Different types of DVI to VGA adapters

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VGA is analog signal, there's one type of it. I don't understand how there could be different versions of a DVI to VGA adapter, but there are.

For example Matrox Triplehead2Go Digital has 3 DVI-I (both analog and digital) output ports. I want to connect to my projectors via VGA, so I need a DVI to VGA adapter.
I'd assume there would be 1 type of DVI to VGA adapter, but there are two types I've found is sold.
The manual only mentions you need a "DVI to HD-15 adapter" ( http://www.matrox.com/graphics/media/pdf/products/gxm/en_triplehead2go_guide.pdf ,page 10 )

Here are the two types:

enter image description here

There are a lot of these on eBay and in local stores. Notice there are no pins on the top and bottom of the lone "horizontal line pin" (the C1, C2, C3, C4 pins are missing). I think they are needed to pass analog signal.
So what are these adapters? Do some weird devices use VGA ports to accept digital signal? Is that the explanation?

The second type looks like this:

enter image description here
enter image description here

Notice that all the pins are there, including the C pins.

Er, what's the difference? If it weren't the necessary analog pins missing on the first I'd assume it's for single link or dual link DVI, but that's not it…

There's also the third kind missing the middle pins…
enter image description here

I'd assume it's for single link DVI, but again, VGA is analog and the C pins are missing too…

Best Answer

There are a lot of different types because there are a lot of different DVI types. DVI-D, DVI-I to name a few.

Some DVI types have 4 pins on the left side near the -. They are analog signals, and the Sub-D (VGA) connector links to just those pins. Some connectors don't have them, then the signal is internally linked to a digital signal. It is up to the monitor to understand the digital signals then.

You should note that a VGA monitor that does NOT have a DVI port, does not understand digital signals, and will fail if the analog signals are not connected (missing those 4 pins near the -) it will simply say cable unplugged. These are only the old monitors from 10 years ago.