This cable connects RS232 output of UHBX-R-PSE to VSA-X21 or EMX-AMP RS232 in and from RS232 out to a projector. It is used to control the internal functions of VSA-X21/EMX-AMP such as changing volume or mute/unmute. There is also a connection from the VSA-X21/EMX-AMP RS-232 output to a DB9 Male connector that can be used to for controlling power ON/OFF state of a video projector. The RS-232 output command from VSA-X21/EMX-AMP can be initiated based on RS232 input command, video detection or in case of VSA-X21/EMX-I AMP Telnet or ON/OFF buttons in its internal Web GUI.
Note that the RS232 output connector on the cable is a DB9 Male. If the projector has a DB9 Female connector, you can plug the cable directly to it, but some projectors may have unique RS232 control ports such as a Female MiniDin 6-pin connector. In that case, a DB9 F to MiniDin6 Male cable would be needed. Such cables are commercially available.