Multiple Monitor Mystery

[DallasDon]

Okay, I am hoping some of you power users can save me reading 10,000 words to solve this mystery.

My buddies and I recently acquired a need for multiple monitors on a single PC (Don't Ask). Not interested in spanning or cloning, just use them as independent. All windows load on the primary, the move them to the secondard monitor.

In tinkering with various builds, I find that onboard video combined with and add on card sometimes works, sometimes don't. The burning question is why?!?!?

Here has been my experience so far:

Enpower rig
MSI MS - 6378, onboard Video, plus PNY PCI Geforce 2, dual monitors work fine

EMachine, 800 mhz Celeron, onboard killed DEAD with an add on PCI PNY card. Couldn't even get it back uninstalling the pci card.

IBM Aptiva, 500 MHZ, PII, stock AGP video, add on PCI ATI Rage 7000(?), two monitors work fine.

MSI M2M combo, onboard video disabled with AGP GeForce 4 Dual head card installed. Added a Pci Geforce 2, and THREE monitors work fine.

I have tried to research the mobo manuals, to find a setting in the BIOS to enable both onboard and add on video, but can't find anything.

Going to bed soon, but any thoughts would be appreciated. Especially interested in the M2M combo, the latest build.

TIA

[Kruzin]

With the M2M, the onboard video is using the AGP bus.
AGP only supports a single video card, be it onboard, or a separate card.
Because of this, when you add a separate AGP card, the onboard AGP video is disabled. There is no way around this. If you want to use the onboard video, you must use a PCI card.

[DallasDon]

TVYM, Kruzin. That makes perfect sense, now that it's explained to me.

Does the shuttle MN31N (dual onboard video) also use the AGP bus? In other words, to add a third monitor, you need to add a PCI card, or will the AGP work with the onboard?

[Kruzin]

Most likey it's onboard AGP, so a PCI card would be needed.
Your other option is to use a triple-head video card, like Matrox's P750 or Parhelia, to run all 3 displays.

[DallasDon]

Thank you again. Now I know what kind of combination works, what doesn't, and why.