How do I get my Mac Pro to see the second monitor through a video splitter and/or switcher?

Which video card are you using?

If it's a 5x70 have you considered cabling all the monitors to it (if three monitors there's some rules you need to adhere to)? Then you could use SwitchResX to enable, disable, or mirror* the monitors in any combination.

A point of clarification is needed. When you say it doesn't "recognize" or "see" the other monitor when you switch is that based on the fact the monitor doesn't show it's getting a signal or i based on the monitor doesn't show in the Displays system preferences Arrangement tab? If it does show in Arrangement what happens if you click Detect Displays?

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* Small discalaimer, not sure about the mirroring part, i.e., which monitors get mirrored, but 100% sure of enable/disable monitors since that is one of the main functions of SwitchResX.

but (a not-well documented fact) that doesn't mean one can really get three full-resolution signals out of it at one time, as you alluded to in your message. So, yes, we did try hooking all three up directly to the computer, but the computer would only recognize two of those monitors,

Did you use the corect adapters for three displays?

<http://support.apple.com/kb/TS3477>

<http://support.apple.com/kb/HT4279>

It is possible to mirror two or three of the displays.

<http://docs.info.apple.com/article.html?path=Mac/10.4/en/mh1551.html>

Christopher Dobrian wrote:

Thanks for the response! Yes, it's a 5x70 video card, a 5770 I believe. In any case, it does have the three ports, but (a not-well documented fact) that doesn't mean one can really get three full-resolution signals out of it at one time, as you alluded to in your message. So, yes, we did try hooking all three up directly to the computer, but the computer would only recognize two of those monitors, and the only way to get it to "de-recognize" the second monitor and recognize the third monitor was to physically disconnect the second monitor.

I did say when you want to connect three monitors to 5x70's there are some rules to adhere to. These are covered in the first two apple docs mentioned above, or by title:

Mac Pro (Mid 2010), Mac Pro (Early 2009): Issues with three displays and multiple DVI, HDMI connections

Mac Pro (Early 2009), Mac Pro (Mid 2010): Supported display configurations

The general gist of these rules is that to drive three monitor son a 5x70 you need two active adapters. That is why one of your monitors was disabled when you tried to add a third.

Here's a good explanation of what's going in here and why you need two active adapters on these cards:

Active vs. Passive Displayport adapters *the truth*

If you use DVI monitors and any of them have resolutions less than 1920 x 1200 then you can use single link active adapters. The "AMD Eyefinity Validated Dongles" chart shows some active (and passive) adapters that are acceptable. For example of a single link DVI active adapter is the Accell B087B-006B. An example of a dual link DVI active adapter would be the one from monoprice.com if you need to connect monitors with a resolution greater than 1920 x 1200.

I'm doubtful that SwitchResX would be able to convince the computer that there were only two monitors connected and direct the second monitor signal to the correct place, but it certainly seems worth a try, so thanks for the idea! Any other ideas are still welcome in the meantime.

I'm not doubtful. It is a very powerful utility for dealing with all aspects of multiple monitor configurations (they call them "Display Sets") complete with forcing EDID information if necessary. The different configurations can be controlled with a (contextual) menu and/or hot key.

SwitchResX is academic at this point if you want to have all three monitors connected. You would need to get that working first. Then use SwitchResX to vary the configuration combinations. So if you go this route the "name of the game" is figure out the active adapters you need.

I only described in general terms above about the adapters since you never said exactly what kind of connections these monitors require (single link DVI, dual link DVI, hdmi, mini-displayport, vga).

You can't run three displays from the card using Mini DisplayPort to HDMI adapters since they are passive adapters. You need active adapters.

<http://support.amd.com/us/eyefinity/Pages/eyefinity-dongles.aspx>

You could use active Mini DisplayPort to DVI adapters and DVI to HDMI adapters or cables.

Christopher Dobrian wrote:

I think our next step should be first of all to confirm that we are indeed using the right adapters, per the instructions to which you linked me. I'm pretty sure we are, but I'll make doubly sure now based on the documents to which you pointed. Then the next step will be to see what we can do with SwitchResX, which sounds very promising, first in the three-display-port configuration and then in the two-display-port-with-splitter configuration.

Does that sound like a good plan to you?

More or less what I said except for the setup with the splitter. Why would you need the splitter if you have three displays to juggle around where SwitchResX can do the juggling?

As for the adapters there are no eyefinity active hdmi adapters on the list although I can suggest an alternative that works (I know I use it) below. But I have to ask, do all these monitors only have HDMI inputs? Are there alternative connections like DVI, and if so, are those monitors large enough (> 1920 x 1200) to require dual link DVI? Do they all require audio?

As for the active HDMI adapter I use the following for my TV:

DVI & S/PDIF Digital Coax/Optical Toslink Audio to HDMI® Converter

This is sufficient to qualify as an active adapter. I have it on the DVI port. My other active adapter is a mini-displayport to VGA for a 17" VGA monitor. VGA adapters count as a second active adapter so I fulfill the requirements allowing me to use my 27" apple display on the other mini-displayport. I use SwitchResX to enable/disable the TV and always leave the converter box on since it doesn't use much power.

If the mirrored HDMI displays both use 1080p you could use this adapter

<http://www.monoprice.com/products/product.asp?c_id=101&cp_id=10114&cs_id=1011409 &p_id=8117&seq=1&format=2>

It would use only one port on the graphics card for two displays, so you could use a third display without the restrictions on adapter types needed when all three ports are used.

starting with the computer connected directly to the displays without the splitter, then replugging everything to insert the splitter.

That sounds like you are using the wrong output ports on the splitter, or the splitter is not sending EDID (Extended Display Identification Data) back to the computer. Splitters often have only one output port capable of sending EDID back to the computer. If there is no display connected to that port when the computer starts, the computer won't recognize the display. Check the manual for the splitter to see if it has one special port (probably the first port).

If the splitter doesn't support EDID you could put a DVI Detective N

<http://www.gefen.com/kvm/dproduct.jsp?prod_id=4714>

between the computer and the splitter, but it might be cheaper to use the three ports (with different adapters).