A shared line transmitter is operative to accept as a message one or more bytes of data in parallel from an input-output channel for predetermined buffering and phase encoding for transmission to a shared line except upon receipt of signal indicating that another transmitter is attempting to gain access to the shared line. It includes an output buffer operative to receive the message from the input-output channel and hold it for a predetermined period; a memory operative to receive the message from the output buffer and hold it for a predetermined period for subsequent transmission in serial form; a phase encoder operative to receive the data in serial form for conversion to a predetermined phase-encoded form for outputting to the shared line. and means for aborting the conversion upon receipt of a message-collision signal for transmitting a signal onto the shared line to abort all concurrent message transmissions, and backoff logic operative to receive the abort signal from the phase encoder to generate a random number representing the period that must elapse before the next transmission.