Patent GB-660274-A (GB)
Title: Private automatic branch exchanges
IPC: H04Q3/62
Filed: 1948-08-17
Abstract: 660,274. Automatic exchange systems. AUTOMATIC ELECTRIC LABORATORIES, Inc. Aug. 17, 1948 [Sept. 12, 1947], No. 21724/48. Class 40 (iv). A P.B.X. incorporates the following features: (1) separate switch trains for incoming and local calls; (2) incoming calls are received on an operator's position connectable to the incoming repeaters, the operator then setting up the call over the incoming switch train, the connection between the incoming line and the local line being completed in the repeater when the local subscriber answers; (3) the called subscriber may then make an interim call with another local subscriber and subsequently return to the incoming line. (4) The incoming call may be transferred to the other local subscriber under the latter's control. Further features include the restriction that only members belonging to one group of privileged subscribers may make outgoing calls some of these being enabled to make toll calls and the others not; that only members belonging to another group of subscribers may receive incoming calls; and that only members of a further privileged group may break in on existing connections. If an incoming call is extended to an unprivileged subscriber the connection is forcibly released. Operation of finder, Figs. 1, 2, 3, 7, 8, 9. When the calling subscriber A completes his loop the combined line and cut-off two step relay 110 operates partially, grounds the lefthand contact of the finder vertical marking bank 290 and brings up the start relays 750, 760. The first allotter start relay 840 then comes up, disables its mate 940 in the second allotter and causes the finder to step vertically due to interaction of vertical magnet with relay 820. Ground from the vertical offnormal spring 238 over allotter wiper 977 causes the second allotter to hunt for a free finder. When the finder reaches the required vertical level ground on the left-hand marking bank brings up the vertical stepping relay 830 and transfer relay 835 comes up to complete rotary hunting circuit, relay 820 and the rotary magnet 220 interacting until test relay 815 operates in series with the line relay which then operates fully. Relay 810 then opens the stepping circuits and brings up the finder switching relay 240. Relay 825 also comes up to restore the allotter to normal and step it to the next idle finder. If all finders are busy serially related off-normal contacts 236 bring up 710 which disconnects earth from the later terminals of bank 877 to stop the allotters hunting, and when 760 operates in response to a new call busy tone is returned over the line relay. The second allotter, Fig. 9, may be made to replace that of Fig. 8, by operation of keys 865, 965. Operation of selector and local connector. The selector is set vertically by the first dialled digit and hunts for a free outlet in the normal manner with the exception that the rotary hunting only takes place if the restrict relay 380 is operated. For local calls this takes place over the left-hand'vertical marking bank 390. In the connector after the line, release and release slave relays 410, 420, 430 operate, the vertical and rotary magnets are stepped in series with relays 510, 450 respectively on the reception by the line relay 410 of the last two digits. Busy tone is normally connected up directly unless the outlet tests free in which case test relay operates in series with the called subscribers line relay during the slow release of 510 to remove the busy tone and connect up ringing tone and current. Ring cut-off and back bridge relays 540, 530 operate when the called subscriber answers. P.B.X. hunting. The auxiliary control bank 480 consists of a metal arc having metal pins protruding from it to make contact with wiper 482 in all P.B.X. line positions except the last of a group. If the first line is busy 510 interacts with relay 450 the circuit for the latter being completed via the metal arc 480 and the rotary magnet 465. Similar arrangements obtain in the incoming connector, Figs. 14, 15, whose other features are described hereafter. Break-in facilities. Up to ten privileged persons in each hundreds group may cut in on busy calls. They are connected to the first rotary positions of levels which are marked by ground on the right-hand contacts of the vertical marking bank 290. To cut in the subscriber dials 1, the resulting release of the line relay 410 bringing up the monitoring relay 520 over the fourth wire 77, and right-hand vertical bank 390 of the selector, the contacts 239 of the line finder which open only at rotary positions 2 to 11, and the vertical bank 290. Relay 520 completes a talking circuit and replaces busy tone by tick tone which is thus introduced into the existing conversation to give warning of the intrusion. If these parties then hang up 550 operates to call the wanted subscriber and releases 520 the call then proceeding in the usual manner. Main exchange calls. Subscribers privileged to make such calls have their EC wire in their finder banks grounded either directly or through a resistance, the subscriber in the latter case being empowered to make toll calls over the main exchange network. The selector is then set by the dialled digit 0, the restrict relay 380 being operated at this level by the above-mentioned earth to permit rotary hunt. ing. When a free outlet to an exchange line repeater, Figs. 10, 11, 12, is seized, switching relay 350 connects the EC earth through to the fourth wiper via the right-hand vertical bank 390. In the repeater, line relay 1185 operates followed by hold relays 1210, 1220, 1230. The succeeding dialled digits are transmitted over the main exchange line by relay 1185 with 1130 operating during each train to improve the impulsing loop. If full ground is present on the EC wire restricted relay 1050 does not operate as its differential windings are equally energized. If the EC wire is grounded via a resistance however, 1050 operates, thereby grounding the positive wire to the exchange as a signal that a toll call is required. Overflow arrangements. The first eight outlets of the selector " 0 " level are connected direct to exchange line repeaters. The last two are connected to overflow repeaters via a secondary hunting switch, Fig. 6 (not shown). Incoming calls. Ring up relay 1240 responds to incoming ringing current, locks and brings up pilot relay 1610 in series with the call lamp L3. In response to the alarm sounded by the pilot relay the operator throws the corresponding answer key 1655 to bring up the operator connect relay 1250 and its auxiliary 1260, followed by 1620 in the operator's circuit. Relay 1250 connects the operator's set to impulse relay 1185 which operates and brings up hold relays 1210, 1220, 1230 which busy the repeater, light the busy lamp L1 connect the exchange line to the bridge comprising the hold coil IC4 talking condensers C6, C7 and relay 1185. Relay 1230 releases 1240 to extinguish the call lamp L3 and release pilot relay 1610. The operator having ascertained the wanted local number, momentarily operates the transfer key 1664, causing 1180 to operate temporarily due to unbalance of the speech conductors, thereby bringing up 1150 which holds the exchange line with resistance R16 and connects the operator's telephone towards the No. 1 finder 1070 which has access to incoming selectors, Fig. 13 (not shown). Relay 1150 also operates the No. 1 finder start relay 1010 to cause the finder 1070 to hunt for a free outlet whereat the finder is stopped by the operation, over wiper 1075, of the test relay 1020 which switches through and disconnects 1010. The incoming selector operates in a normal manner in response to the first digit dialled by the operator, which is repeated to it by relay 1185, to seize an incoming connector, Figs. 14, 15. If the called subscriber is entitled to receive incoming calls, his fourth wire in the incoming connector multiple is earthed to operate the restrict relay 1530 when the connector is set and the call is set up in the normal manner between the operator and the called subscriber. The operator then releases the answer key to release 1250 and 1620. Relay 1185 is then released followed by 1210 and 1130 comes up to hold 1260. Release of 1210 brings up guard relay 1170 which maintains the main exchange loop and the holding earth for the local selector and connector. Relay 1170 also brings up call pass relay 1110 and partly operates the twostep cut-through relay 1410 in the connector over the fourth wire. Relays 1270 and 1160 also come up followed by the No. 1 call switching relay 1120. Relays 1150 and 1270 release and 1185 comes up over the called line loop followed by 1210. Slow release relay 1220 has stayed up during this period. Relay 1185 releases 1260, 1130 and 1170, the last releasing the call pass relay 1110, and brings up cut-through relay 1410 completely by removing the shunt over the lower winding from the fourth wire. The operator is now disconnected from the call and the exchange line is connected through to the wanted subscriber, release being controlled by the called subscriber's loop which holds 1185. Operator withdraws before called subscriber answers. In this case when the operator restores the answer key only relays 1530, 1550 remain operated to earth placed on the test wire by guard relay 1170. The repeater circuit is placed in a waiting condition as follows: release of 1250, 1260 and 1185 is succeeded by that of the hold relays 1210, 1220, 1230, the second of which connects up an interruptor to flash the busy lamp L1 and the third releases 1130, 1110, 1160. Cut-through relay 1410 then operates completely as before. When the called subscriber answers to bring up the ring cut-off relay 1540, 1185 operates to the called subscriber's loop followed by 1210, 1220, 1230 and 1170 releases. The connection is thus completed and 1220 causes the busy lamp L1 to burn steadily.  Operator cut in. If the called subscr