Patent GB-778025-A (GB)
Title: Improvements in or relating to identifying devices for automatic telephone exchanges
IPC: H04M15/08
Filed: 1955-09-16
Abstract: 778,025. Automatic exchange systems. TELEFONAKTIEBOLAGET L. M. ERICSSON. Sept. 16, 1955 [Sept. 17, 1954], No. 26657/55. Class 40 (4). A circuit to identify a calling subscriber includes first and second groups of gas discharge tubes and contacts associated with each subscriber's calling circuit which make in response to a call and connect and fire a tube of the first group in series with a tube of the second group, the combination of tubes being characteristic of the calling subscriber, each discharge tube having an ignition electrode and means being provided to prevent more than one tube of the first group from conducting across its ignition gap at any given time. Each line relay LR has a single contact, such as 11, which when made connects the cathode of a first group tube AG1 to the anode of a second group tube BG1, the AG tubes marking the rows and the BG tubes marking the columns of a matrix in which the relay contacts are elements. The anodes of the BG tubes are held at earth potential when idle by resistors R0, R1, connected to a negative supply, and a common resistor R27 connected to a positive supply and to the BG anodes by the forward resistance of rectifiers L0, L1. The ignition electrodes of the AG tubes are connected via resistor R21, to a voltage divider including resistor R22 and transistor T and have a potential sufficient to fire an AG tube across the ignition gap when the cathode is connected to a BG anode. As the voltage across the ignition gap is constant and independent of current any increase in the potential of the anode of BG1 due to increased conduction through R1 will increase the potential of the ignition electrodes of all AG tubes and increase the possibility of a second AG tube breaking down. If, however, the current through R1 in the idle condition is greater than the current in the ignition path of tube AG1 the potential across R1 will remain constant by virtue of rectifier L1 except for a change across R27 which, as it is common to R0, R1, will be small. During the ignition interval, therefore, the circuit is guarded against intrusion. When the tube AG1 strikes across its main electrodes, conduction through R1 increases beyond the idle condition to block rectifier L1 and increase the potential of the anode of BG1 which strikes to its ignition electrode to transmit conduction between the main electrodes. To prevent firing of a further AG tube once the rectifier L1 is blocked, conduction current between the main electrodes of an AG tube raises the potential on the emitter of a transistor T by virtue of resistor R23 and thereby increases the collector resistance which, as it is in the potential divider supplying the AG ignition electrodes, drops their potential below break-down. This voltage is further reduced by varistors R25, R26, which are in parallel with R23 and form a potential divider with R24 for the transistor base. In this condition of the circuit relays A1 and B1 in the main discharge paths of tube AG1 and BG1 are pulled up and a positive potential is fed to digit relays of a marker over contacts a1, b1 and a break contact of a relay K. The digit relays determine the units and tens of the subscriber and establish circuits for selection of a free connection path to the subscriber, a free register, and a free link. As soon as connection is made with a register, cut-off relay BR attracts, contact 32 of which breaks the series connection of tubes AG1 and BG1. Tube AG1 is extinguished but tube BG1 continues conducting over rectifier L1 and resistor R27, the potential on the anodes of tubes BG falling considerably and producing a negative impulse which is inverted and amplified in a transformer TR to ignite and fire a tube CG. The main gap of tube CG conducts over a contact d of the digit relays and over the winding of relay K to apply a positive impulse, via a capacitor and rectifiers L10, L11, to the cathodes of tubes BG which are thereby extinguished. Energization of relay K releases the marker and digit relays, contact d extinguishing tube CG to leave the identification circuit idle. Resistors R21 and R28, common to the leads connecting the ignition electrodes of the AG and BG tubes, prevent the firing of more than one tube should more than one contact such as 11 be closed at once.