Thus, to ensure it is safe to cross, particularly where the line of sight along the track is restricted, there is a need to warn prospective users if a train is imminent and thereby assure the user that it is safe to cross in the absence of a warning, e.g. for a safe crossing time of 20 sees for a train travelling at say 112 km/hr it would be necessary to warn a user if a train was within 640 metres of the crossing .

The object of the invention is to provide a warning system which is particularly suitable for use at railway crossings and which can be adapted to other applications e.g. for trackside workers, or for train drivers.

According to this invention a track occupation warning system comprises a master unit positioned at a predetermined position along a length of track, eg at a crossing, or signalling point, and a slave unit for each directional working of the track positioned along the track at a predetermined distance from said master unit, said master and slave units each being electrically connected to the running lines of the

track, said master unit comprising operating means for initiating , generating and transmitting a predetermined series of pulses forming a unique encoded message to the or each slave unit via said running lines and first receiving means for receiving and verifying a response from the or each slave unit, and the or each said slave unit comprising second receiving means for receiving and verifying an encoded message from said master unit, and response means for generating and transmitting a response in the form of its own unique encoded message only if an encoded message from said master unit is verified by said second receiving means, the system being such that, if a vehicle is either closely approaching a slave unit, or is within the section of track between said slave unit and master unit, any encoded message ^" from said master unit will be attenuated by the vehicle contact with the running rails sufficiently to prevent verification by said second receiving means. Preferably, the master unit includes display means which are connected to said first receiving means so as to provide a visual indication to the user as to whether or not the track length is occupied by a vehicle. Conveniently, the operating means of the master unit and response means of the or each slave unit comprise an electrical circuit including an inverter for generating via capacitors different levels and/or states of impulse, e.g. positive and negative going

impulses, and an encoder controlling said positive and negative impulses and producing a predetermined sequence thereof, said sequence constituting a unique encoded message which is transmitted between units via said running rails, each electrical circuit also including decoder means for receiving and verifying encoded messages.

To initiate the master unit, its electrical circuit may conveniently include a press-button or other suitable switch, particularly for crossing or track-side applications. If the system is used as a simple signalling application the circuit may be initiated by a remotely operable switch in response to an infra-red or other suitable remote control signal, e.g. operated by the driver of an approaching train and directed via a trackside relay. Alternatively, the system may be interrogated and operated automatically by an approaching train. As a further alternative for little used lines, or in addition to remote operation, the system may be operated manually by the driver alighting from his vehicle and actuating an appropriate switch.

In order that the invention may be readily understood, one preferred embodiment of system, for a level crossing, will now be described with reference to the accompanying drawings, in which:-

Figure 1 is a schematic view of the level crossing system and showing the message protocol,

Figure 2 is a schematic circuit arrangement of

the master unit.

Figure 3 is a schematic circuit arrangement of a sub-master unit, and

Figure 4 is a schematic circuit arrangement of _: a slave unit, and

Figures 5a, b and c are schematic views of alternative level crossing arrangements using the system.

Referring to Figure 1, the system is associated with a level crossing 1 for a single track 2 comprising a pair of running lines 3 along which trains can pass in both directions under signal control. The system itself comprises a master unit 4 on one approach to the crossing 1, a sub-master unit 5 on the other approach to the crossing, and two slave units 6, 7 positioned at a predetermined ^" distance along the track on each side of the crossing. It will be noted that the master and sub-units are electrically connected via a link 8 and that the master and slave units are electrically connected via pairs of cables 9, 10, 11 respectively to the running rails 3.

Furthermore, it will be noted that the electrical connection is such that data 12, 13 can be transmitted from the master to each slave unit and that data responses 14, 15 can be transmitted from each slave unit back to the master unit.

As discussed above, the distance X of the slave units along the track on each side of the crossing is determined so as to provide a minimum safe time period

for a user to cross. Thus, if the running time of a train along the track in the vicinity of the slave unit would be such as to reach the crossing within the safe time period a warning of the presence of the train would be displayed to the user.

Referring to Figure 2, the master unit 4 has an operating circuit 16 comprising sequential logic 17 which when initiated by pressing a button 18, or other appropriate switch, connects the circuit to a battery 19 for activating, in sequence, an encoder 20, inverter 21, decoder 22 and a display 23. The inverter 21 is connected via capacitors 24 and a switch 25 controlled by the encoder 20 to the running lines 3 via the connecting cables 9. The circuit also includes a receiver 26 between said cables 9 and the decoder 22, for controlling ^' an output signal from the decoder indicative of the presence or absence of a train, which is fed to the display 23.

Referring to Figure 3 the sub-master unit 5 is not connected to the running lines but only to the master unit 4 via a cable or remote control link 8. No operating circuit is provided; only a display 27 to which the link 8 is connected. However, the link 8 includes a push button 28, or other appropriate switch, which can be pressed by a user on that approach side of the crossing, which will initiate the operating circuit 16 of the master unit. Also, if the crossing is for vehicular traffic, impulse cables, inductive loops, or other appropriate remote operating means can be laid

across the approaches to the crossing and connected into the operating circuit 16 to avoid the need for the driver alighting from the vehicle.

Referring to Figure 4, each slave unit 6, 7 has a response circuit 29 which is basically the same as the operating circuit 16 of master unit 4 and also its own battery 19S and receiver 26S. Thus, the response circuit includes sequential logic 17S, an encoder 2OS, inverter 2IS, decoder 22S, capacitor 24S and an encoder switch 25S, the circuit being connected to the running lines 3 via the cables 10, 11 respectively. It will be appreciated that neither an operating switch, nor a display is needed for the slave units.

In operation of the system, when the push button 18 or 28 of the master or sub-master unit 4, 5 is pressed by a user to activate the operating circuit, the inverter 21 generates a voltage to produce a series of positive and negative charges from the capacitors 24 in the form of spiked voltages and these latter are sent in a timed sequence controlled by the encoder 20 via the switch 25 to the running lines 3. In this way a unique encoded message comprising a particular sequence of positive and negative spiked voltages are transmitted via the running rails 3 to receivers 26S of the slave units 6, 7. Each receiver 26S is programmed to activate its associated response circuit 29 only if it recognises and verifies the particular encoded message. Thus, if no train is approaching i.e. in the vicinity of, a slave unit, or no train is within the

section between the slave units, the encoded message from the master unit will be verifiable. However, if a train is present, its wheel contact with the rails will act as a shunt and will cause such a significant attenuation to the signal that it will not be

D verifiable.

Assuming no train is present, each slave unit will generate its own encoded message in the response circuit for transmitting back to the receiver means of

10 the master unit 4 for verification and, to ensure the messages are not adulterated by interference with each other, preferably the electrical circuit of one of the slave units includes a delay so that its message can be processed by the master unit after it has processed the

15 message from the other slave unit.

It will 'be appreciated that if the length of track section exceeds the normal working extent of the system described above, the system may be extended by the use of further slave units, each with its own

20 receiver and response means so that a message from the master unit to the outermost slave units and response messages from the outermost slave units can be passed serially through the intermediate slave units via the running lines 6.

I D Referring to Figures 5, three alternative arrangements for use of the system where two crossings are situated close to each other are shown.

In Figure 5a, it will be noted that the two crossings represented by master units 4a and 4b are

sufficiently close that both can interrogate a common slave unit 7c located between them. In this arrangement, each master unit has its respective outer slave unit 7a, 7b, and it will be appreciated that the j two master units would provide an encoded message which may be common to both master units. Alternatively, each master unit may have its own unique encoded message, in which case the receiving means of the common slave unit 7c would be designed to receive and

10 verify both encoded messages so as to be able to respond to the appropriate master unit.

As shown in Figure 5b, the spacing between the crossings is close enough for the two master units 4a and 4b to interrogate each other with a common, or different encoded message, hence dispensing with the need for slave unit 7c.

As shown in Figure 5c, where the master units 4a, 4b are very close, each can nevertheless have two associate slave units 7al, 7a2 and 7bl, 7b2

-_υ respectively, in which case the slave units are effectively overlapping each other. It will be appreciated here that the two master units would have different encoded messages to ensure correct interrogation by their respective slave units.