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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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