Final Words, and a Refresher on the Benefits of Serial Buses

We hope you enjoyed this brief tour of the Xbox 360's motherboard, being the last weekend before Microsoft's official retail launch, we figured it would be a good send-off.

We'll actually end this tour of the 360's motherboard with a little comparison point in the advancement of bus technologies. For the past several years, the PC industry has seen a transition away from older parallel bus interfaces (IDE, Parallel Ports, traditional FSBs, PCI/AGP) to much faster serial buses (SATA, USB, Hyper Transport, PCI Express). The Xbox 360's motherboard actually gives us an opportunity to present a good comparison of the benefits of a serial interface, from a layout perspective.

The link between the Xenon CPU and the Xenos GPU in the Xbox 360 is actually two 10.8GB/s serial buses, which you can clearly see from the picture below. Note the clear definition of the traces the clean routing, to the point where you can count the individual data, address and clock lines:

This is similar to the type of routing you'd see connecting two Hyper Transport ports on an AMD motherboard.

Now let's take a look at the parallel GDDR3 interface between the ATI GPU and the Samsung GDDR3 memory:

Note the significant increase in traces, attributed to a much wider parallel interface than what exists between the CPU and GPU. The more complex routing of parallel buses becomes an even bigger problem as devices demand more bandwidth, which is why we have seen such a large scale transition to serial interfaces.

Up Close and Personal with the CPU, GPU and... Yonah?
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  • Bally900 - Monday, June 2, 2008 - link

    Hi everyone.
    I have removed the gpu clamp from the bottom of the xbox motherboard, there seems to be some blown components, therefore I was hoping there was some good photo's of the motherboard with gpu xclamp removed?? Any help is v much appreciated.
    Reply
  • Kensei - Sunday, November 20, 2005 - link

    FYI... have a look at last Friday's WSJ article on the XBox 360. Lots of interesting information on how the XBox is made in China.

    Kensei
    Reply
  • Clauzii - Sunday, November 20, 2005 - link

    I think there is too much ´splatter´ on that GPU-tingy.. Reply
  • agnot - Saturday, November 19, 2005 - link

    Why was Hypertransport classified as a serial bus? As implemented on K8, it's a 16-bit wide data bus in each direction, so 32 pins in total for data per HT link. Moreover it doesn't have any SERDES logic (serializer/deserializer that converts parallel data to serial data and vice versa). This gives Hypertransport a latency advantage over serial links that require these extra steps, and low latency, as I understand it, was one of the main concerns when developing Hypertransport. Reply
  • segfault7 - Sunday, November 20, 2005 - link

    I wondered the same thing. Althought the author is confused on this point he does make a nice comparison between serial and parallel buses.

    "Note the clear definition of the traces the clean routing, to the point where you can count the individual data, address and clock lines:"

    This statement also is a little misleading since every serial bus that I have looked at (PCIe/SATA/IB) dervives the clock from the data stream. Additionally the address and data are typically sent on the same lines in the form of a packet.

    Good article. I'm looking forward to the flurry of xbox 360 hacking that is about to ensue.
    Reply
  • TheInvincibleMustard - Friday, November 18, 2005 - link

    I like the inclusion of the little thumbnails with red circles on them. It's a simple yet effective method of communicating just where something is on the motherboard. Props to whoever proposed that idea!

    -TIM
    Reply
  • icube - Friday, November 18, 2005 - link

    I'm pretty sure that mystery chip is the custom hdtv encoder/scaler created by the old WebTV team for the xbox360. Maybe it has some other functions as well though.

    See: http://blogs.mercurynews.com/aei/2005/08/a_walk_th...">http://blogs.mercurynews.com/aei/2005/08/a_walk_th...

    -----------------

    Robbie Bach, the chief Xbox officer, even came down for a visit once to make sure all the WebTV folks stayed aboard and helped with the 360. He knew that they had a lot of options in the valley. One of the chips they designed was a TV encoder that would support the TV-side of the system.

    -----------------
    Reply
  • yacoub - Friday, November 18, 2005 - link

    That comparison between serial and parallel on the last page was very informative. Sometimes a picture really does speak for a volume of words. Very cool. Reply
  • mlittl3 - Friday, November 18, 2005 - link

    This is a great breakdown of a very custom, sophisticated motherboard. I was wondering if Anandtech could do the same thing with another custom, sophisticatd moetherboard...the one in the Powermac G5. That's got to have some pretty cool features also. Reply
  • stmok - Friday, November 18, 2005 - link

    I'm always curious...We can modify the current Xbox 1 to run Linux...How about the 360? :) Reply

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