Method and apparatus for interfacing computer light pens

A method for reducing the positional jitter that occurs when using a light pen to detect an illuminated pixel on a video display screen of a raster scanning video display monitor in a computer system, comprising the steps of:

• resetting a first count value on the occurrence of an active vertical synchronization signal from the computer system;
• incrementing said first count value on each occurrence of an active horizontal synchronization signal subsequent to the vertical synchronization signal;
• resetting a second count value on each occurrence of the horizontal synchronization signal;
• incrementing said second count value on each cycle of a high frequency oscillator output signal;
• detecting the illumination of a pixel on the video display screen within the field of view of the light pen and providing a light-detected output signal responsive thereto;
• storing a first count value and a second count value on the occurrence of said light-detected output signal and generating an interrupt signal to the computer system;
• generating a first interrupt signal to the computer system when said light-detected signal is activated;
• responding to said first interrupt signal to transfer said first and second count values to the computer system to identify the location of the illuminated pixel and saving said transferred values;
• deactivating said light-detected output signal;
• repeating said steps of incrementing said first count value, resetting said second count value, incrementing said second count value, detecting the illumination of a pixel, storing said first and second count values, transferring and storing said first and second count values, and resetting said light-detected output signal;
• selecting one of said first count values and one of said second count values to represent the location of the light pen on the video display screen, said selected one of said second count values being the smallest of said stored second count values;
• and
• generating a second interrupt signal to the computer system on each occurrence of the vertical synchronization signal, the computer system responding to said second interrupt signal to select said selected one of said first count value and said selected one of said second count value.