GB902172A - Electrical identification system - Google Patents

Abstract

902,172. Automatic exchange systems. STANDARD TELEPHONES & CABLES Ltd. Dec. 19, 1958 [Dec. 20, 1957], No. 41153/58. Class 40 (4). In a line identification system in which a plurality of line terminals are cross-connected by condensers to combinations of a plurality of identification terminals, the line terminal to be identified being energized by A.C., the identification terminals are connected to amplifierdetectors having an input impedance low compared with that of the condensers. It is stated that by this means so much power can be obtained at the identification terminal that only a single stage transistor amplifier is required for operating a relay in the amplifier-detector whilst at the same time substantially zero power is fed to the identification terminals not connected directly by a condenser to the energized line terminal. The invention is described with respect to a system having m<SP>p</SP> (=10<SP>3</SP>) line terminals and mp (=3x10) identification terminals each line terminal, which is fed with 20 kc/s. current when identification is required, being connected by a 0.02 ÁF condenser to one terminal in each of the three groups of ten. The line terminals are required to have substantially infinite impedance to earth and the identification terminals find earth through the input impedance of the amplifier detector which is of the order of 4 #. Fig. 3 illustrates a detector in which each point B corresponds to a different value digit, the digits being registered in a 2-out-of-5 code on relays such as Sar, Sbr. The incoming signal feeds a 14: 1 step-up transformer having a 1 K# shunt on its secondary side, equivalent to about 5# on the primary side. Rectifiers W operate in push-pull to feed transistors OC73, OC<SP>1</SP>73 whose emitter-collector currents operate relays Sar Sbr. The 2k# base resistors also in shunt with the secondary transformer winding reduce the input impedance to about 4#. The collectors of the transistors are fed via normally closed contacts of a relay Hr, which relay forms part of a potential divider. In the event of the earth side of the potential divider becoming disconnected Hr releases, thereby preventing the full source voltage from being applied to the transistors. Specifications 683,794 and 810,168 are referred to.