How It Works - IBM 3270 Operator's Manual

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Extras for the 3270
How It Works
The point of the pen contains a device that can detect the light
of a character on the screen. This device checks for the presence
of a character only when a switch, also located in the point of
the pen, is activated. The switch is spring-loaded and is activated
by placing the point of the pen on the screen and pushing the
barrel of the pen toward the screen.
Figure 41. When using the selector pen, correct positioning of the tip of the
pen is important.