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I have a mid-2010 15" MacBook Pro. I have purchased a mini DisplayPort to DVI-D adapter onto which I have connected a DVI-D to VGA (D-Sub) adapter which then connects into my new monitor. The Displays screen seems to recognised the monitor, but the monitor says "No Signal Detected". Is it valid to connect mini DisplayPort -> DVI-D -> VGA?

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D-Sub (D-subminiature) is more commonly known by the protocol it's most used for, VGA. – CajunLuke Jun 18 '12 at 23:53
You bought a new monitor that only has VGA!? – NReilingh Jun 19 '12 at 0:11
@Graeme, NReilingh is right: why did you buy a new monitor that only has VGA? Are you sure it doesn't have a DVI, HDMI, or DisplayPort inputs? If these other ports are available, you'll get a better picture if you use them instead of VGA. – CajunLuke Jun 19 '12 at 13:50
It also has DVI-D of which I have purchased a cable. – Graeme Jun 20 '12 at 8:46
up vote 6 down vote accepted

No. DVI-D only carries digital signals. Most (if not all) DVI -> VGA converters use the analog signal DVI-I carries. A mini DisplayPort to DVI connector will only pass on digital signals (as you correctly identified by referring to it as a DVI-D signal), so your DVI to VGA adapter is left out in the cold. You need to get a mini DisplayPort to VGA adapter.

The reason the computer sees the display is that one of your adapters is identifying itself as a display.

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