Patent GB-350900-A (GB)
Title: Improvements in or relating to long distance working in telephone systems
IPC: H04Q3/42
Filed: 1930-03-10
Abstract: 350,900. Semi - automatic exchange systems. STANDARD TELEPHONES & CABLES, Ltd. and SKILLMAN, T. S., Columbia House, Aldwych, London. March 10, 1930, No. 7724. [Class 40 (iv).] The receiving equipment for voice-frequency alternating-current signals transmitted over a longdistance junction L2, Fig. 1, from a toll-operator's keyset KS1 is associated at the distant exchange with common apparatus such as a register-controller R2. The same register-controllers R2 are also used for setting up connections over switches AS3 back to the calling party in the case of toll calls in the opposite direction over a junction L1, and attenuating pads AT may be inserted to equalize the levels of voice-frequency currents received by the register-controllers. Supervisory signals may pass between apparatus S individual to the junctions and may be in the form of code-combinations of voice-frequency A.C., long and short pulses of voice-frequency A.C., long and short pulses of low-frequency A.C., direct-current loop signals with long and short interruptions, or direct-current leg signals. The digit-signals transmitted from the keyboard KS1 may comprise varying numbers of impulses of a single voicefrequency or code-combinations of a number of frequencies. The groups of junctions L1, L2, may be combined for two-way working in the manner described in Specification 348,545. Detailed operation, Figs. 3-6. In this embodiment, code-combinations of four voice-frequencies W .. Z are used for all supervisory signals as well as for numerical selection. When the toll operator inserts plug PG2 into junction jack J2, relays S2, OJA operate and lamp LP lights. Relays OZ, OQ, OR, OAG, operate and relay OU transmits a calling pulse of frequencies XY over the junction, the relapse of relay OR terminating the pulse. At the distant exchange, relays B, C, F, G, Fig. 5, operate releasing relay L which locks up relay Y. At the end of the pulse, relay AD operates completing a circuit over conductors 9, 11, for the starting relay Fs of finders PF associated with link circuits, Fig. 6. When the calling junction is found, relay Rl shunts relay Fs. An idle register-controller R2 is then associated over a finder switch with the link circuit whereupon a ground connection over wiper rb causes the operation of relay KPR which by releasing relay EJ, causes the operation of relays BE, P, Q, S, U, and the transmission of a pulse XY back over the junction. Relay OL responds, energizing OY which by disconnecting the low-resistance winding of OJA causes relay S2 to release and lamp LP to go out. The toll operator having thrown key RK2 keys the wanted number, the depression of each key 1 .. 0 causing the transmission of a single pulse of a particular frequency-combination, each pulse being of constant length determined by the release time of a normallyenergized relay PJ. The pulses pass directly over the junction and associated apparatus to receiving relays Sa .. Sd, Da .. Dd, in the register-controller. When the complete number has been received, relay Rs operates, and the circuit from conductor 9 is transferred from wiper d to wiper c, so that relay To operates releasing KPR and causing the transmission back of a pulse YZ which releases relays OK, OY and lights lamp LP. This is immediately followed by the extinguishment of the lamp owing to operation of relay AC which releases EJ again. When selection is completed relay Rs releases and relay St operates energizing EJ which sends back a " light " signal to lamp LP. If the wanted line is busy relay Rs is operated intermittently to flash lamp LP and the operator may break in in known manner by the repeated transmission of ringing-current. If the wanted line is free, lamp LP lights steadily and the operator rings by pressing key RI which impresses frequencies YZ on the junction. Relay N, Fig. 5, releases in response and operates EA which connects up 20-cycle ringing current to the wanted line. When the subscriber replies, relay Rs operates over the called loop, relays St, EJ release, and lamp LP is extinguished. When the called subscriber hangs up relay Rs releases, and relays St, EJ, operate to light lamp LP. On the removal of the plug PG2, relays OJA, OZ, OQ, release and relay OT sends a pulse WY over the junction releasing relay K which unlocks Y, Fig. 5. Relays OJ, J, which are always operated when signals are being received at the corresponding end of the junction, maintain relays OQ, Q, respectively, in their existing (operated cr unoperated) condition in order to prevent the transmission of a signal in the opposite direction at the same time. The extension of the call back to the calling subscriber over jack J1 takes place in similar manner under the control of a registercontroller identical with R2.