READING A LIGHT PEN
A light pen can be connected to one of the controller ports. On the A1000, the light pen
must be connected to port 1. Changing ports requires a minor internal modification. On
the A500 and A2000 the default is port 2. An internal jumper can select port 1. Regardless
of the port used, the light pen design is the same.
The signal called "pen-pressed-to-screen" is typically actuated by a switch in the nose of
the light pen. Note that this switch is connected to one of the potentiometer inputs and
must be read as same as the right or middle button on a mouse.
The principles of light pen operation are as follows:
1. Just as the system exits vertical blank, the capture circuitry for the light pen is
automatically enabled.
2. The video beam starts to create the picture, sweeping from left to right for each
horizontal line as it paints the picture from the top of the screen to the bottom.
3. The sensors in the light pen see a pulse of light as the video beam passes by. The pen
converts this light pulse into an electrical pulse on the "Beam Trigger" line (pin 6).
4. This trigger signal tells the internal circuitry to capture and save the current contents of
the beam register, VPOSR. This allows you to determine where the pen was placed by
reading the exact horizontal and vertical value of the counter beam at the instant the
beam passed the light pen.
- 232 Interface Hardware -
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