Each scanner needs its own specific TWAIN driver provided by the manufacture.
You can create a generic TWAIN application that can scan from any TWAIN enabled device.
There are several examples of sample application with source available.
Sample Data Source & Application (TWAIN 2.0)
TWAIN developers Toolkit’s, Twacker(TWAIN 1.7)
http://twain.org/downloads.shtm
Jim Watters
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JFL Peripheral Solutions Inc.
http://www.jflinc.com/
Your Expert Source of Custom Software and Services
for TWAIN Applications, Data Sources,
Peripheral Drivers and Support Software.
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