ViewSonic TD2421 User Manual page 12

Control of the Touch Function
1. Before using the touch function, make sure the USB cable is connected and the
Windows operating system is started.
2. When the touch function is active, make sure there is no foreign object in the
areas encircled in the figure below.
Make sure there is no foreign object in the encircled areas.
NOTE:
1. The touch function may need about 7 seconds to resume if the USB cable is
re-plugged or the computer resumes from sleep mode.
2. The touchscreen can only detect up to two fingers simultaneously.
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