A method and arrangement of transferring data at high speeds over a parallel data bus provides for calibration of the data without interrupting the data communication. Consistent with one aspect of the present invention, a method of parallel data communication is provided for a circuit arrangement including a parallel arrangement of data paths for passing data in parallel between at least two nodes. The method compares a sequence of data over an additional calibration/spare path relative to a matched sequence of data being passed on one of the multiple paths. In response to detecting skewed data, the transmission time for the subject path is adjusted in response to the comparison. Other example aspects of the invention are directed to procedures for rotating the calibration procedure through each of the data paths while using the spare to maintain communication integrity, and various embodiments for controlling the calibration procedure and for detecting whether the data is skewed.