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I'm not sure if this question belongs here, so please be kind if it doesn't. I just happen to have two question that are related to the same project, that's why I combined them. I'm building a custom PC and I need to know this.

1)When they say

PCI Express 3.0 x16
    2 (single @ x16 or dual @ x8)

PCI Express 2.0 x16
    1 @ x4

In the specs why does it say PCI Express 2.0 x16 then 1 @ x4. Why does this say 1 @ x4 under x16 (isn't this contradictory)? What does 1 @ 4x mean. Also what does 2 (single @ x16 or dual @ x8) mean. Why is there a difference between single and double. Is 16 and 8 the pci express speed.

2)How do you know how many watts you need for a power supply. Is it mostly intuition or do you go by a certain rating.

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migrated from electronics.stackexchange.com Feb 25 '12 at 19:38

This question came from our site for electronics and electrical engineering professionals, students, and enthusiasts.

    
This is off topic here but is on topic at superuser. –  Kellenjb Feb 25 '12 at 18:54
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1 Answer

up vote 2 down vote accepted

1:

From the description, I assume this is a motherboard with three physical PCI-E 16x slots.
In this case it means that the PCI-E lanes are distributed in a certain fashion:

  • Option A:
    1. PCI-E 16 Lanes
    2. No PCI-E Lanes
    3. PCI-E 4 Lanes
  • Option B:

    1. PCI-E 8 Lanes
    2. PCI-E 8 Lanes
    3. PCI-E 4 Lanes

It's important to note that the physical connector for all three slots is a PCI-E 16X connector, so you can plug a PCI-E 16X device into any/all of them. However, the available bus bandwidth (e.g. the amount of data that can be transfered over the PCI-E bus in a specific period of time) varies, depending on the number of lanes.

PCI-E devices automatically adjust to the number of available lanes. Plugging your main video-card into the 4-lane slot will probably affect the graphics performance.

Here is an article from tom's hardware looking at how PCI-E bandwidth (which is a direct function of number of lanes) affects performance.

Different processing loads effect how much bandwidth a graphics card uses. Here is a discussion on how a GPU configured as a dedicated PhysX processor is effected by available bandwidth. PhysX works far better under low bandwidth situations then normal graphics-card loads.
However, it's probably safe to assume that the only time someone would want to place a GPU in the third slot is after they already have GPUs in the first two, and therefore, using the third GPU as a dedicated physics processor is a fairly reasonable configuration, which is probably why the motherboard is laid-out in the manner it is.


2:

With power supplies, you need to ensure that the wattage of every component in your computer added together is less then the power supplies ratings.

Therefore, you need to look-up the power rating for your CPU and graphics cards (which are the primary power consumers in a modern computer), and add something like ~10W (It should be specced on the drives, but they're generally pretty power-efficent) per CD Drive and hard drive. Finally, add 100W or so for headroom, and there you go.

It's generally a good idea to have a bit of headroom, as running a power supply right at it's wattage ratings will stress it more then running it below it's rated power. If you have a 500W power supply, and you draw 500W from it, it's lifetime will be shorter then if you only draw 400W from it.

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Let me check that I get this, the physical size is 16 lanes, but the number of available lanes is 4 for the first, etc... So basically there is a difference between the size of the slots and physical lanes, and the literal lanes, and the card will adjust? –  rubixibuc Feb 25 '12 at 3:54
    
@rubixibuc - Yes. The first slot has 16 lanes if there is no card in the second, and 8 if there is a card in the second. The third always has 4 lanes. Basically, look at it this way. There is a total of 20 lanes. 4 always run to the third slot. There are 8 that always run to the first slot, and an additional 8 that are switched between the first and second slot, depending on if there is a card in the second slot. –  Fake Name Feb 25 '12 at 4:23
    
However, all the connectors are physically capable of supporting a 16 lane card. The cards auto-negotiate the number of required lanes. If you follow the link above (Specifically here, to toms's hardware), you will see that the way they restrict the number of lanes to a card is to tape over some of them. –  Fake Name Feb 25 '12 at 4:25
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