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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  • repoman27 - Monday, September 26, 2011 - link

    Well, the display itself is driven by DisplayPort. The ATD does require Thunderbolt though, otherwise they couldn't have included all the other stuff. Apple is releasing this as an accessory for new Macs that have Thunderbolt ports. It does not make anything obsolete. You can continue to use pretty much any display on the market with either an older Mac that only supports DisplayPort or a new Thunderbolt equipped Mac although you might need an adapter or three. Most display manufacturers are probably not going to go the Thunderbolt route, and will stick with HDMI for 1920x1080 panels, DisplayPort for the higher resolution jobs, and DVI ports until the cows come home.

    Apple may soon cease production of discrete DisplayPort sink devices, but that in no way means that they've turned their back on the standard. Thunderbolt ports are indeed backward compatible with DisplayPort devices, but DP ports have no way of being forward compatible with Thunderbolt devices.

    I find it odd that there are a lot of folks asking where all the Thunderbolt devices are, and then when one is released, everyone complains that they would need to buy a new Mac in order to use it. Thunderbolt devices are designed to be used with Thunderbolt enabled PC's, you're either in the market for them or you're not, end of story.
    Reply
  • eureka_swe - Monday, September 26, 2011 - link

    If you have say 2-3 FireWire 800 disk connected to the Display, do i need to Eject this on the Macbook Pro evry time i will disconnect the Display or is it just to pull out the Thunderbolt cable and the disk is still good ?

    its a big question for me that have 7 FW Disks :)
    Reply
  • Constructor - Monday, September 26, 2011 - link

    That you need to unmount volumes which are about to be disconnected doesn't change.

    The external FireWire controller is basically indistinguishable from one on the motherboard for the OS. And the file system is still the same.

    So unmounting any external FireWire, USB and (directly) Thunderbolt device is a must and will remain so until the file system is fundamentally altered in that regard.
    Reply
  • iSayuSay - Monday, September 26, 2011 - link

    Agreed on some Anand's points. Hooking a Macbook Pro/Air with Thunderbolt onto that display may looks cool, it might instantly look like a desktop. But the real performance is never going to be excellent, it only become acceptable - very good range.

    If I going to cash in such amount of money (consider basic 11" MBA for $900 + 27" Apple Thunderbolt display for another $1000, and for elegance purpose .. don't forget TrackPad/MagicMouse + Wireless keyboard for another $140) ..

    In total you already spend same amount with hi-end iMac 27" which performs much better, yeah sure .. it's not portable, but I don't carry around my MBA too much either :p

    So I say .. while looks nice and cool, I'm not ready to follow world trend to go mobile with today's performance
    Reply
  • dgingeri - Monday, September 26, 2011 - link

    I flat out refuse to buy any Apple products. I have 3 reasons behind this now. At first, it was because they did so much business with Foxconn, and the horrible living conditions of the Foxconn employees who built Apple products. Then I found out about their excessive patent applications on a great many things that have been common habits of electronics manufacturers for over a decade. Now, I add on the excessive legal activity and flat out cheating in court trying to ban competing products.

    Apple is quite simply an evil company. Do not buy their stuff. Do not support the attempt at becoming a dictatorship of the world under the guise of business.
    Reply
  • Mystermask - Sunday, October 09, 2011 - link

    You're a hypocrite if you accuse Apple for what happens in chinese factories.
    1. Name me one brand that does not go for cheap as possible production. And why? Just have a look at those endless discussions where people try to prove that they can build a PC that has the same specs like e. g. a MacBook Pro but is even cheaper. How do you believe this is possible?Aren't you happy when you can buy a PC for €300 when all other cost 500€? Who do you believe is paying that bill? The "race to the bottom" has a long history that started in the PC industry when almost all HW vendors decided to go with DOS / Windows and vendors could only distinguish themselves in the market by being cheaper than others. And consumers gladly bought the cheapest - unable to distinguish value and price - effectively cutting into their own flesh because this has cost all production jobs in the PC industry in western countries.
    2. Have a look at yourself. Do you wear Nike, Addidas, Reebook, Lacost, you-name-it? Do you use Dell, Acer, HP, HTC, Samsung, .. How and where are they produced?
    ..
    All of a sudden, reality looks not as easy and religios blind Apple bashing is certainly neither a solution nor would that change anything for Foxconn or countless other factory workers in China, Vietnam or wherever people have to offer their work for cheap to survive because of western ignorance and greed.
    Reply
  • The_Countess - Monday, September 26, 2011 - link

    i cant help but notice that you could do all this with just 2 cables for far less money and without the expensive proprietary technology.
    1 display cable + 1 usb3.0 cable connected to a USB hub.

    it'll even be fully backward compatible with practically every laptop still in operation on the planet!

    you could even run the display over usb3.0 although I'm not sure how much bandwidth would remain after that.
    Reply
  • repoman27 - Monday, September 26, 2011 - link

    You can get more than 800 MB/s of throughput from a RAID connected to a USB 3.0 hub?... While getting over 780 Mbps over a USB 2.0 Gigabit Ethernet adapter?... (There are no USB 3.0 ethernet adapters in the wild yet.) While recording 720p video from a camera connected to the hub?... While playing back audio from a USB audio interface?... And how are you going to connect a FireWire device to your USB 3.0 hub? There's no adapters for that, and besides, you're out of ports on your hub because the only USB 3.0 hubs on the market have just 4 downstream ports. Looks like you'll need to get another hub and cable to connect a keyboard and mouse or USB disk.

    And yes, you could connect your version to any laptop and enjoy all of those devices sharing less than 40 MB/s of bandwidth, or connect it to a USB 3.0 enabled machine and get less than 400 MB/s. Thunderbolt gives you 2500 MB/s. A 2560x1440 display such as this requires 5.8 Gbps of bandwidth, USB 3.0 can't even hit 3.2 Gbps of real world throughput yet.
    Reply
  • Constructor - Monday, September 26, 2011 - link

    USB ist not remotely capable to drive a display at the same performance level as the built-in GPU in the computer can. Those USB graphics adapters are orders of magnitude slower than that.

    And both 10Gb/s input + 10Gb/s output at the same time as a high-res display (or even two of those with a small output performance hit) are completely out of range for even USB3.

    Add to that the horrible latency problems you've got with USB, which are completely absent with Thunderbolt. (Which is one reason why you couldn't even have a full-performance FireWire port through USB3 since proper FireWire has very low latencies as well, which USB simply can't emulate.)

    USB is a very complicated and not too fast peripheral interface.

    Thunderbolt is effectively a part of the motherboard channeled through a thin cable to pluggable external motherboard expansions (the "motherboard" in the Thunderbolt display is effectively made a part of the computer's motherboard once you plug it in to the Mac).

    Completely different deal.
    Reply
  • AnnonymousCoward - Tuesday, September 27, 2011 - link

    You're comparing USB to a GPU? ok...

    "orders of magnitude"? USB is 5Gbps; apparently the alternative is 500Gbps!! Sorry, it's not even 50Gbps, or even half of that.

    "horrible latency problems you've got with USB" - I have no perceivable latency on my USB mouse, and I'm sensitive to it.

    "USB is a...not too fast peripheral interface." - 5Gbps isn't fast? No external connection aside from raw video can utilize that!

    Face it, you don't know what you're talking about.
    Reply

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