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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  • bguidee - Friday, December 02, 2011 - link

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  • EmmanuelDécarie - Thursday, January 19, 2012 - link


    FYI, I just bought a Thunderbolt Display yesterday (2012-01-19) and I have plugged a Creative Xmod USB sound card (http://us.store.creative.com/Creative-Xmod/M/B000I... in the monitor and it works perfectly. I can use my own speakers that are much better than what comes with the monitor. So from my point of view, I don't miss the audio port on the monitor.

    I guess the Griffin iMic (http://store.griffintechnology.com/imic) could also works as a USB sound card to output sound from the monitor to your own speakers. But this solution is a bummer since this sound card don't add anything to the quality of the sound as do the Creative Xmod. So, you're right to say that's an oversight from Apple not to have an audio port. And Apple could have added more USB ports too.

    Thanks for your great review that helped me to take the jump to this magnificent display.
    Reply
  • EmmanuelDécarie - Thursday, January 19, 2012 - link

    Your CMS mangled the URLs.

    Creative Xmod

    http://us.store.creative.com/Creative-Xmod/M/B000I...

    Griffin iMic

    http://store.griffintechnology.com/imic
    Reply
  • anthony11 - Friday, July 20, 2012 - link


    "today I have no less than seven cables"
    Reply
  • snow peak - Saturday, July 21, 2012 - link

    I have similar annoying issue here, while playing songs which in iTunes on my new MBA(mid-2012) out via TB display's speaker.
    The audio data stream is routed from MBA to ThunderBolt display, and just one apple's USB keyboard connected to ThunderBolt display.

    You might be interested in trying experiment without external storage attached on TB display, and get same result with mine.

    The symptom is little static noise came with music play at beginning, and then the music gradually merged by static noise after a few of minutes or a couple of hours.

    I think it caused by hardware power ground layout issue, so no confidence apple could fix it via firmware update and no idea if I should return both my new MBA and TB display.

    This is really annoying!!
    Reply
  • paulrmc - Tuesday, August 07, 2012 - link

    Hi Anand,

    I don't know whether you looked at comparing directly connected USB drives to same drives hooked up to the TB Display. I was shocked today when I found out the difference. I'm using a USB 2.0-connected LaCie 2GB drive as a secondary drive to my dual-SSD mid-2011 Mac mini Server. Using BlackMagicDesign's Disk Speed Test I came to following results:
    - directly connected to the back of the Mac mini: 29MB/sec write, 33 MB/sec read
    - connected to the TB Display: 7.5MB/sec write, 11.7MB/sec read.
    I'm running OS X Mountain Lion 10.8 on the mini, and the tests were repeatable, after fresh power down - reboot cycles.

    Any ideas? Anyone?

    Paul
    Reply
  • hiscore - Tuesday, August 20, 2013 - link

    I have just discovered the source of my Thunderbolt display distorted audio issue. It is being created by my Drobo which is interfaced through iSCSI. I can replicate the issue every time when moving files to it. Granted the files are moving from my Pegasus array, so I guess there may still be some questions.

    I have been in constant contact with Apple support, Promise support, and Drobo support. Hopefully a fix will be supplied from someone.
    Reply
  • krakago - Saturday, June 07, 2014 - link

    Unfortunately this display is now a dinosaur, and a very expensive one. As of 2014 there are finally more devices and computers appearing with Thunderbolt, but peripherals tend to be very expensive and meanwhile USB 3 has become pretty much ubiquitous. That makes this Apple display unacceptably crippled. If it were cheap that could be overlooked, but it sells for a premium price. I've been looking for a display to use with my Macbook Pro, and comparing the price and features of the Apple offering to their competitors I can't see a good reason to choose the Thunderbolt Display. Apple really needs to bin this thing and bring out a modern version. Reply

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