696,503. Automatic exchange systems. TELEFONAKTIEBOLAGET L. M. ERICSSON. Jan. 12, 1951 [Jan. 12, 1950], No. 948/51. Class 40 (iv). Signal sending relays at one end of an order line common to a group of speaking channels mark a signal-sending speaking channel and send a V.F. signal characteristic of the speaking channel to signal receiving relays at the other end of the order line to energize a relay associated with the receiving end of the speaking channel. Digits dialled by a subscriber A are set up in a register REG, Fig. 2, the last digit operating RG5 which energizes RG6. Selector GVI is set in known manner over wires a, b, after which RG6 restores. Apparatus RG then connects voice frequency combinations from generators G1-G4 to wires a, b, and each time RG6 releases a new combination is selected by the register for operating the next selecting stage. Group selector GVI selects a junction circuit and line Fa, Fb, relays FR1-FR3, Fig. 3, energizing in known manner and operating the related relay HR1 in a group HR1-HRn corresponding to the several junction lines connecting exchanges I and II. HR1 cuts out the other relays in the group and shunts its own lower coil to energize FR4. Voice-frequency transmission. Relays GR1- GR6 are used in common by the group of junction lines and cause transmission of voicefrequency combinations over an auxiliary junction line Ha-Hb, Figs. 4, 5, which is provided with a differential transformer at both terminations DT1, DT2 so that signals can be sent simultaneously in both directions. In the example shown, relay HRI energizes GR6 to connect generator G6 over wires h3, h4 to terminations DT1, DT2, amplifier F2, Fig. 5, actuating the corresponding tuned signal receiver M26 so that MR26 energizes the corresponding junction line circuit, Fig. 6, at exchange II, by operating FR11, FR13 to connect signal receivers M6, M1-M4 to transformer FT2 associated with the junction line Fa, Fb from exchange I. FR11, FR13 also energize FR15, MR6, the latter energizing FR12 and causing generator G7 to return a voicefrequency signal over wires g2, g3 and auxiliary line Ha, Hb to amplifier F1, Fig. 4, to actuate signal receiver M7 so that relay MR7 closes circuit through the upper winding of FR3 to energize RG6, Fig. 2. At the same time, V.F. signals pass from the selected generators G1- G4, Fig. 2, over wires a, b, a1, b1, Fa, Fb, amplifier Fo to receivers M1-M4, so that one or two of relays MR1-MR4 energize to signal group selector GVII over wires a2, b2. Selector GVII selects a junction line to exchange III over a circuit similar to that of Figs. 3-6 and releases MR6, Fig. 6. M7 and RG6 also restore, the latter causing the register, Fig. 2, to change its tone combination for the next selecting stage. Release of MR6 releases FR11 and the new tone combination passes over wires a2, b2 to group selector GVIII. The switching process is repeated for the new stage but as the group selector GVIII is in the terminating exchange, the operated receivers M1-M4 energize relay MR5, Fig. 6, in exchange III in addition to selecting a line finder LV, the latter releasing MR6. RG6 in the register again releases to change the tone combination, the line-finder is connected, MR6 in exchange III reoperates, and generator G7 again returns a signal to reenergize RG6, this time to prepare disconnection. When the line-finder extends the call to the called subscriber, MR6, Fig. 6, and RG6 restore to release NR4, Fig. 2, to dismiss the register and release the auxiliary lines Ha, Hb. When subscriber B answers, FR16 (exchange. III), Fig. 6, is energized by an impulse on wire d2 and energizes FR17 and KR1 before falling back, KR1 being one of a chain KR1-KRn relating to the various lines in the group. In a manner similar to the original signalling over the auxiliary line, selected generators in a group G11-G16, Fig. 6, signal over the second auxiliary line and MR relays, Fig. 4, to operate FR5 (exchange II), Fig. 3, and then similarly over the first auxiliary line to operate FR5 in exchange I. FR5 energizes FR6 and then falls back at the end of the answering signal to operate FR7 and both windings of the current feeding relay NR3, Fig. 2. When subscriber B hangs up, an impulse on wire d2, Fig. 6, clears down by signalling over the auxiliary lines in the manner just described so that FR5 reoperates to restore FR6, NR3, the end of the signalling restoring FR5, FR7. When subscriber A hangs up, NR1, NR2, Fig. 2, and FR1, Fig. 3, fall back and FR3 energizes HR1 and FR4 to signal again over the auxiliary line to cause clear down in the various switching stages. Modification. Figs. 7-10 illustrate a variation of Figs. 3-6, in which a 4-wire junction line is used. When the junction line in exchange II is seized, contacts 640, 650, 660, 670, Fig. 10, determine whether exchange II is the terminating exchange or not. In the former case MR3, MR4 operate together to energize MR5 and FR10. Operation then of MR6 causes generator G7 to return a signal to operate MR7, Fig. 8, and the switching takes place in the manner previously described. If, on the other hand, MR5 does not energize, a junction line to the next exchange is connected. Generators G7, G8 return signals over the auxiliary line to energize MR7, MR8, Fig. 8. Connection of the junction line releases MR6, MR7 but MR8 is slow releasing so that FR8 pulls up, releasing the auxiliary line and energizing a starting device SS, Fig. 7, whereupon a number of line finders VS hunt for the calling junction circuit, the successful finder energizing its relay RU in series with FR9. Voice-frequency signals from the register pass over wires a1, b1, Al, B1 and the selected junction to exchange III, generator G6 returning a signal over wires A2, B2 to receiver MU, so that relay UR, Fig. 7, repeats the signal to RG6 to cause change of tone combination.