Epson Perfection 640U Reference Manual page 22

Once EPSON TWAIN is installed, the Scanners and Cameras
icon appears in Control Panel, and your scanner is listed in the
Scanners and Cameras Properties dialog box similar to the one
shown below.
Note for Macintosh users:
On your Macintosh computer, you cannot assign any application other
than EPSON SMART PANEL for Scanner to the buttons on the
scanner.
Assigning an application to the buttons
Make sure the application supports "Scanner events", and then
follow the steps.
1. Double-click the Scanners and Cameras icon in Control
Panel.
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Using the Buttons