688,197. Automatic exchange systems. AUTOMATIC ELECTRIC LABORATORIES, Inc. Sept. 7, 1948 [Sept. 24, 1947], No. 23634/48. Class 40 (iv). In an inter-exchange system, a selector 800 gives access to trunks requiring a local battery supply and to trunks not requiring such a supply and connects battery when it is required. A " dry " relay 850, Fig. 8, is normally held operated by distant battery when a trunk is selected. Local battery is fed through relay 875 only when the loop is " dried " by disconnection of the distant battery. A toll relay 810 is operated to cut out a repeater 825 and to provide a clean metallic circuit when no local battery is required. Toll relay 810 is controlled by contact springs in selectors 800, 600, Fig. 1. In addition to off-normal springs, the former has springs operated at the second and eighth levels and at the eleventh rotary step. The latter has springs operated at the fourth, fifth and sixth levels and at the eleventh rotary step. Selector 800 selects a level in response to a single digit and hunts. Selector 600 normally selects a level and a group within the level in response to two successive digits before hunting but at the fourth level hunts without awaiting a second digit. Selector 800 is arranged to absorb the digit 2. Simplex signalling is used in both directions over junction 210. A ringing signal is sent forward over one leg of junction 1110. Loop signalling is otherwise employed in the forward direction. Battery reversal is used for backward signalling over junctions. Separate conductors are used for backward signalling within an exchange. Junctions 210, 1110 are used both ways. Fig. 1 shows the general arrangement of the system and Figs. 3, 4 and 8 show details of the toll-board 300, of the toll line circuit 400 and of the toll transmission selector 800. Figs. 3 and 4 should be arranged sideby-side. The trunk 420 is common to Figs. 4 and 8. Call from distant office 10 to station T201 in central exchange. When the operator at 220 selects a junction 210, a toll line circuit 400 is seized by a simplex signal over the junction. A signal relay 385, a pulse relay 470, a hold relay 485 and a switch relay 460 operate in the circuit 400 and a supervisory lamp 379 is lit at the toll board 300 in the central exchange. An answer relay 475 is looped with a line relay 825 to seize a toll transmission selector 800. The line relay is followed by a hold relay 840, a dialling relay 800, a dry relay 850 (which detects the removal of battery from the forward loop), and a control relay 860. The operator dials 24XX (or alternatively 14XX). Impulses are trans. mitted as simplex signals over the junction 210 and are repeated as loop signals over trunk 420 at contact 471 of pulse relay 470. In the selector 800, the impulses step the vertical magnet 807. Off-normal springs 884, 885 are changed over at the first step and dialling relay 880 is subsequently held by impulses. Contact springs 889 are changed over at. the second step causing the release magnet 810 to operate when the dialling relay 880 relapses after the first digit 2. The wiper carriage returns to normal, digit absorbing relay 830 operates and the release magnet restores. When dialling relay 880 relapses after the second digit 4, contacts of the springs 889 are closed, and the rotary magnet 808 operates, the step magnet 820 having been operated at the first step over springs 885. The step magnet and the rotary magnet interact to step the switch 801 to test for an idle trunk 240. When a trunk tests free, the test relay 890 disables the driving circuit, and contacts 891, 892 extend the loop from the dry relay 850 to the combination connector 235, relay 850 now being held by forward battery. At contact 826 the line relay 825 repeats the third and fourth digits over the loop to the connector 235 which completes the connection in known manner. If the called station is idle, the connector disconnects battery from the loop and releases dry relay 850. Control relay 860 follows and connects transmission battery and ground through a supervisory relay 875 and a tone relay 870. When the called party replies the supervisory relay operates and the tone relay transmits a splash of tone from conductor 824 to identify the call as a toll call. The answer signal is repeated by 875 over a signalling conductor 424 to a supervisory relay 430 which operates and extinguishes the supervisory lamp at 220 by means of a simplex signal over the junction 210. Flashing is effective over the same signalling circuit. Disconnection relights the supervisory lamp at 220 and the connection is released when the operator at 220 withdraws. If the called station is busy, 875 is not operated and the connector 235 returns 60 pulses per minute to give a characteristic signal in the supervisory lamp at 220. If the switch 801 finds all trunks busy, it rotates to the eleventh step where contact springs 886, 887 are closed, the switch is stepped and 120 pulses per minute are transmitted from conductor 823. If the operator dials 14XX the call is routed through an auxiliary toll selector 600. The first digit 1 steps the selector 800 to the first level where it hunts for an idle line 710 which normally gives direct access to the selector 600 without operation of the ringing circuit 700. The second digit 4 steps the selector 600 to the fourth level where, with contact springs 634 (not shown) closed, it hunts for a free trunk 240, giving access to the connector 235. The loop from 800 extends to the connector 235 which completes the connection on receipt of the last two digits. Battery is disconnected at 235 and reconnected at 800. Answer, busy, flashing and disconnection signals are transmitted as before. Call from distant toll board 220 to central toll board 300. The operator at 220 can gain access to the toll board 300 through auxiliary selector 600 and one of three similar circuits 301, 304, 500 by dialling 111, 121 or 131. The first digit 1 controls selector 800 to select a free auxiliary selector 600. The second digit 3, for example, steps the selector 600 to the third level and the third digit switches it to the first contact of the wanted group 1 in which it hunts for a free trunk. When a free trunk is found, a backward signal over an extra conductor 715 operates toll relay 810, the loop from the repeater 395 in the line circuit 400 is extended to a line relay in the circuit 500 and a supervisory lamp 373 is lit in the toll board 300. When the operator answers by inserting a plug 321 the circuit 500 is signalled by battery on sleeve conductor 513 and the signal is repeated by grounding an extra conductor connected through conductor 714 and wiper 805 to supervisory relay 430 which signals the board 220 over the simplex circuit already described. The last contact of each group in the selector 600 is grounded so as to operate a busy relay if all lines in the group test are busy. The busy relay arrests the switch and returns 120 pulses/ minute to flash the supervisory lamp at the toll board 220. An all trunks busy condition in 800 is signalled as before by 120 pulses/minute. The operator at 220 can recall the operator at 300 by momentarily removing the plug to transmit a single pulse which operates a slowrelease recall relay in 500. The marginal relay 345 is operated and the lamp 334 is lit pending release of the recall relay. Repeated removal of the plug produces flashing. When the operator at 300 withdraws the plug 321 a disconnect signal is given at 220 by the supervisory lamp which is re-lit over the same signalling circuit as the answer signal. The operator at 220 may still recall 300 by tapping the forward plug. In these circumstances, the lamp 373 is lit and a tone signal is returned to 220 to confirm the signal. Answer supervision is again given at 220 when the operator at 300 replies. Call from distant toll board 220 to toll board 604 in remote exchange 40. The operator at 220 dials 151. The auxiliary toll selector 600 is seized as before in response to the first digit 1 and is stepped to the fifth level by the second digit 5 where it hunts for an idle ringdown circuit 601. Springs 633 (not shown) closed at the fifth level operate a relay to insert a line relay and a repeater in the line in the ringing circuit 700. At the repeater battery is temporarily connected to the line 720 to start a ringing cycle in the circuit 601 and the ringing is indicated at 220 by a splash of tone, and by the momentary extinction of the supervisory lamp. The supervisory lamp is extinguished again when the operator at 300 replies. The lamp is lit to indicate disconnection at 604 and release is initiated when the plug is withdrawn at 220. Call from distant toll board 220 to toll connector 705 in central exchange. The operator dials 161X or possibly 16XX. In response, the auxiliary selector 600 steps to the sixth level. With springs 633, 634, 635 (not shown) operated, the third digit controls the selection of a group in which the selector hunts for a free trunk 706. The supervisory lamp at 220 is extinguished during hunting to delay dialling of the remaining digits. Toll relay 810 is operated an answer is signalled by battery reversal which operates answer relay 475 in the line circuit 400, the signal being repeated to 220 over the simplex circuit on junction 210. Disconnection is likewise signalled by battery reversal. Call from toll board 220 to station T1201 in exchange 30. The operator at 220 dials 84XX. A free trunk 910 is selected in response to the first digit 8 and toll relay 810 is then operated over springs 888 closed at the eighth step. The loop is terminated at a repeater in circuit 900 and a selector 1215 in exchange 30 is seized by closure of successive loops over junction 1110 and trunk 1210. A connector 1217 is selected. by 1215 in response to the second digit 4 and the last two digits complete the connection to T1201. When the wanted line is found