| 摘要 |
PURPOSE:To prevent an erroneous detection, by erasing for a prescribed time only the characters within a light pen detecting field which are displayed on a CRT in the input mode of the light pen and then detecting the changing degree of luminance when the erased characters are displayed again. CONSTITUTION:A code parallel signal 2 given from a refresh memory 1 is read out synchronously with the timing signal given from a timing producing circuit 7 to be supplied to a character generator CG3. The generator CG3 produces a display pattern of the character corresponding to the signal 2, and this pattern is converted 4 into the series signals to be supplied to a modulating circuit 6. The signal 2 contains the designating bit to identify the field code and the designation bit of the light pen detecting field. With application of a switch signal 10 from an external light pen, a video signal 8 is not delivered from the circuit 6 for a prescribed period of time by the signal given from the circuit 7. Thus the characters in the corresponding filed of a CRT9 are erased. When the erased characters are displayed again after a prescribed time, the changing degree of luminance is detected. Thus the shift is suppressed for the position of the luminous point of the CRT to prevent the erroneous detection. |
