Patent GB-1102500-A (GB)
Title: Improvements in or relating to automatic telephone exchanges
Inventor: BARNABY BERNARD SYDNEY
IPC: H04Q11/04
Filed: 1965-07-06
Abstract: 1,102,500. Automatic exchange systems. GENERAL ELECTRIC CO. Ltd. 6 July, 1966 [6 July, 1965], No. 28583/65. Heading H4K. Each line circuit of a time division multiplex exchange has a ferromagnetic core 7 and a channel store 6, coincidence between a calling condition from the subscriber's line and a free time slot, as signalled from a free channel store 14, causing the identity of the time slot to be registered in the channel store 6 to control the multiplex gate 4 to the highway 1. The exchange serves up to 100 subscribers and provides an over-ride busy facility on actuation of a special key at the calling substation. The free channel store 14 records those out of the twelve time slots available in the system that are free and pulses a free time slot highway 15 in the corresponding positions in the exchange cycle. The subscriber's line circuit is shown in greater detail in Fig. 2 where the winding 46 on core 7 is supplied with a bias current from the free channel store as well as with oppositely acting pulses in the positions of the free time slots. When the line 3 is looped by a calling subscriber potential across resistor 42 causes transistors 41 and 44 to conduct and applies a bias to winding 45 of core 7 to cancel the bias from the free channel store on winding 46. Relieved of effective bias the core 7 switches to the first occurring free time slot pulse on winding 46 and stays set in that state. Switching of the core produces an output in winding 47 to cut-off conduction in transistor 52 and register the free time slot (in a manner described in Specification 1,102,499) in the subscriber's channel store 6. At the same i time, transistors 54 and 55 conduct for the duration of the time slot, conduction of 55 opening the symmetrical transistor 38 to gate the line to highway 1. In the same time slot of the next exchange cycle store 6 produces a further interruption in the conduction of transistor 52 to operate the highway gate 38, and so on for each succeeding cycle. If store 6 is of a sort that reads out destructively, conduction of transistor 54 effects automatic rewriting. As shown in Fig. 1, the time slot pulses produced by the subscriber's channel store 6 are also applied to a gate 19 which, primed by the off-hook condition as relayed by a so-called holding circuit 17, passes the pulses to a busy channel store 22. Pulses produced by store 22 in the seized time slot are applied to gate 23 together with the time slot pulses current in free channel store 14. When a time slot has just been seized, coincidence at gate 23 causes erasure of the time slot from store 14 and, by way of a control circuit 24, causes the time slot to be registered in a store 12 to open a highway gate 11 in synchronism with the line circuit and thereby give access to a common register 9 and signal such access by starting a dial tone generator 25. Substations have tone senders and the two digits identifying a wanted subscriber are detected at 26 in the register 9 and set up in store 28. A register control circuit 29 counts the digits received and after the second digit gates the time slot allotted to the connection to the store 28. The digits set up in store 28 connect up that one of a hundred outlets which is connected to a winding 48, Fig. 2, of the core 7 of the wanted subscriber; the pulse from control circuit 29 causing the core 7 to switch in the time slot of the calling connection. Output from winding 47 of the core causes the called subscriber's channel store 6 to register and produce a pulse in the time slot of the calling connection. Coincidence between the production of time slot pulses and the on-hook condition presented from the called subscriber's holding circuit 17 causes gate 32 to open and ringing gate 34 to be pulsed so as to connect ringing tone generator 35 to the highway 1. The pulses that connect ringing also signal the control circuit 24 that the register is no longer required, the time slot of the connection being erased from the store 12. Reply by the called subscriber removes the on-hook condition to gate 32 and consequently trips ringing. On release of the connection by a subscriber, transistors 41, 44, cut-off and a capacitor 56 discharges through reset winding 49 to restore the core to its idle condition. Should a called subscriber be busy, busy tone is connected but no description of this is given. A calling subscriber can break into a connection, over-riding the busy condition, by actuating a special substation key. This key signals the register to reset all subscriber cores 7 which it does by increasing the bias through windings 46. All subscribers engaged in a connection, including the wanted busy subscriber, have their cores 7 switched but the core of the wanted busy subscriber is switched back from the register by store 28 and winding 48 in the time slot of the intruding caller and a pulse in the position of this time slot is consequently injected into the channel store 6 along with the pulse in the time slot of the existing connection. The register 9 is released by a timing circuit and the increased bias to the cores 7 is gradually restored to normal so that successive free channel pulses step the cores of busy subscribers back to their set states, each step being so small as to produce no disturbance to the channel stores 6. The technique employed in breaking into a connection by switching all cores 7 to their idle condition may be employed in the normal establishment of a connection, the register 9 resetting all cores 7 between the receipt of the first and second digits of a wanted number. By the time the register connects the core winding 48 of the wanted subscriber all cores of busy subscribers will have been restored to the set condition. This ensures that the cores of idle subscribers are in the corresponding reset state.