GB661866A - Coupling circuit arrangement for coupling a two-wire direct-current telegraph line to a two-channel trunk - Google Patents

Abstract

661,866. Electric telegraphy. STAATS. BEDRIJF DER PASTERIJEN, TELEGRAFIE EN TELEFONIE. Dec. 31, 1947 [Sept. 9, 1946], No. 34973/47. [A Specification was laid open to inspection under Sect. 91 of the Acts, Dec. 31, 1947]. Class 40 (iii). . A two-wire/four-wire telegraph termination is arranged to avoid some disadvantages of the termination described in Specification 661,867. The current in the local line is stabilized at a constant value by a selfbiassed valve in series with the line. Signalling in to the local line is by short-circuiting instead of breaking that line since in the latter case there is a delay before the valve B<SP>2</SP> cuts off, and signalling in the four-wire trunk circuit is by two-tone carrier currents. When a 40 mA. line current is flowing the grid of a control valve B<SP>2</SP> receives a 1v. negative bias with respect to its cathode which is connected to a point of fixed potential on a potentiometer R3, R<SP>4</SP>, R<SP>17</SP> of which R<SP>17</SP> may comprise filament heaters. A decrease in line current increases the negative bias and the increase in potential at the anode resistor R<SP>9</SP> increases the positive bias of a regulator valve B<SP>3</SP>. This therefore draws more current until the stable mA. condition is attained. This current stabilization allows the termination to be connected to local lines of different resistances. Transmission from local line. When the transmitter contacts ZC are opened for a mark the current through resistors R<SP>1</SP>, R<SP>2</SP> ceases and the blocking potential is removed from a rectifier G<SP>3</SP>. The tuning circuit C<SP>1</SP> ... C<SP>3</SP>, R<SP>5</SP> ... R<SP>7</SP> of a feed-back oscillator B<SP>1</SP> is thereby altered by the added resistances R<SP>8</SP> and RI ... R<SP>4</SP> and the oscillator changes from a spacing to a marking frequency in the outgoing trunk 3, 4. Reception from trunk circuit. Mark and space frequencies in the incoming trunk 5, 6 are amplified, limited and applied to the input transformers TR<SP>2</SP> and TR<SP>3</SP> which are tuned respectively to the mark and space frequency. From the secondaries rectified current is applied to the grid and cathode of a shortcircuiting valve B<SP>4</SP> which is thus made non- conductive or conductive respectively. When the valve B<SP>4</SP> conducts it balances out the normal line current to record a mark on the receive relay OB. The regulator valve current is maintained however at 40 mA. and the bias applied by resistors R<SP>1</SP>, R<SP>2</SP> to the rectifier G<SP>3</SP> remains unchanged so that a normal spacing tone is still generated and transmitted. By increasing the voltage of battery V<SP>2</SP> the signalling in the local line becomes of the double current type.