USB-to-VGA adapters

I have a user who wants a laptop and insists they need 3 monitors.    I am looking at a usb-to-vga adapter to add an additional monitor (laptop, External, USB).  

I am just not sure how these work.  Do they piggyback off of your existing graphics card or do they use the computer cpu and memory and run as a separate video card?
qvfpsAsked:
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Gary CaseConnect With a Mentor RetiredCommented:
They do NOT use the current graphics adapter for anything.   They're a stand-alone graphics device that uses the USB bus for its interface to the PC instead of a PCIe bus.    Clearly that limits the bandwidth to the adapter -- but within the bounds of those limits they do very well.   You can use as many of these as you have available USB bandwidth.
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pjamCommented:
Not sure how the work either.  But I believe that with a Docking station the maximum monitors he will see is 2.
I have two monitors and I cannot use the notebook monitor and both of them so it remains closed on the docking station.  The display adapter is nVidia NVS3100M, so that may be the curtailing hardware.
We had a USB to VGA adapter to give a desktop with integrated graphics two monitors, and soon replaced the complete setup.
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Joe Winograd, Fellow&MVEDeveloperCommented:
I have an HP 8460w laptop in an HP advanced docking station and it supports three monitors, but only after I figured out the trick — when I first tried it, I could get only two monitors working. The solution was to get an active adapter, not passive. I first purchased this DisplayPort (male) to DVI (female) Active Adapter for one office:
http://www.amazon.com/gp/product/B004SUO1GM/

It works perfectly. I now have three monitors working where before only two of the three worked at a time. For a second office where I was having the same issue, I purchased this DisplayPort (male) to HDMI (female) Active Adapter:
http://www.amazon.com/gp/product/B009YC3W1Y/

This also works perfectly and I now have three monitors working at that office, too.

The first two monitors are connected to the dock via DisplayPort and DVI. I'm driving both at 1920x1200. The third monitor may be either the laptop display or an external VGA monitor.

The StarTech website has a good explanation of why you need an active adapter instead of passive to get the three monitors working:
http://www.startech.com/AV/Displayport-Converters/Active-DisplayPort-to-DVI-Adapter~DP2DVIS

Also, the AMD website has this nice description (and other excellent material) of the basic requirements for connecting multiple displays:

Connecting multiple monitors for AMD Eyefinity technology couldn’t be simpler. There are four easy rules to remember:

1. The first two monitors can connect to the graphics card with any display output on your product: HDMI, VGA, DVI or DisplayPort.

2. The third (or greater) display must be connected to the graphics card via DisplayPort.

3. If your monitor does not have a DisplayPort connection, you will need an inexpensive active DisplayPort adapter for it. DVI to DP adapters can be had for less than $30 USD.

4. Every family of GPUs supports a different maximum number of displays.

Of course, the laptop/docking station must have AMD Eyefinity technology for this solution. Regards, Joe
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_Commented:
The one I install for a third monitor (for someone) works pretty good.
This one had DVI Out.
Can't do heavy gaming, but handles most daily stuff (utube, email, pics, etc.)

These usually let you use up to 6 (depends on what make/model) USB video cards on one system, each in it's own port (or a good hub?).

They just use the USB bus, instead of the PCI or PCI-E. So you need to be careful about "flooding the bus" when running multiple "cards" this way.


edit:
forgot, they come in vga, dmi, hdmi, and display port, versions.
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Gary CaseRetiredCommented:
... also, note that if the laptop has USB v3 you can  get enough bandwidth to support full HD displays -- e.g. http://www.amazon.com/Plugable-Multiple-Monitors-1920x1080-DisplayLink/dp/B008GXQ1OW
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