In display apparatus wherein a picture image corresponding to an information train given by an electronic computer is written in a storage tube by a random scanning system and then the picture image is repeatedly read out of the storage tube by a raster scanning system for displaying it on the screen of a cathode ray tube, means is provided to assure precise correspondence between the information train given by the computer and the position on the screen of the cathode ray tube designated by a light pen thus producing an interruption signal to the computer.