Abstract: A trackball device for controlling a cursor on a display screen employs two rotatable shafts and a single nonrotatable support member in contract with the ball to convert rolling motion of the ball to X and Y positioning information. The nonrotatable support member has a number of spaced bearing surfaces and may be removed from and replaced in the trackball housing to present a different bearing surface to the ball, thereby prolonging the life of the device. The device may be provided with removable apertured phase slit members which can be replaced with phase slit members having different apertures therein, to modify the operating resolution of the device.