Patent GB-355788-A (GB)
Title: Improvements in telegraph systems
IPC: H04L12/04
Filed: 1930-05-21
Abstract: 355,788. Telegraph exchange systems. CREED & CO., Ltd. and GRIFFITH, R. G., Telegraph Works, Croydon. May 21, 1930, No. 15622. [Class 40 (iii).] In a telegraph system in which calling and called subscribers are connected through a central exchange and telegraphic communication is effected by breaks in current in a circuit supplied with signalling current from a source of energy at the central exchange, signals are automatically transmitted from the exchange to the calling subscriber to notify the progress of a call consequent upon operations in effecting a connection. The return signals may be simultaneously printed on typeprinting apparatus located at both the calling and exchange stations, and the number required by the calling subscriber may be typed on exchange apparatus, or may be displayed on a lamp screen at the exchange in accord with the number dialled by the calling subscriber. A subscriber effects a calling signal at the exchange by earthing the line circuit and a clearing signal by breaking the line circuit for a predetermined minimum interval of time. Calling the exchange. On depressing a calling key CFK, a relay A at the exchange is operated and lights a calling lamp CL. The operator inserts an answering plug AP of a cord circuit into a jack AJ, energizing a cut-off relay B and a relay C in series. The relay A is now de-energized and the calling lamp CL is extinguished. An impulsing relay D is connected to the calling subscriber whose motor is started, maintaining energized the relay D which operates a slow-release relay E. A relay G1, operated by the relay E, prevents the resetting of the selector arms NS1 .. NS4. Dialling impulses transmitted by the subscriber effect intermittent de-energization of the relay D, and over contacts E4, E5, impulses pass through a relay F to a digit selector arm W2 and a rotary magnet ROT2, which is stepped according to the first digit of the required number. At the end of the first series of impulses, the relay D becomes energized, breaking the circuit of the relay F which on releasing causes a rotary magnet ROT1, to step the arm W2 of the digit selector DS on to the second contact. At the end of the number transmission, the arm W3 of the selector DS is stepped to its fifth contact, and completes a circuit through the arms W4, stepped according to the number required, and the lamps of a display panel. On inserting the calling plug CP into a calling jack MJ, Fig. 2, of the called subscriber, a supervisory lamp SL is lighted after relay H and the calling-jack out off relay I of the called subscriber have operated in series, and a circuit is completed through the called subscriber's teleprinter. Relays L, M when operated extinguish the supervisory lamp SL and release the relays C, D from the cord circuit. Relay E is de-energized and the selector DS is stepped to its neutral position by the rotary magnet ROT1. Similarly, through the released contacts of relays G1, G2 .., the remaining selectors NS1 .. NS4 are stepped round to their initial positions. Through indication. If the called subscriber is not engaged a circuit is completed through contacts L6, L7, N7, N6, V5, V4 to the winding of a release magnet P. A rotatable member CAM1, when released by P, sends break-current impulses from the contacts Q1, Q2, which cause the word " THRO " to be printed by the calling subscriber's teleprinter. On the completion of the transmission, the contacts Q3, Q5 are closed for,a short time and energize the relay N which releases the " THRO " signalling apparatus from the cord circuit and connects the called and calling subscribers. At this stage the calling subscriber may depress a key or transmit a signal so that the called subscriber sends back a characteristic signal, such as his name or number, in combination with his exchange indication. For this purpose the means described in Specification 351,018 may be incorporated. At the end of a telegraphic transmission the line is cleared by the operation of a key CK, or the automatic actuation of a key ACK, operated after an idle period of, say, two minutes by a device which may be of the type shown in Specification 284,740, [Class 40 (iii), Telegraphs, Electric]. Line engaged. When an answering plug or calling plug is inserted into the answering jack or calling jack of an engaged line, the signalling battery potential is placed on the ring connection and relays R, S are operated in sequence to energize the release magnet T of a " BUSY " automatic signalling unit. The impulses sent to the calling subscriber through contacts U1, U2 record the word " BUSY " on his teleprinter. If it is impossible to connect to the wanted subscriber on account of reasons other than line engagement, a key EK will cause the word "BUSY" to be typed on the calling subscriber's apparatus. Should the calling subscriber require the supervising operator or a trunk connection he dials the numbers 1111 or 2222 respectively, and the circuits through the wipers W2 or W3 energize a relay V preventing the transmission of the word " THRO " when the call has been extended to either of the above numbers, as desired. In an alternative cord circuit, Fig. 4 (not shown), used in connection with the tie circuits, Figs. 5 and 6 (not shown), the trunk exchange (and the number required are typed by the home exchange teleprinter, and a signal is transmitted to the calling subscriber when connection to the distant or trunk exchange has been established. In addition the words " THRO " or " BUSY " are transmitted to the calling subscriber from the distant exchange according to the condition of the called subscriber's line.