Patent GB-781472-A (GB)
Title: Improvements in or relating to code communication systems
IPC: B61L7/08
Filed: 1954-06-15
Abstract: 781,472. Railway signalling systems. GENERAL RAILWAY SIGNAL CO. June 15, 1954 [July 10, 1953], No. 17537/54. Class 105. [Also in Groups XXXV and XXXVIII] In a direct current code communication system of the shunt type, e.g. described in U.S.A. Specification 2,399,734 and having particular application to centralised railway traffic control systems in which the normally energized line wires between the control station and the field stations are intermittently shunted in a distinctive code pattern according to the particular control or indication signal to be transmitted an electronically controlled power supply energizes the line wires and includes means responsive to the output current to cause the output voltage to vary in the same direction as the output-current and also means for causing the output voltage to decrease rapidly when the output current exceeds a predetermined maximum value as when a shunt is applied across the line near the control office, since the total resistance of the system as seen at the control office may be very low and the current in the line is greatly increased. This sudden decrease in line voltage when the maximum has been attained ensures the dropping away of all the line relays at the field stations, the dropped voltage, however, being high enough to permit detection at the control office of the removal of a shunt at a transmitting field station. The rising voltage with rising current characteristic of the supply facilitates the detection at the control office of the application of a shunt and the removal of a shunt at a field station. For control from the control office relay C, Fig. 1, pulls up and remains operated, thus short-circuiting the power supply over b, c, CF14 (back contacts 14 of relay CF) b, c, E15 and f, c, C16 (front contacts 16 of relay C) and the transmission lines 10, 11 so that the line relays LO at the field stations drop away, indicating to the field stations that the control office is to begin the transmission of a control cycle. Relay E at the control office is alternately picked up and released in accordance with the code to be transmitted and at b, c, E15 opens the shunt across the lines 10, 11 which are then energized. The increase in resistance of the external circuit causes a decrease in current from the supply and the voltage of the electronically controlled power supply is decreased, again decreasing the current. At the end of an impulse when relay E drops away, lines 10, 11 and the supply are short circuited and the output current of the supply increased. The two-position biased polar type line relay 2F at each field station (which has current flowing from left to right in its upper winding in period of rest, and on reversal or interruption of this current the armature moves from righthand position to left-hand position) is, by these current changes in the line 10, 11, successively operated between its opposite conditions. These current changes in the supply output cause corresponding changes in the voltage across a resistor 59, Fig. 3, in the controlled power supply output circuit and these voltage changes are effective through an electronic impulse detector to operate a two position polar type relay F at the control office between its opposite conditions. To prevent a shunt removal from the lines 10, 11 at the end of a control cycle when relays C, E are dropped away an alternate shunt is over b, c, E23 and b, c, OR24, relay OR being normally held operated. At the field stations the relays 2F are effective on decoding apparatus to select the station according to the code received. At the beginning of an indication cycle from a field station relay CF at the control office is picked up in response to the shunting of lines 10, 11 at the transmitting field station, the polarities of the lines 10, 11 being reversed over f, c, CF12 and f, c, CF14 to actuate the relays 2F at all the field stations to pick up the relay LO at the transmitting station to designate that station as the one to transmit an indication code to the control office. Subsequent intermittent operation of relay EO at the field station intermittently applies shunt across the lines 10, 11 during which a relay 2F is dropped away. Each code digit is terminated by picking up of relay EE to remove at b, c, EE21 the shunt across the lines 10, 11 and at f, c, EE21 connect relay 2F across the lines and the relay is energized, so that the relay follows the code transmitted from the station and allows a local stepping operation. These coding operations effect current changes in the line wires in response to the coded message applied to the wires and produce a corresponding voltage change across the resistor 59 in the power supply and is effective through the electronic pulse detector to operate the relay F at the control office between its opposite conditions to effect operation of decoding and indicating apparatus. The electronically-controlled power supply comprises a full-wave rectifying pair of triode tubes 29, 30, Fig. 3, fed from an A.C. supply transformer 25. The automatic negative grid bias for the tubes is obtained from a potential divider formed by a tube 61, resistor 39 and a tapped portion of a potentiometer 38 connected between the mid-point of the secondary winding 27 of transformer 25 by a wire 37 and a wire 49 connected to the mid-point of a low voltage secondary winding 28, tube 61, being normally cut off. There is between the wires 36 and 37 a direct voltage dependent upon the negative bias of tubes 29, 30, wire 36 being positive to wire 37. The polarity of rectifiers 40, 41 is so chosen that wire 49 is of negative polarity to wire 37. The voltage regulator tube 50 connected between wires 37 and 49 tends to maintain the bias at a uniform level. Resistor 59 in the output wire 57 has a low value so that the voltage drop across it due to the line current is small. The resistance of the plate cathode circuit of tube 60 included in series in the other output wire 56 is considerably greater than that of resistor 59 and is raised by varying the grid bias to alter the output voltage of the power supply across wires 56, 57. Tubes 61, 62, 63 respond to variations in load across the line wires, to variations in the input voltage and to the current supplied to the load and these tubes control the grid bias of tubes 29, 30 and the grid bias of the tube 60 to produce the aforesaid characteristics to the power supply, adjustments being so made that the output voltage applied to wires 56 and 57 regulates the level of conduction of tube 63 which controls grid bias of tube 60 and thus the voltage drop across this tube and hence the output voltage of the power supply. By this means variations in output voltage due to variations of the input voltage to transformer 25 are compensated. The voltage drop across resistor 59 tends to make the control grid of tube 63 more negative with respect to its cathode so that the greater the output current, the less is the plate current of tube 63 resulting in reduced voltage across resistor 67 thereby causing an increased voltage on the grid of tube 60 with a lower voltage drop across this tube and an increased output voltage on wires 56, 57. The amount by which grid voltage on tube 60 is increased with increasing output current of the power supply depends on the magnitude of the resistor 59 which thus determines the amount of output voltage rise which must be proportionally less, for increase in output current. Tubes 61, 62 cause the line voltage to be reduced when the line current exceeds a predetermined maximum value. The grid-cathode voltage of these tubes is made up in part by the potential across the resistor 59 and the said voltage is so adjusted at the maximum load current that the tubes are non- conductive ; but an increase in the grid voltage by the flow of the increased load current through the resistor 59 will render them conductive, with a voltage drop across resistance 67 resulting in a reduction in the voltage applied to wires 56, 57. At the same time increased conduction of tube 61 varies the grid bias of tubes 29, 30 resulting in a drop in the output voltage between wires 36, 37 reflecting a further reduction in the voltage across wires 56, 57, resulting in a variation of output voltage with output current as shown in Fig. 4. The changes in voltage across resistor 59 are amplified in a push-pull amplifier including the triodes 86, 88, the output being fed to a flip-flop device including gas discharge devices 102, 104 which alternately actuates the relay F between its two opposite positions. On application of a shunt across the wires 56, 57 at a field station, the voltage drop across resistor 59 is increased, thereby increasing the grid bias of tube 86 resulting in a decrease in positive voltage applied to the control grid of tube 96, and a voltage decrease is applied to the grid of tube 88 resulting in increased voltage applied to the grid of tube 100. The plate voltage of tube 96 is increased and that of tube 100 is reduced, this latter having no effect on the then non- conducting gas discharge tube 102, but the former on gas discharge tube 104 causes this tube to fire, resulting in current through the upper winding of relay F moving the armature to the left. On removal of the shunt by the field station, the voltage drop across resistor 59 is reduced, resulting in tube 104 being rendered non-conductive and tube 102 being rendered conductive with a flow of current through the lower winding of relay F. There is also a momentary increase of current through the plate cathode circuit of tube 102, through capacitor 108, resistor 105 and the upper winding or relay F to ground, this current added to the existing flow through the tube 104 increases the voltage at the cathode of tube 104 beyond the extinguishing point of the tube, and the upper winding of relay F is de-energized and the armature moves into its right-h