Our Testing department have been testing the below devices: 

USB Mass Storage, USB Smart Card Reader, USB Printer, USB Scanner, USB Game Controller, USB KB, USB MS, USB Touch Screen, USB Pen Tablet / Sign Pad, Apple iPOD and so on. 

However USB asynchronous devices cannot be supported due to hardware limitation (USB timing) such as USB Fingerprint Reader, etc ...

USB Speaker and USB Microphone can be supported with the latest FW version.

Please note slim version like KE6900ST/KE8900ST and KE9900ST do not support USB touch screen.