If you've got the version with a DVI input, use a HDMI to DVI cable. That'll take care of picture but not sound. You need to hook up an audio cable that finishes in a 3.5mm stereo jack if you want to use the Benq's speaker.
If you have the older version with just a VGA input then buy an Xbox 360 VGA cable. You'll also need an audio adapter; 2x phono/RCA sockets going to 3.5mm stereo jack.
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If you've got the version with a DVI input, use a HDMI to DVI cable. That'll take care of picture but not sound. You need to hook up an audio cable that finishes in a 3.5mm stereo jack if you want to use the Benq's speaker.
If you have the older version with just a VGA input then buy an Xbox 360 VGA cable. You'll also need an audio adapter; 2x phono/RCA sockets going to 3.5mm stereo jack.