Patent GB-680608-A (GB)
Title: Toll switching telephone system
IPC: H04M3/60
Filed: 1949-06-21
Abstract: 680,608. Semi-automatic exchange systems AUTOMATIC ELECTRIC LABORATORIES Inc. June 21, 1948 [July 10, 1947], No. 7216/51 Divided out of 680,521. Class 40 (iv). The description is identical with part of that of Specification 680,521 and the claims are directed to supervisory signalling over a twoway trunk using booster battery over the simplex in the forward direction and the normal exchange battery over the simplex in the backward direction. The invention is applied to signalling between the tandem exchange 20 (Fig. 1) and the main office 40 over the trunk 665. The circuit between the exchangers is completed by the carrier link 70, through signals being repeated at the way station 30. Call from subscriber of tandem exchange 20 to the operator switchboard in main office 40, Figs. 2-5. In the detailed description the first digit of a three-digit reference indicates the figure in which the item appears, digits 4, 5, 6 and 7 corresponding to Figs. 2, 3, 4 and 5 respectively. When the subscriber at T21 initiates a call, the finder of the link 426 seizes his line 421 and in response to the dialled digit " 0 " the selector selects an idle trunk, say 510, leading to the trunk circuit 500. The calling loop is extended to the line relay R610 in the trunk circuit which operates. Hold relay R570 which follows, starts the ringing machine, grounds C513 to busy the trunk, and brings up control relay R590 which interrupts the normally completed circuit for seize relay R585 over the simplex and applies booster battery 616 to bring up signal-out relay R660 via rectifier 667 in series with pulse relay R620. R570 also grounds the line conductors C512, C516 of the trunks 510, 515 respectively and connects up ringing tone from C623. The operation of R660 replaces the direct ground applied to rectifier 666 by ground through its own upper winding, and applies ground to the signal-out conductor C603 bringing up R697 in carrier equipment 694. The resulting restoration of signal-in relay R795 grounds C704 and energizes the lower winding of signal-in relay R705 in the trunk circuit 700 which operates slowly due to the short-circuit across its upper winding. The busy and line lamps L732, L733 are thus illuminated, whereupon the operator selects an idle cord circuit, say 766, and inserts the front plug P748 into the jack J730 terminating the calling trunk circuit 700. Jack relay R710 and sleeve relay R720 pull up, the front supervisory relay R745 remaining down as it is marginal. Depression of the talk front key K734 then bridges the headset 767 across the line. The operation of sleeve relay R720 applies battery to signal-out conductor C703 to initiate an answer signal which effects restoration of signal-in relay R670. The intermediate winding of signal-out relay R660 is thus connected in series with its lower winding and that of pulse relay R620, whereupon the latter relay being marginal releases, holds R590 and operates supervisory relay R560 which disconnects the ringing tone, and brings up timer relay R550. A second timer relay R540 follows, releases R550, removes ground from C526 to stop the ringing machine, holds R570, and brings up reverse-battery relay R530 which reverses the polarity of trunk 515 but not that of trunk 510. No metering therefore takes place on the line 421 at this time. The operator extends the call using rear plug P758. When the subscriber at T21 hangs up, line relay R610 relapses, interrupts the circuit of pulse relay R620 and signal-out relay R660, and applies battery through the upper winding of R620 to the simplex, so that R620 remains down and R660 restores since rectifier 667 will not pass current from the battery. Ground is thus removed from signal-out conductor C603 and the consequent release of signal-in relay R705 introduces the upper winding of sleeve relay R720 in parallel with its lower winding whereupon supervisory relay R745 operates lighting the front supervisory lamp L747. The operator withdraws the front plug P748 releasing relays R745, R720 and jack relay R710. The supervisory and busy lamps L747 and L732 are extinguished and battery is removed from signal-out conductor C703, whereupon the signal-in relay R670 re-operates connecting ground to the simplex to bring up pulse relay R620. Reverse-battery relay R530 and supervisory relay R560 restore followed by R540, R570 and R590. The relapse of R570 removes ground from C513, thus releasing the selector of link 426; and that of R590 releases pulse relay R620, whereupon the seize relay R585 re-operates. The connection is thus completely released. Call from the operator switchboard in main office 40 to a subscriber of tandem exchange 20. Assuming that the operator has answered a call using the rear plug P758 which requires extension to substation T21, she inserts front plug P748 into the jack of an idle trunk, say J730 of trunk circuit 700, having previously operated dial key K7.37 to its front position. Jack relay R710 pulls up, lights busy lamp L732 and removes the short-circuit from the upper winding of signal-in relay R705 to make it fastto-operate over its lower winding; and sleeve relay R720 (energized over both windings) comes up in series with the front supervisory relay R745 which lights lamp L747. Battery is applied by R720 to signal-out conductor C703, whereupon signal-in relay R670 restores, and removes ground via rectifier 666 from the simplex to release seize relay R585. The resulting operation of control relay R590 brings up supervisory relay R560 followed by switch relay R630 which connects the right-hand winding of answer relay R595 across the line conductors C521, C522 of trunk 520 leading to incoming selector 434. This selector is seized and returns ground on C523 to energize the left-hand winding of answer relay R595 and hold switch relay R630. R590 also applies battery to the simplex, but pulse relay R620 and signal-out relay R660 remain down as rectifier 667 will not pass current from the battery. The operator then dials the directory number of the wanted party, say " 121." The dial off-normal springs 761 bring up dial front relay R740 which releases R745 and R720 and lamp L747 is extinguished. Conductors C701, C702 are disconnected from the tip and ring of J730 and looped by resistor 717, and battery is maintained on the signal-out conductor C703 over an alternative path including the impulsing contacts 762 of the dial so that the digits dialled are received by signal-in relay R670. For each impulse received R670 momentarily applies ground via rectifier 666 to the simplex so that the impulses are repeated to pulse relay R620. Dial relay R640 comes up for the duration of each train, holds control relay R590 and shortcircuits the right-hand winding of answer relay R595 to prevent its operation during impulsing and to present a clean loop to the incoming selector 434. Each operation of R620 interrupts this loop so that the incoming selector responds to the first digit " 1 " and selects one of the first group of local connectors, say 427, and this connector responds to the subsequent digits " 21 " to select the line 421. Ringing takes place and on the reply of the called party battery reversal brings up answer relay R595 followed by supervisory relay R580. The latter relay replaces the battery connected to the simplex by booster battery 616 to operate signal-out relay R660 via rectifier 667 (marginal R620 stays down). Ground is thus applied to C603 and the consequent operation of signal-in relay R705 lights the busy lamp L732 and removes the upper winding of sleeve relay R720 from the circuit of the marginal supervisory relay R745 which relapses and extinguishes the front supervisory lamp L747. The operator restores the talk front key L734 to complete the connection. When the called party hangs up answer relay R595, supervisory relay R580 and signal-out relay R660 relapse in turn. Ground is removed from C603 and signal-in relay R705 restores, re-introducing the upper winding of sleeve relay R720 into circuit so that R745 re-operates and lights the supervisory lamp L747. The operator removes plug P748 releasing R745, R720 and R710. The lamps L747 and L732 are extinguished and battery is removed from C703, so that signal-in relay R670 re-operates and connects ground to the simplex to bring up pulse relay R620. Control relay R590 and supervisory relay R560 restore followed by R620, whereupon seize relay R585 re-operates. The incoming selector 434 is thus released followed by the local connector 427.