Dell 2300MP not displaying Optiplex 520 video

The projector will display video from Optiplex 280's, Latitude 600 series laptops, and all HP laptops but will not display from the Optiplex 520's.  The projector will not detect the analog video signal from the 520 video port.  I've tried changing the resolution and refresh rate, but still nothing.  The video drivers and chipset are the same as those in the 280's.  Does anyone have an idea as to why it won't display?

Thank you in advance for your help.

Jay