Power supply that reads power requirement information across a DC power cord from a load

A power supply device that is capable of outputting multiple different output voltages is coupled by a DC power supply cord to an electrical device. An unpowered memory in the electrical device stores information about the power requirements of the electrical device (for example, the DC supply voltage required). In a first time period, when the power supply device is not powering the electrical device, the power supply device sends an energizing pulse train across the power cord. The electrical device captures energy from the pulse train. In a second time period, the electrical device uses the captured energy to transmit the information back to the power supply device across the power cord as pulses. The power supply device uses the information read back from the electrical device to output the appropriate DC supply voltage for the electrical device onto the power cord.