WO 2006/063368 PCT/ZA2005/000180 DATA TRANSFER ON AN ELECTRIC FENCE BACKGROUND OF THE INVENTION [0001] This invention relates generally to an electric fence and more particularly is concerned with the transfer of data along an electric fence for communication purposes. 5 [0002] An electric fence may extend over a considerable distance and, at least in order to monitor the integrity of the fence and to detect intrusions, it is desirable to have the facility to communicate on the fence. [0003] US Patents Nos. 5420885 and 5651025 respectively describe the use of an additional pulse train or a carrier frequency which is superimposed on the fence wires apart 10 from the energizing pulses. US Patent No. 6081198 describes a pulse density modulation technique. [0004] Safety legislation in a number of countries lays down strict requirements regarding pulse repetition rates and pulse energy levels. It is evident therefore that the use of communication pulses in combination with shock (energizing) pulses must be carried out 15 with great care if safety regulations are to be met. This can lead to a complex system. [0005] US Patent No. 6801045 describes a technique in which the fence (shock) pulses are modulated using pulse position principles. This can be effective but it suffers from the disadvantage that it is not possible to synchronise a number of individual energisers in the time domain. This is a requirement, for safety reasons, in adjacent fences which are 20 separately energised by the respective energisers. A further disadvantage is the difficulty of excluding extraneous noise on the fence or fences using threshold detection. [0006] US Patents Nos. 5767592, 6020658 and 4859868 describe energisers which produce unipolar output wave forms. An intention in this respect is to increase efficiency and WO 2006/063368 PCT/ZA2005/000180 possibly energy output and to reduce electromagnetic interference generated by the pulses. Further, such energi'ers often monitor a return fence voltage in order to determine fence loading. Usually the energy for generating the energising pulses is derived from a charged capacitor. The energy which is stored in a capacitor is proportional to the square of the 5 voltage across the capacitor. Thus if an attempt is made to operate at a low load condition the fence voltage may be considerably reduced. SUMMARY OF INVENTION [0007] The invention addresses at least some of the aforementioned problems. [0008] The invention provides, in the first instance, a method of transferring data along an 10 electric fence which includes at least one first elongate conductor, the method including the steps of energising the at least first conductor using a first train of pulses selected from a first unipolar pulse of a first polarity, a second unipolar pulse of a second polarity and bipolar pulses, and using predetermined pulses, selected from the first pulse train, to represent defined information. 15 [0009] Any technique or protocol may be used to associate the pulses with defined information. For example the information may be expressed in binary form with one or more defined pulses representing a logical zero and one or more defined pulses representing a logical one. [0010] The predetermined pulses may be sent in groups at predetermined spacing from 20 each other or in predetermined sequences. [0011] Each group of pulses may be identified in any appropriate way. Thus the existence of pulses which are associated with data or information may be identified while other pulses, which are not so identified, do not represent data or information. An important aspect of the invention in this regard is the realisation that pulses which are used for energising the 2 WO 2006/063368 PCT/ZA2005/000180 conductor can be selected from an available range of pulses in order to represent data while, at the same time, ensuring that the pulses comply with applicable safety regulations particularly in respect of energy per pulse and pulse repetition rates. [0012] Nonetheless it falls within the scope of the method of the invention to make use of 5 pulses of lower amplitudes than the amplitudes of pulses which are used for energisation purposes, for data transfer purposes. [0013] The pulses or groups of pulses may be used in accordance with any defined protocol to represent data. Thus, by way of example, the polarities of the pulses may be varied and bipolar pulses with positive and negative leading edges may be employed, 10 according to requirement, to represent data e.g. a logical zero or a logical one. [0014] The method of the invention may be used with a single energiser or with a plurality of energisers. In the first instance a single energiser may be used to transmit data to a controller or monitor. If an installation includes two or more energisers then a first energiser can be used to transfer data, using the method of the invention which has been described, to 15 a second energiser and the second energiser can be used to relay the information received from the first energiser and, at the same time, to transmit information from the second energiser. The transmission from the second energiser may be directed to a third energiser at which the aforementioned process can be repeated, or to a monitor or control unit. Within reason a plurality of energisers can be placed in communication with each other in this 20 cascaded or daisy-chain method although, due to the constraints of prescribed pulse repetition rates and the energy which is required per pulse in order to satisfactorily electrify a fence the number of energisers which can be cascaded for data information purposes will be limited. [0015] The invention also extends to an energiser which includes a first generator for 25 generating a first pulse of a first polarity, a second generator for generating a second pulse 3 WO 2006/063368 PCT/ZA2005/000180 of a second polarity, and a control unit for controlling operation of the first -and second generators. [0016] The control unit may be such that, in respect of first and second adjacent time intervals, each of which is of a defined duration, a first pulse or a second pulse is generated 5 in the first time interval and no pulse, a first pulse or a second pulse is generated in the second time interval. [0017] The energiser is therefore such that it can produce a pulse train with pulses selected from unipolar pulses of first and second polarities respectively and bipolar pulses with a positive leading edge and with a negative leading edge respectively. 10 BRIEF DESCRIPTION OF THE DRAWINGS [0018] The invention is further described by way of examples with reference to the accompanying drawings in which: Figure 1 illustrates an energiser circuit of known construction i.e. forming part of the prior art; Figure 2 illustrates a half bridge energiser circuit in accordance with the present invention; 15 Figure 3 illustrates a full bridge energiser circuit in accordance with a second form of the invention; Figures 4A, 4B and 4C and 4D respectively illustrate waveforms which can be produced by the circuits of Figures 2 and 3; and Figures 5 and 6 schematically illustrate different electric fence installations in which data 20 transfer can take place in accordance with the method of the invention. DESCRIPTION OF PREFERRED EMBODIMENTS [0019] In the following description reference is made to those components of an energiser and an electric fence which are necessary for an understanding of the invention and in order to distinguish the invention from the prior art. As is known to those skilled in the art 4 WO 2006/063368 PCT/ZA2005/000180 energisers are required to operate at prescribed pulse 'amplitudes (energy content) and repetition rates and these objectives can be achieved in a plurality of manners which are known and which for this reason are not further described herein. [0020] Figure 1 of the accompanying drawings illustrates a conventional energiser 10 used 5 to electrify a fence, not shown. The energiser includes an energy device 12 such as a storage capacitor which is charged from a control circuit and which can be discharged by means of a switch 14, such as a silicon controlled rectifier, through a primary winding 16 of a step-up transformer 18. The transformer has a secondary winding 20 with terminals 22 which are connected, in known manner, to electrical conductors in a fence. The pulse train 10 which is applied to the fence is determined by the circuit arrangement of the energiser and is not readily varied except, possibly, in accordance with timing information which is used to control the switch 14 and in respect of the amount of energy stored in the capacitor 12. [0021] Figure 2 illustrates an energiser 24, according to a first form of the invention, which embodies what is referred to as a half bridge circuit. This circuit includes first and second 15 energy storage capacitors 26 and 28 respectively which are connected in series with two SCR (silicon controlled rectifier) switches 30 and 32 respectively which are individually controllable by means of a control circuit 33. The switches are in parallel to diodes 34 and 36 which are used for control and protective purposes, as is known in the art. A primary winding 38 of a transformer 40 is connected between the junction point of the capacitors 26 20 and 28 on the one hand and via an inductor 41 to a junction point of the switches 30 and 32 on the other hand. A capacitor 42, used for protective and smoothing purposes, as is known in the art, is connected in parallel to the primary winding. [0022] A secondary winding 44 of the transformer is connected to the wires in an electric fence, not shown. 5 WO 2006/063368 PCT/ZA2005/000180 [0023] It is evident that when the switch 30 is closed' energy from the capacitor 26 is discharged and passes through the primary winding 38 in a first direction producing an output pulse of a first polarity. If the switch 32 is closed then energy from the capacitor 28 is passed in a second direction through the primary winding and an output pulse of an 5 opposing, second plurality is produced. Figure 4A illustrates a positive unipolar pulse 45 which is produced by discharging the capacitor 26. Figure 4B shows a negative unipolar pulse 46 produced by discharging the capacitor 28. [0024] If the discharge of the capacitor 26 is followed at a predetermined time by the discharge of the capacitor 28 then a bipolar pulse 47 with a positive leading portion, of the 10 type shown in Figure 4C is produced. A bipolar pulse 48 with a negative leading portion, of the type shown in Figure 4D, is however produced when the capacitor 28 is discharged first and the capacitor 26 is thereafter discharged. [0025] The control unit 33 controls the charging and discharging of the capacitors 26 and 28 using as control parameters information which is input relating to the unit from suitable 15 sensors, not shown, relating to the operational aspects of the fence which are to be monitored, and in accordance with the type of communication protocol which is adopted. [0026] Figure 3 illustrates an energiser 50 according to a second form of the invention which is referred to herein as a full bridge circuit. Components which are used in the energiser 50 and which are the same as the corresponding components used in the 20 energiser 24 bear like reference numerals. [0027] The energiser 50 includes two additional SCR switches designated 30A and 32A respectively which together with the SCR switches 30 and 32 make up a full bridge circuit. Two storage capacitors 52 and 52A are connected across the bridge and use is made of diodes 54 and 56, 34 and 36, and 34A and 36A, respectively, for protective, operative and 25 control purposes. 6 WO 2006/063368 PCT/ZA2005/000180 [0028] A positive unipolar pulse of the type shown in'Figure 4A can) be generated by allowing the capacitor 52 to discharge through the switches 30A and 32. A negative unipolar pulse, of the type shown in Figure 4B, is achieved by closing the switches 30 and 32A to cause discharge of the capacitor 52A . A positive bipolar pulse 47 of the type shown in 5 Figure 4C can be achieved by the closure of the switches 30A and 32 followed by a timed closure of the switches 30 and 32A. The Figure 4D negative bipolar pulse can be achieved by closure of the switches 30 and 32A followed by a timed closure of the switches 30A and 32. [0029] Figure 5 illustrates a single energiser 60 which may include an energiser 24 of the 10 kind shown in Figure 2 or an energiser 50 of the kind shown in Figure 3. The secondary winding of the respective transformer is connected to fence wires which are symbolically represented and which are designated 62 and 64. A ground connection 66 to the transformer or energiser is also made. [0030] A receiver 68 is connected to the wires at a remote point. The receiver includes a 15 detector 70 and a monitor 72. The detector 70 has an attenuator 76 and an analogue to digital converter 78. The attenuator reduces the voltage applied to the analogue to digital converter which samples the waveform on the fence wires and produces a train of digital pulses which are applied to the monitor and interpreted in accordance with a defined data transfer protocol. The detector can also be used to monitor one or more of the fence 20 voltage, the fence current and the degree of electromagnetic interference generated through the use of the energiser. [0031] The detector can include a circuit or algorithm so that it only measures signals in a certain time window to exclude time interference from any extraneous source or any other fence. 7 WO 2006/063368 PCT/ZA2005/000180 [0032] The monitor 72 is capable of detecting the pulse polarity and the current sequence. Assume that use is 'made of a communication protocol in which a positive leading edge bipolar fence pulse 47 from the energiser 60 indicates that the monitor 72 must synchronise its clock to that of a master clock so that the following fence pulse is then regarded as a 5 known bit positioned in the data stream. [0033] The energiser 60 is considered as a master and it generates a bipolar pulse (47) with a positive edge first at some arbitrary point in time (Figure 4C). In this example the system is not reliant on an external clock. After detection of the synchronisation pulse a positive unipolar pulse (45 - Figure 4A) represents a logical one and a negative unipolar 10 pulse (46 - Figure 4B) represents a logical zero. The energiser pulse rate is known or it can be determined by the monitor 72. An arbitrary number of data bits can be transmitted by the energiser to the monitor along the wires 62 and 64 to communicate information. [0034] In the arrangement shown in Figure 6 a fence installation 80 includes energisers 60A, 60B, 60C and 60D respectively. Each energiser has a respective built-in detector, 15 generally of the kind described in connection with Figure 5, designated respectively 70A, 70B, 70C and 70D. Assume that the communication protocol is as follows: either a positive leading bipolar pulse (Figure 4C) or a positive unipolar pulse (Figure 4A) represents a logical one and either a negative leading bipolar pulse (Figure 4D) or a negative unipolar pulse (Figure 4B) represents a logical zero. This approach has an advantage in that the 20 energisers may run at reduced energy levels when there is little fence loading. [0035] Each energiser is used to electrify a corresponding fence section designated 62A, 62B, 62C, and 62D respectively. A monitor or control unit 84 is connected to the energiser 60A which is treated as a master in the system. 8 WO 2006/063368 PCT/ZA2005/000180 [0036] The arrangement shown in Figure 6 is typical of a security installation in which a defined area is bounded by the fence sections each of which is electrified through the use of a separate energiser. [0037] The control unit 84 is used to give instructions to each energiser and to monitor the 5 status of each energiser. [0038] The detector 70B monitors the fence pulses generated by the energiser 60A; the detector 70C monitors the pulses produced by operation of the energiser 60B; the detector 70D monitors the pulses produced by operation of the energiser 60C; and the detector 70A monitors the pulses produced by operation of the energiser 60D. 10 [0039] Each energiser is aware of its respective status (i.e. a master or slave) in terms of its location in the sequence with respect to the master energiser 60A and the total number of energisers in the system, and each of the energisers 60B, 60C and 60D can interpret commands from the master energiser 60A. [0040] After a clock synchronisation sequence is transmitted by the energiser 60A the 15 remaining three energisers can synchronise their clocks to identify a particular bit position in the data stream and transmit their respective data. [0041] An arbitrary protocol of a three bit binary code is used for this example. The protocol may be changed in order to decrease the response time or to increase the amount of information which can be transmitted. If the energiser 60A receives information which is 20 not identical to the information which it transmitted, due to a possible communication loss, it can attempt to resynchronise the system. [0042] If communication is for any reason lost then it is possible for a subsequent energiser in the sequence to initiate data transmission so that the faulty zone (i.e. a zone in which a possible communication fault occurred) can be identified. The circuitry in each respective 9 WO 2006/063368 PCT/ZA2005/000180 detector 70 may be so designed, or include an algorithm, to measure signals which are in a certain time window 6nly so as to exclude interference from other energisers or extraneous sources. [0043] If all the energisers are turned off then no communication is possible in the system. 5 It is however possible to design each energiser with a detector which has significant dynamic range so that although the energiser output is reduced communication is maintained. This has the benefit that the leading edge of any waveform can be positioned in time so that all energisers remain in synchronisation in order to comply with pulse repetition rates. [0044] In each energiser use may, if required, be made of an external reference such as a 10 clock or GPS receiver to ensure that within reason absolute synchronisation is achieved. This makes communication simpler and has advantages under certain fault conditions. [0045] The following table represents a three bit data sequence, for each energiser, in the installation 80 of Figure 6. ENERGISER PULSE TRANSMISSION 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 TxA A1 A2 A3 Al TxB Al B1 A2 B2 A3 B3 Al B1 TxC Al B1 C1 A2 B2 C2 A3 B3 C3 Al B1 TxD Al BI C1 D1 A2 B2 C2 D2 A3 B3 C3 D3 A1 RxA Al B1 C1 D1 A2 B2 C2 D2 A3 B3 C3 D3 [0046] in the table: 15 Al, A2, A3 are three respective bits for a first word of information for the energiser 60A; 10 WO 2006/063368 PCT/ZA2005/000180 Al, A2, A3 are three respective bits for a second word of information for the energiser 60A; TxA is the information transmitted by the energiser 60A; TxB is the information transmitted by the engerizer 60B, etc.; 5 the uppermost row 0, 1, 2 . is the energiser high voltage pulse sequence on the fence, in predetermined time slots or with the pulses in defined positions. Data pulses are shown and the open blocks or slots can be occupied by pulses representing any arbitrary excitation of the fence i.e. not conveying information. [0047] The energiser 60A transmits its control information in the slots 0,4 and 8. The 10 remaining slots of the energiser 60A can have an arbitrary value. The energiser 60B receives the information sent by the energiser 60A in the corresponding slots 0,4 and 8 and transmits this information in the slots 1,5 and 9. The energiser 60B ignores the pulses received in all other slots. It also sends the three bits of information pertaining to itself in slots 2,6 and 10. The remaining slots of the energiser 60B (i.e. slots 0,3,4,7,8 etc.) can have 15 an arbitrary value. This process is repeated by the remaining two energisers. [0048] Another aspect which flows from the energiser of the invention relates to its energy saving characteristic and reduced electromagnetic interference level. Assume, for example, that use is made of the energiser shown in Figure 2 in the arrangement of Figure 5. [0049] If the fence 62, 64 of Figure 5 is lightly loaded then only the SCR switch 30 is fired. 20 A train of positive unipolar pulses (Figure 4A) then appears on the fence. On each firing only the energy stored in the capacitor 26 is used. [0050] If the fence loading increases and the switch 30 is fired followed, at the appropriate time, by a firing of the switch 32, then the amount of energy transferred to the fence is doubled. The energy consumption would also double as both capacitors 26 and 28 are 25 discharged. In a modification of this aspect voltage control can be exercised over the capacitors to vary the amount of energy stored by the capacitors. 11 WO 2006/063368 PCT/ZA2005/000180 [0051] It is known that the frequency content of a bipolar wave is less than that of a unipolar wave and consequently the use of the bipolar pulses contributes to a reduction in the amount of electromagnetic energy interference generated by the fence. 12