Patent GB-625893-A (GB)
Title: Improvements relating to teleprinter exchanges
IPC: H04L12/00
Filed: 1945-12-19
Abstract: 625,893. Telegraph exchange systems. JENNINGS, H. W. K. (Telefon Fabrik Automatic Aktieselskab). Dec. 19, 1945, No. 34457. [Class 40 (iii)] A telecommunications exchange system of the type in which when an outgoing call is made the calling subscriber's equipment transmits a calling potential which causes the called subscriber's equipment to send back a response impulse of similar polarity, is characterised by the fact that a privilege subscriber has additional equipment responsive to the response impulse to transmit a control impulse of opposite polarity which only if the called subscriber is also a privilege subscriber gives rise to a reverted control impulse of said opposite polarity whereby the operative connection of the two subscribers' instruments is effected. In Fig. 5 a subscriber (A) is connected via a translator device (not shown) to the points A1, A2 of the privilege equipment MA and a subscriber (B) is similarly connected at B1, B2. When a call is initiated by subscriber A, relays TA and ZA operate in known manner. Point A1 is connected to the send leg a over za, xa1, ta and the subscriber A selects his required connection over switches represented by 1W, 2W, 3W. When the desired connection is made to privilege equipment MB, relay TB is operated over the test leg c and a positive calling potential is applied at point A1, over the send leg a, to operate relay UB. A positive response potential is applied to the return leg b over zb, yb2, xb1 normal and tb1 up. Relay UA therefore operates over tal made and at ua1, ua2 energizes relays XA, YA respectively, the latter being slow to operate. Contact xa1 connects a momentary negative control potential over the send-leg a to operate relay MRB, and contact ya2 restores the positive potential to relay UB before it has had time to release. At mrb1 subscriber B's local equipment is connected to line, and relay ZB is operated via a translation device (not shown). Relays XB, YB operate over zb1, ub1 or ub2 and a momentary negative control potential is applied over line b to operate relay MRA which at mra1 connects the local equipment of subscriber A to line. The desired operative connection is now complete and at the end of a communication is broken down in known manner, all relays releasing. The subscriber B may be replaced by a group of privilege subscribers reached through a selector AVB. If a privilege subscriber B is called by a common subscriber A, the response impulse does not give rise to any control impulse and no connection of the subscribers' local equipment takes place. Similarly, if a privilege subscriber A calls a common subscriber B, the first control potential does not give rise to a reverted control potential at B and the local equipments are not interconnected. The privilege equipment MA or MB may be additionally provided with delay means which breaks down the initial connection after a predetermined time if no control potential is received. The control impulse may consist of a train of impulses of one or two polarities. If a privilege subscriber A is to be allowed to communicate only with selected " special " privilege subscribers, an auxiliary selector is added to his privilege equipment and is responsive to the dialled impulses from subscriber A. Unless the selector is responding to the digits representing a " special " subscriber, it makes no connection to the distant exchange. A key may be provided so that a privilege subscriber can cut out his privilege equipment and receive calls from common subscribers.