public void run ()
		{
			while (directory_prefix != null) {

				// No matter how we learn that something may have
				// changed, we do the same thing to figure out
				// exactly what changed:  scan usbdevfs
				scanForDevices(matchUSBvendorID,matchUSBproductID);
				//while (scan ())
				//  continue;

				try {
					Thread.sleep (POLL_PERIOD_MS);
				} catch (InterruptedException e) {
					// set dir to null to cause a clean exit
					directory_prefix = null;
				}
			}
		}