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Abidal

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May 17th, 2024 15:42

What does mean x4 electrically?

Hi,
Can someone explain to me concisely as possible what this mean: PCI Express 3.0 x16 (x4 electrically) card slot (slot 1)  and what is the difference between it and that: PCI Express 3.0 x16 card slot (slot 2)
I found this in the T7910 manual.
Thanks a lot.

ProfessorW00d

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2.1K Posts

May 17th, 2024 17:00

x16 is the form factor of the slot, indicating the physical length and maximum number of PCIe lanes, while x4 electrically indicates that only 4 PCIe lanes are actually wired on the motherboard

Abidal

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17-05-2024 20:40 PM

@ProfessorW00d​ Thank you for replying.
Whats it's the logic behind having 4/16 lanes wired? For example, if I have a GPU x16, if I use it in this PCIe, it will not work? 
Thanks in advance.

ProfessorW00d

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17-05-2024 23:25 PM

@Abidal​   the primary logic is that it is cheaper.  I am not familiar with the Precision Workstations particularly. I am familiar with Alienware, and for many years they were putting out Aurora gaming rigs where the primary PCIe slot was x16 mechanically, x8 electrically. So the graphics cards were functioning with half the band width of a fully wired slot. There was a lot of complaining in the Alienware Community around this fact, until we learned that half the band width was most likely more that you would ever need for the gaming applications. So I think usefulness of the PCIe slot that is x4 electrically would depend on your application and the band width requirements. It may well be sufficient for the workstation purposes.

mazzinia_

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May 18th, 2024 01:57

It's a bit more complicate, and not about being cheaper. Cpus have a maximum number of pcie lanes they can handle. In the case of the xeons in the 7910 , each cpu handles 40 lanes.
In this specific case :

SLOT1 PCI Express 3.0 x16 (x4 electrical), 16GB/s
  SLOT2 PCI Express 3.0 x16, 16GB/s
  SLOT3 PCI Express 2.0 x16 (x4 electrical), 16GB/s
  SLOT4 PCI Express 3.0 x16, 16GB/s
  SLOT5 PCI
  CPU2 SLOT1 PCI Express 3.0 x16 (requires second CPU)
  CPU2 SLOT2 PCI Express 3.0 x16 (requires second CPU)

As you can see, the slots handled by cpu1 ( aka the one that must be anyway present to have the system to work ) use all 40 lanes ( 4/16/4/16 )

ProfessorW00d

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18-05-2024 03:51 AM

@mazzinia_​    thanks for clearing that up!

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