| 摘要 |
PURPOSE:To attain the input through a light pen regardless of the state of a screen by flashing the entire screen at the luminance higher than a prescribed level in case it is detected that the position corresponding to the light pen on the screen is set at the luminance less than the prescribed level. CONSTITUTION:When a light pen 1 is set oppositely to a dark screen of a CRT2, a contact signal SW is supplied to a shift register 41. However, a photodetecting signal SG is not obtained and accordingly a count stopping signal CS is not obtained from a waveform shaping circuit 11. Then an FF44 is reset and a dark screen detection signal BP is set at H level. A full screen flash signal F2 is obtained from an AND gate 45 in a period when a display synchronizing signal -BLANK and a field discriminating signal -EVER given from a CRT controller 21 are kept at H level, respectively. Thus the entire screen is bright for a field. Therefore signals SG and CS are obtained in a field, and a partial flash signal F1 is obtained in the next field. Thus the same processing as that carried out with a bright screen is performed. The FF44 is reset at the rise of the next signal -EVER, and the signal BP is set at L level.
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