Multi-monitor with the Thunderbolt Display

The Thunderbolt Display has a single Thunderbolt port for daisy chaining additional devices. Each Thunderbolt port can support 7 additional devices, which means 6 devices once you connect the Thunderbolt Display. These devices can be anything Thunderbolt, however you cannot connect a DisplayPort monitor to the Thunderbolt Display directly. If you want to connect another monitor directly to the Thunderbolt Display it must be another Thunderbolt Display. To understand why, we need to look at the architecture of a Thunderbolt controller.

This is the first Thunderbolt controller Intel introduced, codenamed Light Ridge:

You see it has four Thunderbolt channels and two DisplayPort inputs. It also has a single DisplayPort output as well as a DisplayPort passthrough option. What's the difference? If there are only two devices in the chain, the computer and a DisplayPort monitor, you can use the DisplayPort passthrough option bypassing the majority of the logic entirely. This is how the 2011 MacBook Pro can connect directly to a DisplayPort display. Put a Thunderbolt device in between those two devices and you can no longer use the passthrough mode. You have to send a Thunderbolt signal to the Thunderbolt device, and it can then extract the DisplayPort signal and output it. Simply passing DisplayPort through won't work.

I originally believed Apple used the smaller Eagle Ridge controller in its Thunderbolt Display, but now believe that to be incorrect. Upon closer examination of our dissection photos it appears that the 27-inch panel is driven by an embedded DisplayPort (eDP) connection. If I'm right, that would mean the DisplayPort output from the Light Ridge controller is routed to the eDP connector in the display. With its sole DP output occupied by the internal panel, the Thunderbolt Display cannot generate any more DP signals for anything connected directly to its Thunderbolt Port. 

If you connect a Mac to the Thunderbolt Display what is sent is a Thunderbolt signal. DisplayPort is broken off and sent to the display but there's no way to propagate an additional DisplayPort signal to any other non-TB displays in the chain. The output on the Thunderbolt Display is literally a Thunderbolt output, it can't double as DisplayPort.

However, if you connect another Thunderbolt device that uses Light Ridge you can split any additional DisplayPort signals out of the chain. In other words, if you connect the Thunderbolt Display to a Promise Pegasus you can then chain on another DP panel. If you own a 27-inch Cinema Display and were hoping to add the Thunderbolt Display to it on the same Thunderbolt chain, you will need another TB device in between.

There are also the obvious GPU limitations. The 13-inch MacBook Pro only supports two displays (Ivy Bridge will up this to three). If you manage to connect two to the 13 however, Apple will just blank the display on the notebook and drive the two external panels. The MacBook Air is a different story. Not only does it only support two displays, but the Eagle Ridge controller only has a single DisplayPort input so you're not driving more than one external display via a MBA no matter what you do.

I tested multimonitor functionality with a 27-inch LED Cinema Display as well as a second Thunderbolt Display. In the case of the Cinema Display, as expected, I couldn't get video out of the port on the Thunderbolt Display. Connecting the Thunderbolt Display to a Promise Pegasus and then connecting a Cinema Display to it worked however. I also woke up the MacBook Pro's internal display and confirmed that I could get all three functioning simultaneously. There's a definite slowdown in UI frame rate with two 27-inch panels being driven by the MacBook Pro's integrated Radeon HD 6750M. It's not unbearably slow but kiss any dreams of 30 fps goodbye.

I also confirmed that two Thunderbolt Displays worked on the MacBook Pro regardless of the connection configuration.

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  • JasperJanssen - Saturday, September 24, 2011 - link

    Not every product merits an in-depth review. 1 in-depth equals about 4 or 5 short reviews, and every site has to choose carefully how to spend its time.

    The first time we saw SF2000, there were *massive* reviews. If Samsung would suddenly put out an SSD that is a few dozen percent faster than the current fastest SSD, (and not cost the earth) it'll no doubt get an extensive review. When Apple puts out something very new that could well be the way of the future for the industry as a whole, they get a large review. Makese sense to me.

    And before you jump all over me on that score: Just look at the Sony implementation of Light Ridge on the current Vaio Z. That's a docking station that has everything this thing has (apart from the display) and some more.

    The market as a whole may well not go for integration into displays and integrated power supplies, but Thunderbolt/Thunderbolt-style docking stations seem likely to sweep the world.
    Reply
  • Zink - Friday, September 23, 2011 - link

    +1 top notch video review in every way Reply
  • Sunrise089 - Friday, September 23, 2011 - link

    Agree as well. While I definitely hope the text reviews will continue and receive the most resources, these video reviews are a great supplement, especially with less "numbers driven" products like displays and mobile devices.

    I wish all the CNET, television, etc. reviewers would take a look at this simple, engaging, intelligent review style and adopt it as well. Most reviewers spend way too much time trying to be entertaining and talking down to their audience.
    Reply
  • Anand Lal Shimpi - Friday, September 23, 2011 - link

    The text reviews will continue, this is simply something we're experimenting with to offer an augment :)

    It's always been my opinion that we should treat the readers as equals. It turns out that if you don't treat your readers like idiots then you'll find that you actually attract some really smart people :)

    Take care,
    Anand
    Reply
  • Constructor - Friday, September 23, 2011 - link

    The video was quite good indeed.

    And I like the eye-level-approach. Especially since you had a pretty good balance between talking about the actually interesting bits and still keeping it easy to follow.

    Maybe in some places you might want to slow down just a tiny little bit. ;-)
    Reply
  • Iketh - Friday, September 23, 2011 - link

    NOO!! If anything Anand, talk even faster! Throw it at me at full GigE! I'm a busy man! Reply
  • Constructor - Friday, September 23, 2011 - link

    Just play the video in fast forward then. ;-)) Reply
  • pmcg - Friday, September 23, 2011 - link

    I agree too. What an impressive setup and continuous, cogent presentation. I have never seen a video of Anand but now that I have, I demand more! Should there be an anandtech.tv? Reply
  • jleeworking - Saturday, September 24, 2011 - link

    Wow! nicely done, can we have more of those videos? Reply
  • mymoon - Saturday, September 24, 2011 - link

    Also agreed! Well done Anand on the video review. Felt my time well spent. Reply

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