A data communication system includes a master and a slave. The master transmits a first subject signal including a first subject data to the slave via a transmission line. The slave extracts a clock signal from the first subject signal by performing a clock data recovery process and determines the first subject data based on the first subject signal. The slave transmits a second subject signal including a second subject data to the master during an existing period of the first subject signal without interfering an extracting of the clock signal and a determination of the first subject data. The master receives the second subject signal and cancels a waveform component of the first subject signal from a waveform of the second subject signal, and then determines the second subject data based on the second subject signal.