22
Abstract: 674,243. Automatic exchange systems. AUTOMATIC TELEPHONE & ELECTRIC CO., Ltd. May 23, 1950 [July 22, 1949], No. 19389/49. Class 40 (iv). In a system using a single voice frequency for the setting up and supervision of connections and having arrangements for end-toend signalling over a plurality of trunks, the incoming end of each trunk in a connection is released by a single pulse signal from the outgoing end of the connection and the outgoing end of each trunk is released by a single pulse signal from the incoming end of the associated trunk due to the initiation of release thereat. The outgoing equipment may be seized from an operator's position or by transit selectors. The incoming equipment gives access to selectors. A signalling frequency of 2520 c.p.s. is used. Outgoing call, Figs. 1, 2. When the operator plugs into the jack OJ, battery over the sleeve wire connects up S which busies the relay set to the selector levels SM. Relays SS, SR, FLS operate in turn, FLS actuating contacts to indicate the next idle junction. Relay SS brings up R for the operating period of AX to transmit a 40-60 m.s. seize signal. When the operator throws her dialling key, low resistance battery connected over the tip conductor energizes RR, which completes a circuit for DT to operate R, A and to shunt down RR. Relays A, RR when operated short circuit the signalling source. In response to dialling, relay A sends impulses of the signalling fre-' quency. Restoration of the dialling key momentarily connects battery to the. sleeve wire to bring up LR. Relays DT, R fall back respectively lighting the supervisor lamp and disconnecting the signalling source. Setting up a connection through a tandem exchange. Receipt of the seize signal by the incoming relay set, Figs. 3, 4, causes VFA to operate E and C. At the end of the signal, VFA, C release, G busies forward and AA, AB and AD switch through. The dialled impulses are repeated by AA to extend the connection to a further junction, whereupon earth from the associated outgoing relay set, Figs. 1, 2, operates TRA, which locks up relays AA, AB, AD, disconnects a resistor YFA and the bridge comprising D and I from across the speaking circuit and earths a lead PS to operate a pad switching relay if required. In the seized outgoing relay set TR is operated to switch through the talking connection, disconnect relay VF, earth a pad switching lead PS and to connect up R. R, AX interact to transmit the seize signal. Relay RG energizes followed by ET, the latter busying the operator's circuit. Subsequent signals of short duration have no effect at the tandem exchange. Incoming call at terminating exchange, Figs. 3, 4. In response to the seize signal, VFA brings about operation of E, AA, G, AB, AD as described above, but TRA stays back. The dialled impulses are repeated by AA, the impulsing circuit being improved by the dialling relay C shunting I, D. Answer. Relay D polarized by rectifiers MRB, MRC responds to the line current reversal and RA interacts with a sluggish relay DE to send back a 60-90 m.s. answer signal. RA in falling back completes a circuit for DF to remove a resistor YFA from across the line. In response to the answer signal VF temporarily locks SS to prevent the operator prematurely releasing by withdrawing her plug. M pulls up and, when VF releases at the end of the signal, MA disconnects the supervisory lamp. Trunk offering. By actuating the ringing key the operator connects low resistance battery to the tip conductor to operate RR, whereupon TO earths the start lead 12 of a generator producing 10 impulses per second. TO connects TP to the generator lead 11 and completes a circuit for R so that the signalling frequency impulsed by a contact of TO is sent forward. Relay R also connects 0.75 secs. pulses at 0.75 secs. intervals over lead 13 to AY and subsequently AZ. These relays interact with RG to determine a 4 secs. period after which R, TO are disconnected to terminate the signal. At the incoming end, VFA in responding to the signal energizes C which releases AB, AD and connects in parallel the opposed windings of TOA. After a delay dependent on the charging period of condenser QEA, TOA operates so that when C releases at the end of the signal the line wires are earthed to establish trunk offering conditions in the final selector. Release. The relapse of D when the called party replaces his receiver brings up. RA which reverts a 60-90 m.s. clear back signal terminated when the release of the differentially wound relay DE disconnects RA. DF in falling back removes a holding earth from AA. At the outgoing end the clear back signal momentarily operates VF and M, MA release, the latter shunting the high resistance winding of S to light the supervisory lamp. Should the called subscriber immediately remove his receiver, the answer signal will again be reverted to extinguish the supervisory lamp. When the operator removes her plug, S, SS, AX release in turn and R pulls up to transmit a 4 seconds clear forward signal, the four seconds being measured by the interaction of AY, AZ, RG in response to the pulses from lead 13 connected by the release of SS. At each incoming relay circuit relay VFA is operated for a time amply sufficient to shunt down E and at the end of the signal G releases, thereby connecting up RA during the release period of C to send back a 180-250 m.s. release guard signal. At the tandem exchange the release of G disconnects TR, TRA and AX is disconnected. The release guard signals are transmitted independently from the incoming relay sets in the connection and if such a signal is not received by each outgoing relay set an alarm lamp LP is operated in the appropriate set. In response to the release guard signal, relays RG, ET relapse and the equipment is freed. The outgoing relay set can be busied from the incoming end by backward transmission of the signalling frequency which is effected by inserting a plug into a test jack, not shown, which operates RA. The Provisional Specification describes a similar system using a 1¢ secs. release signal and having no trunk offering facility.