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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  • decryption - Friday, September 23, 2011 - link

    No mention of how the built in Intel graphics perform on the high res screen? It was an initial concern of mine when the new MBA was released - how it would perform at that high resolution when paired up to a Mac running the 3000 HD graphics?

    Any UI lag? (e.g: mission control, swiping full screen apps, etc.)
    Reply
  • boblozano - Friday, September 23, 2011 - link

    Been using this combo for a couple of days and it's just about perfect. No lag, no complaints.

    I'd probably like usb3 and an sd reader as well, but those are nits.
    Reply
  • MonkeyPaw - Friday, September 23, 2011 - link

    I am surprised there's no card reader, but that's amendable by the USB. I don't understand the lack of audio ports. Macs are primarily media machines--how could they forget audio ports?!

    I guess you could use a USB sound card?
    Reply
  • JasperJanssen - Saturday, September 24, 2011 - link

    My guess is, Apple considers the humble analog audio port to be a dodo, except for use while actually on the move. But that's just speculation.

    All of the recent Intel-based macs that have been released, except the MacBook Air (and the Pro, which separates them), have an analog port that doubles as an optical output by plugging in the right (mini-TOSlink) optical cable -- maybe they would only want to include an audio port if it also included that optical part, which may well either make the port too deep for the regular port-location, and/or simply be too expensive. You could put a deeper port on the side of the monitor instead of the rear, but that would be both ugly and pretty expensive, as you'd have to add a whole extra PCB including a cable that goes to it.

    Come to think of it, I'm not sure that at the location of the other port there is enough depth even for a regular 3.5mm jack, let alone a miniTOSlink variant. A 3.5mm jack is about 15-20 mm long and might well simply not fit between the aluminium back and the display backlight.
    Reply
  • TypeS - Friday, September 23, 2011 - link

    I really don't understand why people ponder this. Intel's integrated graphics don't do much for gaming at high resolutions (do AMD and NVIDIA's IGPS either?), but they've been fine for years now for high resolution monitors. Reply
  • Ryan Smith - Friday, September 23, 2011 - link

    If you check the video Anand made for this article, he goes into more detail about MBA performance hooked up to such a large monitor. Overall the performance is respectable, but there are points where the GPU can't keep all of the GUI animations at 30fps. Reply
  • cactusdog - Friday, September 23, 2011 - link

    Its nice to have less cables but this kit would cost around $2500? You can buy a similar res screen+notebook for around half that and take a few extra seconds to plug in a second or third cable.

    For work purposes most people would only really need to plug in the display cable (hdmi/displayport) anyway, and maybe ethernet if theres no wireless...

    I dont think its worth the price premium unless you're getting it for free.....
    Reply
  • TypeS - Friday, September 23, 2011 - link

    Really... half? Dell wants $100 more for their 27" IPS 2560 x 1440 (16:9) display. And doesn;t come close to matching the capabilities of the Thunderbolt display. And a comparable notebook from Acer or Sony aren't much cheaper than the Macbook Air. So.. where are you getting this ultra portable notebook + 27" IPS display all together for only $1250?

    I don't like Apple much taking "facts" out of your arse is just as dumb as Steve Jobs' "magic".
    Reply
  • cactusdog - Friday, September 23, 2011 - link

    What are you talking about? The thunderbolt display is $1200 and the Dell is $899 but with discount you can get it for $750.

    The macbook starts around $1,300 with 13" screen and integrated graphics, then 15' + graphics is $1,699-$2000.

    You can get a similar size/spec Asus or Samsung with discrete for around $900
    Reply
  • cactusdog - Friday, September 23, 2011 - link

    BTW, this is an apple advertisement. People should understand how things work by now and understand how Apple target their marketing. Reply

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