Patent GB-315482-A (GB)
Title: Improvements in or relating to automatic and semi-automatic telephone systems
IPC: H04Q3/00
Filed: 1928-04-18
Abstract: 315,482. Standard Telephones & Cables, Ltd., Wright, E. P. G., Baker, J. H. E., and Cameron, A. H. April 18, 1928. Automatic exchange systems. - If, in establishing a call, a group selector or the final selector encounters a busy condition, a busy signal is transmitted from the group selector and the transmission is initiated bv the movement of the selector encountering the busy condition to a special position in which the current flowing over the a- and b-wires is reversed. If the wanted line is idle, the opening of the loop at the final selector initiates the transmission of ringing- current from the group selector. The invention is described with reference to a system in which the group and final selectors are of the singlemotion step-by-step type provided with two sets of wipers and controlled by common marking switches. Selector-repeater, Fig. 1. Relay K energizing over the incoming test wire T associates the selector with the common control circuit, and relay BB operating in series with K marks the control circuit busy by opening the shunt circuit to battery over wiper SC3 of the marking switch. Relay A connects relay AA in series with the lower winding of relay I and the left-hand winding of a shunt-field relav U, and incoming impulses are repeated by relays A, AA to magnet SCDM. As soon as the marking switch moves from normal, magnet SDM is energized over wiper SC1 but the main switch is prevented from getting ahead of the marking switch by relav DD which operates over wipers CA (or CB) and SC2. At the end of the digit the relapse of relay CC renders the circuit of magnet SDM independent of relay DD, and connects up the test relays HA, HB to wiper SC3. When an idle line in the marked group is found, relay HA (or HB) operates, locks in series with magnet SDM, disconnects relays K, BB, and connects a bridge through relays I, U and resistance R across the outgoing conductors. A restoring circuit for the marking switch is closed over off-normal bank SC1. All lines in wanted group busy; statistical metering.-If no idle line is found, the mainswitch wipers pass to the end of the bank at which relay DD operates over a directly-grounded terminal, and connects up, over wiper SC3, a test relay HA or HB. The latter energizes in series with relay II which operates a statistical meter over wiper SC4. The last terminals in the banks of the talking wipers have battery and earth connected in reversed sense so that relay U operates and connects up a busy tone over relay A and a holding circuit for relay B. Final selector Fig. 2. Relays KK BBB, operate over the incoming test lead and extend the loop to relay CCC and magnet TCDM which however, do not operate on account of resistance R. During the repetition of impulses, resistance R is intermittently short-circuited by relay A so that magnet TCDM responds to the impulses and relay CCC remains operated for the duration of each digit. At the end of the tens digit, twostep relay EEE, controlled by relay CCC, switches over the loop to the magnet UCDM of the units-control switch, energizes relay III, and completes a circuit for the main-switch magnet CDM. When the marked group is reached, relay DDD operates over wipers C, TC1, energizes relay FF, and cuts the drive. At the, end of the units impulses, a second two-step relay GG completes a new circuit for magnet CDM. When the wanted line is reached, relay HH operates over wipers C, UCI, and connects test relay H in series with relay III. Busy test, If the wanted line is busy relay H does not operate, relays III, HH, release and the main switch moves on to the end of its bank at which relay HH operates again, and relay H operates over contacts KK5, PG3. The talking wipers encounter reversed -battery connections so that relay U, Fig. 1, operates and gives a busy tone. Relay H releases relays KK, BBB, whereby restoring circuits are closed for magnets TCDM, UCDM. Ringing. If the wanted line is idle relay H operates and extends the incoming loop to the talking wipers which, however, do not find battery feed. Relay I, Fig. 1, therefore releases and connects up ringing current over relay F When the called subscriber replies, relay F and back-bridge relay D respond. Release. When relays A, B, release, relay C operates to maintain the selector busy until fully restored. The release of the test relay completes a restoring circuit for magnet SDM, At the final selector the removal of earth from the test lead releases relay H. Permanent loop. If no impulses are received by the group selector, Fig. 1, a time-pulse lead TP successively operates relays EE, HA, and a supervisory alarm connected to the normal terminal of wiper A is operated over the loop including relays I, U. In the final selector, Fig. 2, lead TP successively operates relays TA. PG, and if the units digit has not been received by this time, the main switch moves to its penultimate contacts at which relay H operates and releases the control circuit. The release of relay KK connects an alarm lamp to the grounded incoming test lead T.