651,521. Automatic exchange systems. WESTERN ELECTRIC CO., Inc. Jan. 23, 1948, No. 2127. Convention date, May 1, 1942. [Class 40 (iv)] Exchange originated coded digits consisting of combinations of voice frequencies transmitted in impulses are prefixed by a distinctive code impulse of predetermined duration which controls the connection of a signal receiver to a register at an intermediate exchange, each digit represented by an impulse being stored in turn in the receiver which remains locked until the register indicates that registration of the digit is completed. Signal receiver. As shown in Figs. 1 and 2, a trunk T, R taken into use by a calling exchange operator requiring a connection to a subscriber B at an exchange Y<SP>1</SP> or to a subscriber C at an intermediate exchange X<SP>1</SP> is connected to an idle register-receiver at the intermediate exchange through a cross-bar switch SS and an operated relay SW. The coded digits are transmitted from the calling exchange in combinations of two frequencies selected from the voice frequencies a-f by means of a keysender KS. At the intermediate exchange the coded digits are applied through a variable attenuator AP and high-pass filter HPF to a volume limiter VL comprising negatively biassed push-pull connected tetrode valves L1, L2 which amplify the input signal to a constant power output level controlled by the potential on the screen grids. The volume limiter output is applied across a resistance network N to a group of band-pass filters BPFa-BPFf and to a transformer SPT connected to the control grid of a positively biassed tetrode valve S which feeds the secondary winding of relay SPR. This relay is normally inoperative but operates on the reduction of anode current accompanying a signal, thereby energizing the primary windings P of channel relays Ra-Rf providing the conductors J and L are connected in the register as described later. A choke SPI and a condenser SPCN make relay SPR slow to operate and guard against false operation by transients. relays Ra-Rf are similarly controlled by tetrodes Ta-Tf fed from filters BPFa- BPFf. Battery is connected over switches (not shown) to conductors BAT1 and BAT2, Fig. 3, when the register is seized by the calling trunk, relays CK1, CK2 and CK3 being energized over their secondary windings S but not operated. Battery is also applied to the screen grids of valves Ta-Tf over resistors A2-F2 and to the primary winding P of a polarized relay KP1 which does not operate due to battery D connected to its secondary winding S being earthed at relays Rc and Rf. The valve filaments are continuously energized. The register conductors A-F are extended from the channel relays to the register over contacts of relays KP2, KP3 which operate and lock under control of the slow-to-operate relay KP1. Operation of relay KP3 also energizes series connected tertiary windings T on the channel relays, these windings aiding the release of the relays by tending to neutralize the primary windings and assist the secondary windings. Preparatory signal. Operation of relays SPR, Rc and Rf by a preparatory signal KP consisting of frequencies c and f removes earth from the secondary winding of relay KP1 which commences to operate. Receipt of another signal during the operating lag of KP1 provides an earth which releases the relay. Relays KP2, KP3 operate and lock when either of the channel relays Rc and Rf release at the end of the preparatory signal. Receipt of a digit. On completion of the KP signal any further signal in the digit code will operate the appropriate channel relays, thereby disconnecting direct earth from the screen grids of the valves responding to the signal and inserting a previously short-circuited connection to earth over the corresponding resistances A3-F3 and the primary windings P of relays CK1, CK2, CK3. The potentiometer circuit so formed from battery on conductor BAT1 reduces the screen grid potential and hence further reduces the anode current of the signal energized valves so that the channel relays remain operated after the completion of the impulse until their primary windings P are disconnected. The current through the primary windings P of relays CK1, CK2, CK3 depends on the number of channel relays operated. Relays CK1, CK2 only operate providing two or more frequencies are received simultaneously. Relay CK3 operates if three or more frequencies are received and connects earth over a conductor RO to a re-order sending circuit. Registration of a digit. Two different types of digit registers, Figs. 4 and 5, are described, necessitating the wiring connections X and Y respectively in the receiver circuit shown in Fig. 3. The register shown in Fig. 4 records coded voice frequency impulses transmitted from the local or a distant exchange, while the register shown in Fig. 5 also provides facilities for the registration of locally transmitted D.C. impulses. In the register shown in Fig. 4, steering relays TR1, TRA are operated by a momentary earth over conductor 20 when the register is seized, both relays locking to earth over contacts on relay TRA. The other groups of steering relays TRB, TR2 to TRN, CI operate in turn over contacts on the preceding relay group and lock to earth. Relays TR1, TRA extend the signalling conductors A-E to the lower windings of the first digit register relays 1-5. Operation of the relay SPR energizes the primary windings of the channel relays over back contacts of a spring-biassed polarized relay TO and contacts of the operated relay C1. After receipt of the KP impulse, the first digit register is operatively connected to the channel relays over contacts of relays KP2, KP3. The first digit code signal operates two channel relays and earth applied to conductor H by relay CK1 releases relay TRA and holds relay TR1. Two register relays 1-5 are operated over their lower windings from earth at the operated channel relays and lock over their upper windings, the primary winding of'relay TO, and back contacts of CKA and CK3. The relay CKA releases slowly after operation of relay CK2 and delays the locking of the register relays to enable relay CK3 to function if a third signal frequency is received. Relay TO operates provided two register relays operate, locks over conductor J, relay SPR and resistor CH2 and releases the operated channel relays. If the coded impulse ceases before registration is completed, relay SPR releases but the operated channel relays are maintained by resistance earth over relay CK2 until relay TO operates. If the impulse continues after registration is completed relay TO remains operated over its secondary winding S until relay SPR releases. Relay CK1 releases when the channel relays are released and disconnects earth from conductor H, thereby releasing relay TR1 which locks the operated register relays and connects the signalling conductors to the next digit register. On receipt of the final digit, relay C1 releases and disconnects conductors J and L to prevent further operation of the channel relays. Relay C1 also disconnects battery from relay TO and by further contacts (not shown), starts the sender. The operation of GK3 by the simultaneous reception of three or more frequencies a-f disconnects earth from conductor K to prevent the register relays from locking. If a start signal ST comprising the frequencies e and f used with the register shown in Fig. 5 is received during registration, a re-order signal is given from earth at the channel relay Rf. The relay SW, Fig, 2, is not required. Alternative registration arrangement. In the register shown in Fig. 5, the steering relays AL, AC . . . NL, NC are operated as before when the register is seized, the translator conductors TP, RP, RM and TM being connected from translator relays TA-TF to, the first digit register relays 1, 2, 4, 5. Relays ACP and ON operate in any suitable manner for calls using voice frequency signals. The operation of relay SPR in response to any frequency connects earth to the primary windings of the channel relays over conductors J and L and contacts of a polarized relay PT. Receipt of the KP signal connects the channel relays to the translating relays TA-TF. When relays CK1, CK2 subsequently respond to a two-frequency signal, relay CK1 connects earth from conductor J to conductor H and relay TG operates, and locks directly to conductor J. The operation of relay CK1 also connects resistor CH2 in series with the primary windings of the channel relays to reduce their sensitivity as a protection against false operation by transients and cross-modulation. Relay CK2 transfers the earth on conductor M to conductor J, which is already earthed over relay SPR. Removal of earth from conductor M releases relay PT, the release being delayed by a condenser PTC connected in series with the winding R. The slow release interval is longer than the time required to operate the register relays since complete registration does not control a locking relay as in the previously described register. Relay PT when released disconnects earth from the primary windings of the channel relays to effect their release in preparation for the next code signal. The translator relays operate according to the frequency code received, two relays being required to operate before earth is connected to any of the translator conductors. The register relays operate on their lower windings and lock over their upper windings, the number of register relays energized varying from 0 to 3, depending on the digit received. Steering wire TS is also earthed when two translator relays operate together, releasing relay AL. When the translator relays release, relay AC releases and connects the translator conductors to the next digit register. The combined operation of translator relays TE and TF by a start signal ST comprising the frequencies e and f operates the register relays 1 and 4 and provides a start signa