There's no built-in support for accessing third-party devices from web apps, mainly because of security limitations imposed by the browser.
However, since this is a valid requirement, which has arisen in some of the commercial projects we developed (e.g. accessing a scanner), we managed to provide the following solution.
We created a client program, based around a small-footprint web server library (we used Jetty), which accepts predefined HTTP requests from the Tersus application running in the browser, accesses the device (via the APIs provided by the device), and posts the results (usually files) directly to the application server running the Tersus application.
Of-course, the client program should be installed on each workstation, in order for this to work.
For best results, use the Firefox browser..