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Thunderbolt Drives One External Monitor on MacBook Pro

[Editor's note: With the announcement of the Apple 27-inch Thunderbolt Display in July — see “Apple Thunderbolt Display Announced,“ 20 July 2011 — Apple said that any Thunderbolt-equipped 15-inch or 17-inch MacBook Pro could, in fact, use the internal built-in display and one or two Thunderbolt displays. This directly contradicts what Apple told us about general support for Thunderbolt displays across all Mac laptops at the time of this article. We are sorry for the error, but we relied on information provided by Apple.]

Ever since Apple announced the new MacBook Pro models that use Intel’s super-fast Thunderbolt technology, a debate has been running across Twitter, comment forums, and email: Can a new MacBook Pro out of the box drive two external monitors from the Thunderbolt port?

The answer: no. Don’t believe me. I asked Apple — in fact I asked the company twice, the second time after new information appeared to contradict its first answer. I received a definitive statement both the first and second times. No, no, no, no, no. (I also wrote this in “Secrets of Thunderbolt and Lion,” 27 February 2011, but I sought more clarification after particular questions kept arising.)

Where does this confusion arise? It’s the difference between genotype and phenotype. Stay with me. The Thunderbolt spec is the genotype. The MacBook Pro models are the phenotype. The Thunderbolt spec (its genes) says that the standard may support up to two monitors along the chain. This is true. However, the MacBook Pro (the expression of those genes) contains graphics circuitry that is already supporting one display: the internal screen. That allows for just one external monitor.

This became more confusing in part because Apple’s Australian product page for the new MacBook Pros had a footnote (number 4) that read, “The 15- and 17-inch MacBook Pro support two displays; the 13-inch MacBook Pro supports one display.” This footnote was oddly referenced out of sequence under the “Ports with possibility” heading near the bottom of the page. Apple revised the page soon after, and the footnote is now gone.

You can use a third-party DisplayPort splitter, like the Matrox DualHead2Go ($160 to $200 online), just as you can with previous DisplayPort-only Macs. The splitter divides the available resolution into smaller rectangular pieces that Mac OS X treats as separate monitors. Thunderbolt will also ostensibly support more elaborate external adapters that would use the data side of Thunderbolt, just as a few USB-to-video converters work today. For more on those, see “Put More Pixels on Your Desktop with ViBook+” (13 November 2009). [Update: Note that neither Apple nor TidBITS is recommending the Matrox specifically. A reader—see comments—says this model isn't working with a 2011 MacBook Pro, in fact. The Matrox was cited as an example of the kind of gear that should work. Updates may be needed for compatibility, firmware or otherwise.]

I also confirmed that you cannot work around the problem by shutting the lid of an early 2011 MacBook Pro and thus, by avoiding use of the internal display, actively shunt the output to two external monitors.

While Apple doesn’t comment on future products, it’s abundantly clear that:

  • Future desktop machines without internal monitors can and will support up to two external monitors using Thunderbolt, just as they do using DisplayPort today.

  • A different motherboard design that uses separate graphics circuitry for the MacBook Pro’s internal and external displays could clearly allow two external monitors driven by the Thunderbolt port.

There you have it. But I expect the first comment to this article will be: are you sure?


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Comments about Thunderbolt Drives One External Monitor on MacBook Pro
(Comments are closed.)

Adam Engst  An apple icon for a TidBITS Staffer 2011-03-01 20:19
Are you sure about this, Glenn?

Sorry, couldn't resist.
Jeff Carlson  An apple icon for a TidBITS Staffer 2011-03-01 20:31
No, really, are you sure? Are you surely sure, Shirley?
Glenn Fleishman  2011-03-01 20:33
In comments space, no one can hear you scream.
Since Thunderbolt is PCI Express I assume someone could make an external video card enclosure and allow additional monitors that way?

Not sure OS X would support it, but seems like it would be adding a new additional video card in a Mac Pro.
Actually that makes me think you could make a generic enclosure that was just a row of PCI Express slots that you could plug any regular PCI Express card into.
Glenn Fleishman  2011-03-01 21:02
This seems possible. The question is whether you'd need some wacky driver or if it could supported through built-in tools. You can get USB-to-video adapters now which work even over the poky 480 Mbps USB 2.0 connection. So I can't imagine we won't see TB (PCI Express) adapters of the same kind.
pobregizmo  2011-03-01 21:29

Relevant part at 0:10
Glenn Fleishman  2011-03-01 22:40
Thank you for that great laugh!
Concerned two-monitor owner  2011-03-01 21:53
Early Powerbooks had the ability to display to a external monitor INSTEAD of the internal monitor.

Could the new MacBook Pros do this - display to two external monitors with the internal display turned off
Glenn Fleishman  2011-03-01 22:40
No. I'll add this to the main body of the article.
Just for completion, (still) has both the claim and the footnote

And don’t worry about a single drive or peripheral tying up the Thunderbolt port: You can daisy-chain up to six devices, including up to two high-resolution displays.3

3 : The 15- and 17-inch MacBook Pro support two displays; the 13-inch MacBook Pro supports one display.

Glenn Fleishman  2011-03-01 22:40
Apple confirmed in a follow-up to a call that it will be removed.
Hans Erik  2011-03-03 08:22
Well, there remains one thing I don't understand: the MacBook Pro 15" and 17" actually have two graphics controllers: Intel HD onboard (?) and a AMD Radeon chip. Would make sense to me that one of these is driven by the Thunderbolt controller and the other is not.

In that case, one could use Thunderbolt to drive two external displays?
Adam Engst  An apple icon for a TidBITS Staffer 2011-03-03 14:34
Well, that's an excellent point, although it doesn't seem to be the case.
Arne Ruckert  2011-03-08 00:54
My external monitor picture is constantly shaking using the thunderbolt port. Many people have complained about this, is there anything that can be done?
Hi Glenn,

Are you sure about this statement you made

"You can use a third-party DisplayPort splitter, like the Matrox DualHead2Go ($160 to $200 online), just as you can with previous DisplayPort-only Macs'

I am using this config on my 201 MBP 15" and have just tried it on my new 2011 MBP 15". Havent got it working yet. Matrox are currently testing compatibility, but no news from them since their response to my support request over a week ago.

Glenn Fleishman  2011-03-09 20:47
Note the "like the Matrox": Apple said existing devices should be compatible, but they can't provide any guarantees, nor have I tested the equipment.

Such devices will certainly work, but there's no way to know which of them will without updates.

I'll add a note to the article to make it clearer I was citing an example of the kind of equipment that would work, not a specific product recommendation or testing outcome.
Marcus  2011-04-15 21:28
I realize it's been a month since the last post but I have another question. Later when the spec expands to 100gbps could we use two external monitors then? An implied question would be, is my thunderbolt port on my new goddamn mac going to be old tech in less than a year?
Glenn Fleishman  An apple icon for a TidBITS Staffer 2011-04-15 21:46
It may be a while until 100 Gbps hits, and even then it won't suddenly make current technology break!

But as to the double-monitor issue: it sounds completely like a limitation of graphics cards, not Thunderbolt.
Marcus  2011-04-17 12:53
So is the comment I read about future optical cables having the transceiver baked into the cable allowing early '11 models the capability to transfer future speeds correct? If so, then I guess I could always look forward to external graphics solutions.
Glenn Fleishman  2011-04-17 13:23
Seems unlikely. The speed has to be at the motherboard, because you need to talk at that full speed to the bus on the computer. Unless the idea is that Thunderbolt connectors already have 100 Gbps capability that's not enabled by the first-generation cables and equipment. That all sounds unlikely to me, but you never know how clever Intel and Apple are about this.