Third Long PCIe Slot On High-End Z77 Boards Wired to CPU, Works Only with Ivy Bridge

[btarunr]

It turns out that the "third" long (physical x16, electrical x4) PCI-Express slot on most higher-end Z77 chipset-based motherboards, across vendors, are wired to the CPU, and not the Z77 PCH, as the media assumed. Early buyers of these motherboards were greeted by an informative sticker stuck to the third slot, which tells them that to use the third slot, a 3rd Generation Core "Ivy Bridge" processor must be installed, although the motherboard very much supports 2nd Generation Core "Sandy Bridge" processors.

This can be explained by taking a close look at the block diagram of Intel Z77 Express system. Z77, in combination with "Ivy Bridge" processors, allows the CPU root complex to drive three devices. Its single PCI-Express x16 link can be arranged in three ways: x16/NC/NC; x8/x8/NC; and x8/x4/x4. As you can see, the third long slot is taken into the configuration. Intel figured out that since PCI-Express 3.0 x4 offers bandwidth comparable to PCI-Express 1.0 x16 to gen 3-compliant graphics cards, it's wise if the third electrical x4 slot is also wired to the CPU's PCIe root complex. This renders most high-end LGA1155 motherboards with such CPU-driven third x16 (x4) slots 3-way SLI/CrossFireX-capable. Sweet.



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[1c3d0g]

That's a fair trade-off. If you can do triple-SLI, you can afford an Ivy Bridge CPU.

 

[slyfox2151]

That's a fair trade-off. If you can do triple-SLI, you can afford an Ivy Bridge CPU.

lol fair point.

 

[Chaitanya]

anyways most people buying these CPU's in first place wont be using more than 2 video cards.

 

[DonInKansas]

anyways most people buying these CPU's in first place wont be using more than 2 video cards.

Why?

 

[Vulpesveritas]

Why?

LGA1155= Mainstream.

 

[The Von Matrices]

An extra PCIe 2.0 x4 was present in the 1155 Sandy Bridge processors as well (connected to the processor, not the PCH); it just wasn't enabled except when paired with a server chipset. See the Asus P8B WS for example and a review of the server chipsets.

 

[EpicShweetness]

 

[Grings]

I wonder if any motherboard manufacturers will make the 3rd slot switchable between the chipsets pci-e2 lanes and the cpu's?

It would suck if putting a sound card or raid controller etc in that slot makes one of you're graphics cards drop to 4 lanes.

 

[eidairaman1]

LGA1155= Mainstream.

ya and soon to be EOL with 1150 coming up shortly

 

[dj-electric]

Everything that intel makes is soon to be EOL

 

[eidairaman1]

Everything that intel makes is soon to be EOL

aint that right heh

 

[Steven B]

however it isn't true on all board, for the UD3H its wired to PCh, for UD5H its CPU.

 

[RejZoR]

What a rubbish crap. Most higher end boards in the long past had 2x full x16 PCIe. Like long forgotten X58

 

[Jurassic1024]

LGA1155= Mainstream.

More specifically, a lower number of PCIe 3.0 lanes from CPU compared to X79 CPU's. PCIe 3.0 x4 may be equivalent to PCIe 2.0/1 x8, but in a triple gpu setup with these new PCIe 3.0 GPU's, you're still going to run into a bottleneck. x4 will work, but not near as well as x8 or x16.

http://www.anandtech.com/show/5458/the-radeon-hd-7970-reprise-pcie-bandwidth-overclocking-and-msaa

 

[Jurassic1024]

An extra PCIe 2.0 x4 was present in the 1155 Sandy Bridge processors as well (connected to the processor, not the PCH); it just wasn't enabled except when paired with a server chipset. See the Asus P8B WS for example and a review of the server chipsets.

Except all those slots are wired for PCIe 2.0, and we're talking about using the 3rd slot for another PCIe 3.0 GPU in a slot wired for PCIe 3.0 x4. Am I wrong?

 

[Jurassic1024]

I wonder if any motherboard manufacturers will make the 3rd slot switchable between the chipsets pci-e2 lanes and the cpu's?

It would suck if putting a sound card or raid controller etc in that slot makes one of you're graphics cards drop to 4 lanes.

I found it odd, if i put my x1 sound card in my second x16 slot (wired for x8), my GPU in the first x16 slot downgrades to x8.

 

[Aquinus]

Except all those slots are wired for PCIe 2.0, and we're talking about using the 3rd slot for another PCIe 3.0 GPU in a slot wired for PCIe 3.0 x4. Am I wrong?

It is wired for PCI-E 2.0 x4 on the 3rd PCI-E x16 slot.

Asus' Site and NewEgg both says this for Z77 Sabertooth:

Expansion Slots
2 x PCIe 3.0/2.0 x16 (x16 or dual x8) *2
1 x PCIe 2.0 x16 (x4 mode, black) *3
3 x PCIe 2.0 x1
...
*2: PCIe 3.0 speed is supported by Intel® 3rd generation Core™ processors.
*3: The PCIe 2.0 x16 slot shares bandwidth with PCIe 2.0 x1_1 slot, PCIe 2.0 x1_2 slot and PCIe 2.0 x1_3 slot. The PCIe 2.0 x16 slot default setting is in x1 mode.

Source

Same is true of the P8Z77-V DELUXE.

I found it odd, if i put my x1 sound card in my second x16 slot (wired for x8), my GPU in the first x16 slot downgrades to x8.

That is because that first 1x lane is actually the 9th lane on your first slot. They're switched.

 

[eidairaman1]

in all honesty if you want to do Multi Card Configs go with skt 2011 and skip 1155 altogether

 

[Aquinus]

in all honesty if you want to do Multi Card Configs go with skt 2011 and skip 1155 altogether

It has more PCI-E lanes for a reason. What is there not to like about x8/x8/x8/x8 @ PCI-E 3.0?

 

[Morgoth]

i got 4 full 16x slots... anda bunch of 8x slots

 

[Dos101]

in all honesty if you want to do Multi Card Configs go with skt 2011 and skip 1155 altogether

^^ This. Although there are some 2011 motherboards that are only x8/x8.

 

[newtekie1]

anyways most people buying these CPU's in first place wont be using more than 2 video cards.

Actually, I'd say most people buying these probably won't be using more than 1 video card.

 

[Steven B]

why is everyone hating on Z77 lol

 

[Jurassic1024]

It has more PCI-E lanes for a reason. What is there not to like about x8/x8/x8/x8 @ PCI-E 3.0?

Scaling sucks when you go past 2 cards and it has nothing to do with what PCIe version or number of PCIe lanes your CPU has, assuming you're matching the PCIe version your motherboard and/or CPU supports to the version supported by your GPU(s).

why is everyone hating on Z77 lol

It offers very little. Quicksync and PCIe 3.0 benefit only a small few, and 3770K performance is minimal over the 2600K.

It is wired for PCI-E 2.0 x4 on the 3rd PCI-E x16 slot. :mad

So is the third x16 slot on my P8P67 EVO. It has to do with the limited lanes on the 2nd generation Core CPU's if those 4 lanes are coming from the PCH.

in all honesty if you want to do Multi Card Configs go with skt 2011 and skip 1155 altogether

If you want poor scaling, go ahead.