A communication system for transmitting electrical power and data between a first end of a transmission line and a second end of the transmission line , the system comprising, at said first end: DC supply means for supplying direct current to the transmission line ; first switching means for switching said DC supply means; and a first modulator for operating said first switching means, and at said second end: energy storage means for storing and supplying DC power; second switching means for switching power from the energy storage means; and a second modulator for operating said second switching means, wherein: said first modulator operates said first switching means to provide a communication on power signal from the first end of the transmission line to the second end of the transmission line; said second modulator operates said second switching means to provide communications from the second end of the transmission line to the first end of the transmission line; and communications from the second end of the transmission line to the first end of the transmission line are provided only when power from the first end of the transmission line to the second end of the transmission line is not being provided by said signal, wherein a falling edge of the signal indicates a data transfer and synchronizes a demodulator in readiness to receive the data transfer. DC supply means for supplying direct current to the transmission line ; first switching means for switching said DC supply means; and a first modulator for operating said first switching means, energy storage means for storing and supplying DC power; second switching means for switching power from the energy storage means; and a second modulator for operating said second switching means, wherein: said first modulator operates said first switching means to provide a communication on power signal from the first end of the transmission line to the second end of the transmission line; said second modulator operates said second switching means to provide communications from the second end of the transmission line to the first end of the transmission line; and communications from the second end of the transmission line to the first end of the transmission line are provided only when power from the first end of the transmission line to the second end of the transmission line is not being provided by said signal, wherein a falling edge of the signal indicates a data transfer and synchronizes a demodulator in readiness to receive the data transfer. said first modulator operates said first switching means to provide a communication on power signal from the first end of the transmission line to the second end of the transmission line; said second modulator operates said second switching means to provide communications from the second end of the transmission line to the first end of the transmission line; and communications from the second end of the transmission line to the first end of the transmission line are provided only when power from the first end of the transmission line to the second end of the transmission line is not being provided by said signal, wherein a falling edge of the signal indicates a data transfer and synchronizes a demodulator in readiness to receive the data transfer. <EOS>