Patent GB-1328678-A (GB)
Title: Light probe circuit for persistent screen display system
IPC: G06F3/037
Filed: 1970-10-08
Abstract: 1328678 Cathode-ray tube displays RCA CORPORATION 8 Oct 1970 [15 Oct 1969] 48024/70 Heading H4T In a cathode-ray tube display system employing a tube having a screen persistence of between one and two field scan periods for displaying character data (specifically Alpha Numeric characters) by intensity modulation of an interlaced line scanned raster and utilizing a light-pen responsive to the light from a selected character for producing an output signal for subsequent control of the displayed data, spurious operation of the system caused by the production of an output signal by the light-pen in response to the "persistence-light" of the character "aimed at" by the pen when the tube beam may, in fact, be scanning a different screen (and character) location is prevented by circuit means which responds only to the production of a light-pen output signal during the second or more of the successive field scans. The invention is described as applied to a display system of the kind disclosed in Specification 1,235,297 in which a digital-to-video converter 11, Fig. 1, supplies successive ones of a plurality of complete frames of data including a "master index" consisting of a plurality of characters (letters) indicative respectively of the address of each frame of data so that a particular one of the latter may be selected by selection circuits 17 and displayed on a cathoderay tube display 18 in response to a control signal supplied by the light-pen circuitry when the pen is "aimed" at the corresponding character in the index, the latter being displayed initially. Fig. 7 shows the light-pen circuitry and Fig. 8 shows the various waveforms at the positions indicated by reference letters in Fig. 7. In operation a switch 66 on the light-pen 54 is initially operated and via line 67 supplies a reset pulse which causes a frame address register to store the address of the master index frame which is then automatically displayed after a period of two fields during which the existing display is erased from the frame storage device 19 (Fig. 1) as described with reference to Figs. 3 to 6 (none shown). The light-pen is then "aimed" at the index character representative of the data frame which it is required to display and the light sensor circuit 53 generates an output B which sets a flip-flop 56 to produce output pulse E when it exceeds its threshold level TFF. When an instant later, the light-pen is pressed against the face of the display tube a pressure operated switch 68 generates an output C on line 69 to condition AND gate 59 to set a flip-flop 57 on the occurrence of the next inverted (vertical drive) field pulse A which resets flip-flop 56. The first light-pen response 73a (Fig. 8) is thus ignored but on the occurrence of the next pulse B from the light-pen flip-flop 56 is again set and its output E is then passed by AND 61 gate already conditioned by the D output of flip-flop 57 as waveform F. The latter sets a flip-flop 58 which is reset by an "end of character pulse", (of the character "aimed" at) to produce the pulse G which controls the frame selection circuit 17 (Fig. 1) to cause the subsequent (after erasure of the index display) display of the desired data frame. When the lightpen is removed from the sensing position the output C terminates and the next pulse A resets flip-flops 56 and 57 so that E and D drop to zero and terminate the pulses F and G.