Patent GB-682985-A (GB)
Title: Method of line scanning for automatic telephone systems
IPC: H04M15/08
Filed: 1950-05-09
Abstract: 682,985. Automatic exchange systems. STANDARD TELEPHONES & CABLES, Ltd. May 9, 1950 [June 14, 1949], No. 11520/50. Class 40 (iv). In a common line circuit arrangement for a group of substation lines, the exchange battery is fed to each line circuit in such a manner that a calling condition on a line changes the potential of a point in the power supply circuit of that line and connections from these points are multipled in or more stages to a common lead, one or more stages of electrical current gates being applied to the groups of multipled leads and being controlled by electrical pulses in different time positions so that calling potential conditions on the respective individual lines result in pulses on said common lead each in a time position individual to the corresponding line. As shown in Fig. 1, each line such as LA is terminated in the banks of a line finder SL and the a and b wires are connected via resistors X, Y of 15,000 and 30,000 ohms respectively to earth and battery. For each line there is provided a resistor R1 (30,000 ohms), rectifiers Sa, Qa and a terminal B1. Each terminal such as B1 is connected to a source of potential impulses Pa1 to Pa5 (Fig. 2), the source chosen depending upon the final digit of the line number. The leads from the Sa rectifiers are multipled in groups of 5 to common points C1 and  each such group is provided with rectifiers Sb, Qb and a terminal B2 which is connected to the relevant source of impulses Pb1 to Pb5 (Fig. 2). The duration of each Pb impulse is equal to the duration of one cycle of the different impulses Pa. The leads from the Sb rectifiers are similarly multipled in groups of 5 to common points C2 so that there are four points C2 each multipled to 25 lines and each such point is provided with rectifiers Sc, Qc and a terminal B3. Each terminal B3 is connected to the relevant one of the four impulse sources Pc1 to Pc4 (Fig. 2), each impulse of which is equal in duration to one cycle of the five Pb impulses and the four Sc leads are connected to the common point C3. This point is normally maintained at 40 V. by means of potential divider R3, R4 and is connected to amplifier DA, the output of which is fed to wire F permanently connected to a distributer, and to a device DB which operates and holds relay R while it is receiving impulses characterising one or more lines. Operation. When a line is not in use the point A is at full battery potential, i.e. -48 V., and with point B at - 40 V. the rectifiers Sa, Sb, Sc are non-conductive so that no current flows in the scanning device. The impulse sources Pa, Pb, Pc connect - 40 V. to the terminals B1, B2, B3 normally, but -16 V. during, impulses. In the calling condition (a, b looped) the point A<SP>1</SP> is brought to about -16V. and when the three terminals B1, B2, B3 simultaneously receive an impulse, the path Sa, Sb, Sc is conductive and an impulse passes from A to B during the reception of an impulse Pa on terminal B1. As will appear from Fig. 2 this impulse will be sent during a particular hundredth part of the scanning time for the 100- line group and characterises the calling line. This impulse is repeated during each scanning cycle, is amplified in DA and operates relay R which controls the seizure of a free register to which wire F is extended. The register thus receives the.pulse and from its time position determines the identity of the calling line and causes it to be seized by a line finder. On seizing the line. LA the finder SL applies the - 48 V. battery over resistance R2 to the C wire. The value of R2 is such that the point A is brought to a potential between - 44 V. and - 48 V., i.e. more negative than B, whereupon the transmission of impulses characterising the line LA ceases and the device DB then releases relay R. If several lines in the 100-line group are calling, however, the respective impulses are successively received at B and transmitted over wire F. The register seized responds to the first impulse, but relay R remains held so that another register is seized and so on until all the calling lines have been recorded. Relay R restores when there is no longer a line in the calling condition in the group.