Patent GB-815632-A (GB)
Title: Improvements in or relating to communication systems
IPC: H04Q3/00
Filed: 1956-10-26
Abstract: 815,632. Automatic exchange systems. WESTERN ELECTRIC CO. Inc. Oct. 26, 1956 [Nov. 3, 1955], No. 32704/56. Class 40 (4). In an arrangement for scanning two-conductor lines the voltage extended to the scanning terminal of a line is a function of the algebraic difference of the voltage changes of the line conductors of that line. As shown, the right-hand end of resistor 14 extends to a combining circuit comprising resistors 23, 24 and thence through a transistor amplifier 25 to the scanning conductor 28 which is connected to the scanning apparatus (not shown) which may be of the type described in U.S.A. Specification 2,679,551. The righthand end of resistor 15 is connected through resistor 20 to the base of transistor 32 which is also connected via variable resistor 22 to a source of positive voltage. The values of 20 and 22 are chosen so that the collector-emitter current of transistor 32 causes a sufficient voltage drop across resistor 27 to bring point 30 to earth potential when the line is open. In this condition, therefore, the upper end of resistor 23 and the lower end of resistor 24 are both at earth potential, so that terminal 31 is also at earth potential, which potential is repeated by transistor 25 to the output conductor 28. When the line loop is closed the right-hand end of resistor 14 becomes negative with respect to earth potential and that of resistor 15 less negative than the voltage of the battery. The resulting increase in the collectoremitter current of transistor 32 causes the point 30 to become more negative. The feedback resistor 21 renders the operation of the circuit substantially independent of the characteristics of the transistor 32, and the resistors 20, 21, 22 and 27 are of such value that the change in voltage of point 30 is substantially equal to that at the right-hand end of resistor 15 but in the opposite direction. With the upper end of resistor 23 and the lower end of resistor 24 both becoming more negative, the point 31 also becomes more negative, and its voltage is repeated to the output lead 28. If the tip conductor 10 becomes connected to earth (as at 8) sufficient potential change will appear across resistor 15 due to the opening and closing of the line loop to apply a sufficient voltage change to the conductor 28 to enable the changes in line condition to be recognised by the scanning apparatus. Compensating for differences in earth potential.-If the leakage resistors 8 and 9 are present and extend to earth potential different from that at the exchange, then the current components due solely to this difference in earth potential will flow in the same direction in resistors 14 and 15 and will be of approximately the same magnitude. Consequently equal and opposite voltages will be applied to the upper end of resistor 23 and the lower end of resistor 24 and these will produce no change in the voltage of the output conductor 28. In like manner, if the earth potential difference between the earth at resistors 8, 9 and that at the exchange comprises alternating voltages, these will produce no effect on the output conductor 28.