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Observer biswagourav
Observer
14,180 Views
Registered: ‎10-09-2007

PCIe Lane issues

My home pc intel motherboard is having two x1 slots and 1 x16 (that is intended for a GPU card) slot. we will be using an avnet dev kit which has xilinx v5 and x8 pcie card. can the x8 card be installed in an x16 slot?
 
Maybe server boards are different, or does it even matter, just install in the x16 slot and the hardware/software negotiates the proper number of
lanes?
Please suggest.
 
Biswa
 
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6 Replies
Xilinx Employee
Xilinx Employee
14,164 Views
Registered: ‎08-07-2007

Re: PCIe Lane issues

Hi,

Yes you should be able to use the x8 card in the x16 slot and the MB should negotiate the link to a x8. However, occasionally we find that some x16 slots are looking to be used with certain types of cards and if BIOS does not find what it wants, it may not allow the slot to function. I have two PCs I use, one where my x8 ML555 will work in the x16 and another where it gives me problems. Usually it makes this determination based on the Vendor ID and/or Class Code of the card. You will just have to try it and see. If the card is recognized and works, then you will be fine.

-John
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Observer filmil
Observer
14,142 Views
Registered: ‎08-15-2007

Re: PCIe Lane issues

A x8 card should work fine in a x16 slot.  The link training at startup will determine automatically how many lanes are used.

However, there is an issue with some motherboards and their x16 slots. These slots are then usually designated as "PCIe for graphics" or some such (caution: some vendors will 'forget' to specify this bit of info, no way to be sure except to dig into the documentation for the northbridge of your chipset).  The BIOS will (apparently) probe for a video card in the PCIe slot, and if it does not find any, it will turn the PCIe root complex off.  This will prevent your card from begin seen by the OS.  This is not what you want.

So you need a way to keep the PCIe root complex on after bootup.  For my machine (Fujitsu-SIemens Esprimo P, it uses the iQ965 chipset), I found out that the only way to keep the x16 PCIe root port on after bootup if a non-graphic-card is plugged into x16 is to specify in the BIOS that my default display adapter is on PCI Express. (set: "PEG" for graphic adapter in the BIOS.  Looks silly, but read on).

If you do this, and plug some other (i.e. non-video) card into the slot, e.g. your own design, the BIOS will notice that there is no graphic card in the PCIe slot, will revert to using onboard video adapter for display, but will not turn the PCIe root port off.  This will give you what you want: a fully functional x16 PCIe slot.

HTH,
f



Message Edited by filmil on 01-08-2008 09:17 AM
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Observer biswagourav
Observer
14,136 Views
Registered: ‎10-09-2007

Re: PCIe Lane issues

This sounds pretty good. Thanks..Can you specify exactly where i need to make the change to keep the root complex on even without graphics card.
 
Biswa 
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Observer schradi
Observer
14,123 Views
Registered: ‎01-07-2008

Re: PCIe Lane issues

Hello Biswa,

which mainboard do you use?

Greetings,

Schradi
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Observer filmil
Observer
14,114 Views
Registered: ‎08-15-2007

Re: PCIe Lane issues



@biswagourav wrote:
Can you specify exactly where i need to make the change to keep the root complex on even without graphics card.


It probably depends on the BIOS you use.  I don't know if yours will work this way.  On my machine, the menu Advanced->Chipset Settings->Graphics Adapter (or something similar).

f


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Observer biswagourav
Observer
13,952 Views
Registered: ‎10-09-2007

Re: PCIe Lane issues

Hello,
 
Can anybody suggest me a chipset on which the x8 card work fine?
 
Biswa
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