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Potentiometer Specifications
The resistance of the potentiometers should be a linear taper. Based on the design of
the integrating analog-to-digital converter used, the maximum resistance should be no
more than 528K (470K
+/-
10 percent is suggested) for either the X or Y pots. This is
based on a charge capacitor of 0.047uf,
+/-
10 percent, and a maximum time of 16.6 mil-
liseconds for charge to full value (one video frame time).
Light Pen
A light pen can be connected only to the left controller port (port 0). Its connections are
not shown in table 8-3. The pins controller port pins normally used by a light pen are
shown in table 8-4.
Table 8-4: Light Pen Pin Usage
Pin Number
Usage
7
+5V
8
GND
5
Pen-pressed-to-screen
6
Capture beam position
The signal called "pen-pressed-to-screen" is generally a single switch to ground, normally
open, which is 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 treated as such. The signal
called "capture beam position" is connected as the trigger switch of a normal joystick.
The principles of light pen operation are as follows (assuming the light pen has been
enabled):
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 light pen creates a trigger signal at the moment that the video beam passes
the window in the nose of the pen.
224 Interface Hardware
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