Abstract: A light pen computer interface system utilizes a coil located externally of the computer's CRT display monitor to sense the magnetic field which provides the raster for the CRT. Vertical and horizontal sync detection circuits receive the sensor coil output and supply vertical and horizontal sync signals to a timing circuit. The timing circuit in response to an output signal from the light pen and the vertical and horizontal sync signals supplies a pen position signal to the computer which is either representative of the x,y coordinates position of the light pen on the CRT screen or representative of the change of x,y coordinates between successive positions of the light pen.