There is provided apparatus for connecting between a data source or a data receiver and a data line. The apparatus comprises an impedance and an impedance controller arranged to continually adjust the value of the active impedance so as to control the relative impedances of the total source, the total source comprising the data source and the impedance, and the data line or of the total receiver, the total receiver comprising the data receiver and the impedance, and the data line. There is also provided a semiconductor chip, connectable to a data line, the semiconductor chip comprising the apparatus together with a data source or data receiver, as appropriate. There is also provided a method for controlling impedance matching between a data source or data receiver and a data line. The method comprises the steps of providing an impedance connected between the data source or data receiver and the data line; and continually adjusting the value of the impedance so as to control the relative impedances of the total source or total receiver and the data line.