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A method for communicating between a first station and a second station over a wire pair, in which the first station sends an alternating power signal over the wire pair, the alternating power signal comprising a plurality of communication symbols, the second station receiving the alternating power

A method for communicating between a first station and a second station over a wire pair, in which the first station sends an alternating power signal over the wire pair, the alternating power signal comprising a plurality of communication symbols, the second station receiving the alternating power signal and determining therefrom the communication symbols, the second station drawing current from the alternating power signal in a sequence corresponding to at least one further communication symbol, and the first station determining the current drawn in the alternating power signal to recover the further communication symbol.

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What is claimed is: 1. A method for communicating between a first station and a second station over a distribution medium, comprising: the first station sending an alternating power signal over the distribution medium, the alternating power signal comprising a plurality of communication symbols; th

What is claimed is: 1. A method for communicating between a first station and a second station over a distribution medium, comprising: the first station sending an alternating power signal over the distribution medium, the alternating power signal comprising a plurality of communication symbols; the second station receiving the alternating power signal and determining therefrom the plurality of communication symbols; the second station drawing current from the alternating power signal in a sequence corresponding to at least one further communication symbol; and the first station determining the current drawn in the alternating power signal to recover the at least one further communication symbol. 2. The method of claim 1, wherein the alternating power signal has a substantially square wave-form, and has substantially equal proportions of positive and negative components, averaged over time. 3. The method of claim 2, wherein the second station drawing current comprises the second station drawing current during a positive component and a negative component of the alternating power signal. 4. The method of claim 3, wherein the second station is arranged to draw current during the positive component of the alternating power signal in a sequence corresponding to the at least one further communication symbol, and to repeat the current draw in the negative component of the alternating power signal. 5. The method of claim 3, wherein the second station is arranged to draw current during a substantially centrally located portion of the positive component and negative component of the alternating power signal. 6. The method of claim 3, wherein the second station is arranged to draw current adjacent each rising edge and falling edge of the alternating power signal. 7. The method of claim 1, wherein the second station drawing current further comprises the second station not deriving power from the alternating power signal for other purposes while drawing current corresponding to the at least one further communication symbol. 8. The method of claim 1, wherein the communication symbols comprise '1', '0', 'idle', and 'sync'. 9. The method of claim 8, wherein the '0' symbol is represented by equal portions of positive and negative components in the alternating power signal, and the '1' symbol is represented by unequal portions of positive and negative components in the alternating power signal, and the proportion of positive and negative components in the alternating power signal representing a '1' symbol is alternated each time a '1' symbol is sent. 10. The method of claim 1, wherein the plurality of communication symbols form a stream, selected from one of: a command stream comprising a sync symbol, an instruction byte, and a plurality of station instructions; an idle stream comprising a sync symbol, and a plurality of idle symbols; and an instruction stream, comprising a sync symbol, an instruction byte, a station address, and station data. 11. The method of claim 10, wherein there are a plurality of second stations, each second station having an address, each second station responsive to a station instruction or to station data at a position in the command stream corresponding to its address, and each second station draws current during an idle pulse at a position in the command stream corresponding to its address. 12. The method of claim 11, wherein one said instruction byte corresponds to a second station activate/deactivate instruction, each second station activating or deactivating according to whether a corresponding station instruction is a '1', symbol or a '0', symbol, respectively. 13. The method of claim 12, wherein each second station is arranged to count the number of other second stations that have been activated before it, and to activate at an offset from the sync pulse corresponding to the number of other second stations. 14. The method of claim 1, wherein the first and second stations comprise part of an irrigation system. 15. An apparatus for communicating with at least one station over a distribution medium, comprising: means for providing an alternating power signal over the distribution medium, the alternating power signal comprising a plurality of communication symbols; and control means arranged to determine current draw in the alternating power signal to recover at least one further communication symbol from a station. 16. The apparatus of claim 15, wherein the means for providing an alternating power signal comprises a power generating circuit connected to and operating under control of the control means, the power generating circuit operable to provide an alternating power signal over the distribution medium. 17. The apparatus of claim 16, wherein the power generating circuit is arranged to provide an alternating power signal that has a substantially square wave-form, and has substantially equal proportions of positive and negative components, averaged over time. 18. The apparatus of claim 16, wherein the control means is arranged to control the power generating circuit to form the alternating power signal from a plurality of communication symbols forming a stream, selected from one of: a command stream comprising a sync symbol, an instruction byte, and station instructions; an idle stream comprising a sync symbol, and a plurality of idle symbols; and an instruction stream, comprising a sync symbol, an instruction byte, a station address, and station data. 19. The apparatus of claim 15, wherein the communication symbols comprise '1', '0', 'idle', and 'sync'. 20. The apparatus of claim 19, wherein the '0' symbol is represented by equal portions of positive and negative components in the alternating power signal, and the '1', symbol is represented by unequal portions of positive and negative components in the alternating power signal, and the proportion of positive and negative components in the alternating power signal representing a '1' symbol is alternated each time a '1', symbol is sent. 21. The apparatus of claim 15, wherein the control means is arranged to determine current draw during a substantially centrally located portion in each positive component and negative component of the alternating power signal. 22. The apparatus of claim 15, wherein the control means is arranged to determine current draw adjacent each rising edge and falling edge of the alternating power signal. 23. The apparatus of claim 15, wherein the at least one station comprises part of an irrigation system. 24. An apparatus for communicating with a first station over a distribution medium, comprising: means for receiving an alternating power signal comprising a plurality of communication symbols over the distribution medium, and for recovering the communication symbols therefrom; and control means arranged to draw current from the alternating power signal in a sequence corresponding to at least one further communication symbol. 25. The apparatus of claim 24, wherein the control means is arranged to draw current during a positive component and a negative component of the alternating power signal. 26. The apparatus of claim 24, wherein the control means is arranged to draw current during the positive component of the alternating power signal in a sequence corresponding to the at least one further communication symbol, and to repeat the current draw in the negative portion of the alternating power signal. 27. The apparatus of claim 26, wherein the control means is arranged to draw current during a substantially centrally located section in a positive component and negative component of the alternating power signal. 28. The apparatus of claim 26, wherein the control means is arranged to draw current adjacent each rising edge and falling edge of the alternating power signal. 29. The apparatus of claim 24, wherein the control means is arranged not to derive power from the alternating power signal during a prescribed section other than the current draw. 30. The apparatus of claim 24, wherein the means for recovering is arranged to recover a plurality of communication symbols forming a stream, and to decode the stream into one of: a command stream comprising a sync symbol, an instruction byte, and station instructions; an idle stream comprising a sync symbol, and a plurality of idle symbols; and an instruction stream, comprising a sync symbol, an instruction byte, a station address, and station data. 31. The apparatus of claim 30, wherein the apparatus has an address, the means for recovering being responsive to station instruction and station data at a position in the command stream corresponding to the address, the control means arranged to draw current during an idle pulse at a position in the command stream corresponding to the address. 32. The apparatus of claim 31, wherein the control means is arranged to count the number of other apparatus that have been activated before it, and to activate at an offset from the sync symbol corresponding to the number of other apparatus. 33. The apparatus of claim 24, wherein the first station comprises part of an irrigation system. 34. A method for controlling a plurality of stations, comprising: distributing an alternating power signal to the stations via a distribution medium; distributing a reference datum to the stations; selectively activating a plurality of the stations at different prescribed phase offsets from the reference datum so that one of the activated stations draws power from the alternating signal at a different time than at least one other one of the activated stations; sensing a number of activated stations; and selecting the prescribed phase offset from a set of prescribed phase offsets according to the number of activated stations to average the power drawn by the activated stations. 35. The method of claim 34, wherein the alternating power signal has a substantially square wave-form of equal amplitude, and has substantially equal proportions of positive and negative components, averaged over time. 36. The method of claim 34, wherein the distributing of a reference datum further comprises distributing a plurality of activation marks after the reference datum. 37. The method of claim 36, wherein the plurality of activation marks comprise a further predetermined sequence of positive and negative components in the alternating power signal after the reference datum. 38. The method of claim 36, wherein the activating of stations comprises sending instructions to each station specifying whether or not to activate the station, the instructions being embedded in the plurality of activation marks. 39. The method of claim 34, wherein the reference datum comprises a predetermined sequence of positive and negative components in the alternating power signal. 40. The method of claim 34, wherein the stations comprise part of an irrigation system. 41. The method of claim 34, wherein the set of prescribed phase offsets comprises phase offsets of 0 degrees, 22.5 degrees, 45 degrees and 67.5 degrees. 42. An apparatus for controlling a plurality of stations connected to the apparatus by a distribution medium, the apparatus comprising: means for providing power to the stations via the distribution medium; control means arranged to provide a reference datum to the stations and an instruction to instruct each station to activate, each station being responsive to the instruction to activate at a prescribed phase offset from the reference datum so that one of the activated stations draws power at a different time than at least one other one of the activated stations; sensing means for sensing a number of activated stations; and selecting means for selecting the prescribed phase offset from a set of prescribed phase offsets according to the number of activated stations to average the power drawn by the activated stations. 43. The apparatus of claim 42, wherein the means for providing power comprises a power generating circuit connected to and operating under control of the control means, the power generating circuit operable to provide an alternating power signal of variable frequency over the distribution medium. 44. The apparatus of claim 43, wherein the power generating circuit is arranged to produce an alternating power signal that has a substantially square wave-form, and has substantially equal proportions of positive and negative components, averaged over time. 45. The apparatus of claim 43, wherein the control means is further arranged to provide a plurality of activation marks after the reference datum. 46. The apparatus of claim 45, wherein the control means is arranged to provide the activation marks by controlling the power generating circuit to produce a further predetermined sequence of positive and negative components in the alternating power signal after the reference datum. 47. The apparatus of claim 45, wherein the control means is arranged to provide instructions to each station by controlling the power generating circuit to produce a prescribed sequence of alternating positive and negative components in the alternating power signal as the activation marks, each activation mark corresponding to an instruction to a station whether or not to activate. 48. The apparatus of claim 43, wherein the control means is arranged to provide the reference datum by controlling the power generating circuit to produce a predetermined sequence of positive and negative components in the alternating power signal. 49. The apparatus of claim 42, wherein the stations comprise part of an irrigation system. 50. The apparatus of claim 42, wherein the set of prescribed phase offsets comprises phase offsets of 0 degrees, 22.5 degrees, 45 degrees and 67.5 degrees. 51. An apparatus for communicating with at least one station over a distribution medium, comprising: a power generating device for providing an alternating power signal over the distribution medium, the alternating power signal comprising a plurality of communication symbols; and a control device arranged to determine current draw in the alternating power signal to recover at least one further communication symbol from a station. 52. The apparatus of claim 51, wherein the at least one station comprises part of an irrigation system. 53. An apparatus for communicating with a first station over a distribution medium, comprising: a receiver for receiving an alternating power signal comprising a plurality of communication symbols over the distribution medium, and for recovering the communication symbols therefrom; and a control device arranged to draw current from the alternating power signal in a sequence corresponding to at least one further communication symbol. 54. The apparatus of claim 53, wherein the first station comprises part of an irrigation system. 55. An apparatus for controlling a plurality of stations connected to the apparatus by a distribution medium, the apparatus comprising: a power generating device for providing power to the stations via the distribution medium; a control device arranged to provide a reference datum to the stations and an instruction to instruct each station to activate, each station being responsive to the instruction to activate at a prescribed phase offset from the reference datum so that one of the activated stations draws power at a different time than at least one other one of the activated stations; a sensing device for sensing a number of activated stations; and a selecting device for selecting the prescribed phase offset from a set of prescribed phase offsets according to the number of activated stations to average the power drawn by the activated stations. 56. The apparatus according to claim 55, wherein the stations comprise part of an irrigation system. 57. The apparatus of claim 55, wherein the set of prescribed phase offsets comprises phase offsets of 0 degrees, 22.5 degrees, 45 degrees and 67.5 degrees.