I can explain why you only have 2 monitor up and why they will switch when you plug the 3rd one in.
5770 can only output 2 monitors in the same time, if you want 3 monitors running at the same time, you have to use "Mini Display Port to Dual Link Adapter" instead of "Regular Mini Display to VGA/DVI Adapter".
Hope this help.
If 3 monitors running all together, so you can switch the signal on the monitor itself, would it solve your problem
I have a similar (but not exact) problem to yours. I have my main 27" ADC monitor connected to one of the mini displayports (mDP) on my 5870 video card. The other mDP is usually connected to a second monitor - a VGA through a mDP to VGA adapter. But I also want to be able to use my Sony 32XBR6 sometimes. That means, like you, getting up and unplugging the VGA and plugging in the mDP to HDMI adapter (which in turn is connected to my HDTV).
I've done extensive google searches looking for what amounts to a mDP A/B switch (not a KVM -- apparently one mDP KVM I found cannot be run in reverse). So far it looks like such an animal doesn't exist.
So I've resigned myself to having to physically change the plugs. But one concern I had with this is waring out the mDP socket on the video card. So what I am going to do is get a mDP extension cable (i.e., male-to-female), plug that into the video card, and then plug the TV or VGA into the extension cable. I think it's much better to ware out the socket in a cheap extension cable than the expensive video card. In addition it is also easier to switch the plugs at the end of a 3' cable than having to reach behind my mac pro and change the plugs there.
Thanks for that, Alain. No matter what I do to the second desktop monitor, if the TV is plugged in it kills it. I tried shutting off the TV but no go. The only thing that brings back my second monitor is to pull the tv from the mini display port. That 100 bucks is kind of steep. Why would that make all three ports open up simultaneously when a single link cable doesn't?
That's what I'm not sure too. But not only in Mac, in PC, if you want to have three ports open up simultaneously, you have to use something like "Active Display Port" the same stuff as mini display port to dual-link dvi which include an extra usb cable of the adapter. Might be running 3 monitors need some "extra data transfer", that's why we need this kind of stuff.
But I do read from some other thread saying that, when you using VGA instead of DVI, you don't need such an adapter for "that VGA monitor". But I don't have a mini-display to VGA adapter to try if it really works.
For example, if you have DVI*2, VGA*1 monitors, what you need is
1st DVI - plug directly to DVI port
2nd DVI - using mini display to dual link adapter
3rd VGA - mini display to VGA adapter
Might be running 3 monitors need some "extra data transfer", that's why we need this kind of stuff \[adding USB].
It's not the USB data that is needed to run three displays. It's the extra POWER. If you read the Apple article, they say the only solution they endorse is their US$100 Dual-Link adapters. Check the table at the end of the article.
Mac Pro (Early 2009), Mac Pro (Mid 2010): Supported display configurations
But readers here have found that if you have two Brand-X Powered adapters, aka active adapters, it also works fine. These do not always require another USB. If designed recently, they can pick up a power pin in the Mini DisplayPort interface.
That article is a bit information dense without being especially enlightening. But the Note right above the table helps a little:
Note: If you are using an Apple Mini DisplayPort to DVI adapter, only two displays are supported across the three ports. If you are connecting three displays, any DVI displays must be connected to either the Dual-Link DVI port or must use an Apple Mini DisplayPort to Dual-Link Adapter.
So as I read it, you can use the regular DVI for free, but if you want an additional DVI off a Mini DisplayPort, you have to limit yourself to two displays or use the expensive Apple ACTIVE adapter to do the conversion. \[Or use a Brand-X ACTIVE adapter.]
If the adapters are $100 each, could you not just buy a second card and have up to 4 monitors with out the hassle... it would also cost you the same or less than 2 adapters.
Also, referencing the article that grant linked too, can a MP 4,1 (early 2009) install and run 2 5770's? Each using one of the additional power connectors?
Sorry, I'm trying to figure this all out in an evening as I take possession of my first Mac Pro tomorrow morning. It's an early 2009 that I want to do some quick upgrades too.
So it's looking like buying the 100 dollar part from apple is the cheapest solution to have all three ports working at once. Though I have to admit that the technical articles confuse me. Just when I feel like I've been told something I realize that I still don't feel confidant as to what the solution really is. I'm going to put the mini display port to Dual link DVI on my biggest desktop monitor. The other desktop monitor will plug directly into the DVI port on the card and the 40 inch tv will still connect via HDMI over the second Mini Display port.
This page at AMD (makers of the 5770 and 5870) lists the qualified POWERED adapters that meet their most stringent requirements. Look down the page and you will find several qualified Active Mini DisplyPort to various others, such as HDMI. These adapters are available from other outlets, such as Amazon, sometimes for far less than the Apple adapters.
I'm going to put the Mini DisplayPort to Dual link DVI on my biggest desktop monitor. The other desktop monitor will plug directly into the DVI port on the card and the 40 inch tv will still connect via HDMI over the second Mini Display port.
That will work only if you use an active Mini DisplayPort to HDMI adapter. The only one I have found is:
but you should confirm that they are really active before buying 500 of them.
You can connect a DVI to HDMI adapter or cable to an active Mini DisplayPort to DVI adapter but that won't give you any audio over HDMI.
You could connect the DVI displays to the Mini DisplayPorts with active adapters and use a DVI to HDMI cable or adapter from the DVI port to the TV.
If you want audio via HDMI, you can use audio from the Mac Pro's optical output with this DVI to HDMI converter: