Abstract: A data transmission system (200) includes a data transmission terminal (201) and a portable data device (203) that is remotely powered by the terminal. A method is provided for supplying power and data from the terminal (201) to the portable data device (203) using a carrier signal (401), including the step of identifying a slot location into which a modulation discontinuity (403-409) is introduced. The slot location is chosen within a modulation frame, which is based on n bits of the information signal to be transmitted, the number of slots in the frame varying between 1 and 2n. The modulation discontinuity (403-409) is introduced into the chosen slot location to produce an encoded data stream, which is then transmitted to the portable data device.