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Thunderbolt 103: Getting Inside The Controllers

Everything You Need To Know About Thunderbolt
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As a technology, Thunderbolt operates similarly, regardless of the controller you're using. For the sake of our technical discussion, we're using Intel's previous-generation two-port Light Ridge chip.

The Thunderbolt controller on your motherboard is always in host mode, with a second-gen PCI Express interface to either a Sandy/Ivy Bridge-based CPU or PCIe-equipped chipset (and one or more available DisplayPort inputs).

Inside the controller, you find a PCI switch and a collection of DMA engines collectively referred to as the Native Host Interface (NHI). The PCI switch enables connectivity for downstream devices, while the NHI is used for software protocols and device discovery (Plug and Play detection). The Thunderbolt switch, marries, if you will, the DisplayPort and PCIe inputs into a single connection.

Recall that each Thunderbolt port requires two channels, one for device I/O and another for display signaling. In the case of Intel's Light Ridge, four channels output to two Thunderbolt ports.

When you have a daisy chain or an endpoint device, Intel's Thunderbolt controller chip provides a PCIe 2.0 x4 downlink. However, the company also enables broader flexibility for attaching multiple components. With four connected, for example, you could configure the downlink as four individual PCIe 2.0 x1 links. According to Intel, Cactus Ridge (2C/4C) can be configured in the following ways:

  • 1 * x4: one device of four lanes
  • 4 * x1: four devices of one lane each
  • 2 * x2: two devices of two lanes each
  • 1 * x2 + 2 * x1: One device of two lanes and two devices of one lane


Most of the time you'll see one device attached to one Thunderbolt controller, yielding a 1 * x4 configuration. However, there are situations where a single Thunderbolt controller might control multiple devices.

Apple's 27" Thunderbolt Display is a good example. Its controller is responsible for communicating with a USB hub, a FireWire 800 port, Gigabit Ethernet, and a FaceTime camera. Each device requires an interface to the Light Ridge controller, with its internal PCI switch divided into four single-link lanes when it's in switch mode. Each lane is then mapped to a device controller (USB, FireWire, Ethernet, and the camera). This setup doesn't negatively affect the display itself because, remember, I/O and DisplayPort are on different channels.

In its current form, Thunderbolt employs PCIe fanout mapping. This means that daisy-chained Thunderbolt devices are routed through the internal PCI switch of the controller ahead of it in the sequence. As a result, the first device in the chain always enjoys the lowest latency.

The PCI Express protocol also influences latency. For example, a storage device on a desktop PC might negotiate precedence over a capture card, and you'd assume that Thunderbolt should probably operate the same way since it uses PCI Express signaling. However, each device plays a role in the PCI arbitration of devices connected downstream. Thus, throttling is quite noticeable if every piece of hardware in a Thunderbolt daisy chain operates simultaneously.

A downside of device arbitration is wasted bandwidth due to inefficient management. This is potentially an issue with fanout mapping because the PCI switch located in the preceding controller manages downstream devices. It's possible to circumvent the drawbacks to fanout mapping by using PCI direct mapping, illustrated in the diagram above. This method passes the Thunderbolt signal through each controller's internal switch, completely bypassing the PCI pathway. It'd ultimately impose a greater negotiation burden on the first system's PCI switch, but it delivers the benefit of greater control over bandwidth/resource allocation.

Thunderbolt controller firmware, as it is implemented by Intel and Apple, uses fanout mapping. Direct mapping is possible and is fully compatible with the Thunderbolt standard. But there's no word yet on if or when it might be a user-selectable option.

Display all 68 comments.
Top Comments
  • 22 Hide
    shoelessinsight , June 12, 2012 4:52 AM
    Active cables are more likely to have defects or break down over time. This, plus their high expense, is not going to go over well with most people.
  • 21 Hide
    mayankleoboy1 , June 12, 2012 4:35 AM
    thunderbolt will fail after external PCIE standard is implemented
  • 19 Hide
    Pyree , June 12, 2012 4:23 AM
    I was really hoping to see some eGPU benchmark. Oh well, I guess I have to wait.
Other Comments
  • 19 Hide
    Pyree , June 12, 2012 4:23 AM
    I was really hoping to see some eGPU benchmark. Oh well, I guess I have to wait.
  • 21 Hide
    mayankleoboy1 , June 12, 2012 4:35 AM
    thunderbolt will fail after external PCIE standard is implemented
  • 15 Hide
    mayankleoboy1 , June 12, 2012 4:36 AM
    for more insight of thunderbolt fail and Intel's lying :

    http://semiaccurate.com/2012/06/06/intel-talks-about-thunderbolt/
  • 22 Hide
    shoelessinsight , June 12, 2012 4:52 AM
    Active cables are more likely to have defects or break down over time. This, plus their high expense, is not going to go over well with most people.
  • 5 Hide
    A Bad Day , June 12, 2012 5:39 AM
    Looks like I'm going to steer clear of Copperpeak for my future build.
  • 16 Hide
    GI_JONES , June 12, 2012 5:43 AM
    Cost is going to kill this.
  • 12 Hide
    mayankleoboy1 , June 12, 2012 6:25 AM
    ^
    because "thunderbolt" sounds much sexier than "HDBaseT " ?

    and with apple, its all about the sexiness, not functionality/practicality.
  • -1 Hide
    JOSHSKORN , June 12, 2012 6:35 AM
    Prediction: We will see Thunderbolt available on SmartPhones. When we do, this port will be able to handle a monitor, external hard drives, speakers and many other USB devices through its Thunderbolt docking station. Obviously a SmartPhone won't need to be attached to a webcam. This will become the future desktop...that is, if it can run Crysis. LOL Had to add that in there. :) 
  • 3 Hide
    pepsimtl , June 12, 2012 7:02 AM
    I remenber scsi interface ,so expensive ,just the company (server) use it .
    and sata interface replace it.
    For me Thunderbolt is the same song
    I predict a sata 4 (12gb) or usb 4 ,soon
  • 13 Hide
    archange , June 12, 2012 7:21 AM
    Hot, expensive active cables cannot be anything else than niche.
  • 7 Hide
    emad_ramlawi , June 12, 2012 7:31 AM
    Technology for the rich ...

    i can wait a couple of minutes for files to be copied on USB 3.0 which is universal and open standard .

    thanks intel but ill pass
  • 4 Hide
    rex86 , June 12, 2012 7:43 AM
    I really hope that this is going to be another flop. USB3 is just fine for almost everything. I do agree that we need open external PCIe standard. We're already paying too much to Intel.
  • 6 Hide
    beetlejuicegr , June 12, 2012 9:46 AM
    Only thing usefull i can see right now is a laptop with intel/amd gpu using it to get access to high end external discrete gpu. All the other possibilities are not needed to be through thunderbolt.
  • -1 Hide
    Anonymous , June 12, 2012 10:15 AM
    What is 125oF in real measurements?
  • 0 Hide
    chesteracorgi , June 12, 2012 12:29 PM
    Thunderbolt is a wonderful innovation and alternative, but hardly ready for prime-time. Even on the Mac platform there is a derth of devices that use thunderbolt. Will thunderbolt be USB or Firewire?
  • -2 Hide
    A Bad Day , June 12, 2012 1:22 PM
    emad_ramlawiTechnology for the rich ...i can wait a couple of minutes for files to be copied on USB 3.0 which is universal and open standard . thanks intel but ill pass


    And if USB 3.0 is too slow, then use two of them (flashdrives in RAID 0 anyone?).
  • 2 Hide
    Anonymous , June 12, 2012 1:38 PM
    I can't believe how narrow some other people comments are. This new standard is for high end users and later others as well once prices start to drop. USB3 eSata when you are working with files that are 10s of Gigs in size are just too slow. Thunderbolt is fast plus easy plug and play for so many future possibilities. There are already a number of hard drives, raids arrays, Displays and now expansion Link PCIe adapter from Mlogic. Already it's potential is becoming interesting.
  • 1 Hide
    josejones , June 12, 2012 2:04 PM
    What are the costs of these new Thunderbolt ports on new z77 motherboards and are they 3rd party?

    I was considering getting the new Gigabyte Z77X-UP5 TH for my new i7 build but, not because of the Thunderbolt ports, but rather, due to the alleged lower mobo temps, which I'm concerned about with our 85F (31C) indoor temps. I await a serious review. These new boards are supposed to be available by the end of June:

    Gigabyte's Hardcore Thunderbolt Demo with GA-Z77X-UP5 TH Motherboard
    http://www.youtube.com/watch?v=deImUH8aUHQ

    Gigabyte Ultra Durable 5 at Computex, shows much lower temperatures
    http://www.youtube.com/watch?v=rLggGetNR14

    z77 Motherboard Discussion
    http://www.tomshardware.com/forum/308058-30-motherboard-discussion
  • 1 Hide
    josejones , June 12, 2012 2:17 PM
    ^ "are they 3rd party?"

    of course not = Intel
  • 1 Hide
    rantoc , June 12, 2012 2:25 PM
    I don't see this tech taking off in the consumer sector any day soon, its to expensive compared to the alternatives and with active cables it ensures that it will remain so! Few have use of the extra bandwidth provided where an usb3 will remain more than sufficient for the masses and the equipment/cables remain cheap!
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