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After some research, it seems not to be a problem:

Can I be 100% certain that a PCIe x4 card will fit in a PCIe x16 slot?

Does a x4 PCIe mechanical work in a x16 slot in x4 mode?

Even wikipedia says that it works:

PCIe card physically fits (and works correctly) in any slot that is at least as large as it is (e.g., an x1 sized card works in any sized slot); https://en.m.wikipedia.org/wiki/PCI_Express#Physical_layer (Nov 16 2020)

But in the specification of the National Instrument PCIe-6738 PCI Express, 16-Bit, 32-Channel, 1 MS/s Analog Output Device (https://www.ni.com/pdf/manuals/377270b.pdf), in footnote 6 (on page 8), you can read:

"6 Some motherboards reserve the x16 slot for graphics use. For PCI Express guidelines, visit ni.com/info and enter the Info Code PCIExpress."

Then how can we know if it will be compatible on the slot x16 on my motherboard? I purchased the pcie 6738, installed it in the pcie x16 and the PC does not start. Is there a way to solve this?

motherboard:

BaseBoard Manufacturer HP

BaseBoard Product 83F0

BaseBoard Version KBC Version 07.D2.00

pre-built PC model: HP ProDesk 400 G5 microtower pc

There are one PCIeX16 and two PCIeX1

Rémi Baudoux
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  • You purchased what exactly? – Ramhound Nov 16 '20 at 09:39
  • The card PCIe 6738, sorry that it was not clear, I will edit – Rémi Baudoux Nov 16 '20 at 09:50
  • Yes, it will work. – gronostaj Nov 16 '20 at 09:56
  • The PC does not start, and why this statement on pcie 16 by national instrument? – Rémi Baudoux Nov 16 '20 at 10:01
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    So your question isn't _"would this work"_ but "it doesn't work for me, how can I fix this"_? Please [edit] your question to emphasize that and include your motherboard model. Have you tried other PCIe slots on the motherboard? – gronostaj Nov 16 '20 at 10:42
  • no, it is not only my issue as it is pointed out by NI themselves, it seems there is compatibility issue between pcie4 and pcie16 ("reserved for graphic use", which is not proven to be the root cause in my case yet) so I have a general question: are they really compatible all the time? is NI right or not? if they are right, is there a workaround? – Rémi Baudoux Nov 16 '20 at 13:48
  • @gronostaj, I edited the question with the mother board details and tried the only PXIe X16 available on the motherboard – Rémi Baudoux Nov 16 '20 at 14:22
  • Is that a prebuilt PC? Can you add the model? – gronostaj Nov 16 '20 at 14:33
  • https://forums.ni.com/t5/Multifunction-DAQ/Can-I-plug-a-PCI-E-DAQ-card-into-a-PCI-E-x16-slot/td-p/649683?profile.language=en – Rémi Baudoux Nov 16 '20 at 14:42
  • There doesn't seem to be anything unusual about your motherboard, the card should work fine in x16 slot. I've never heard about MoBos not supporting less than 16 lanes in a slot wired with 16 lanes; quite the opposite, I'm aware of graphics cards that have less lanes than that. Check if the NI card works in a different PC. If it does, you can try adapting one of the M.2 slots to PCIe - one of them has 4 PCIe lanes connected (the other one has 1 lane, the card may work but bandwidth will be limited). – gronostaj Nov 16 '20 at 19:22
  • @RémiBaudoux Does the BIOS have an option to force it to use on-board graphics? If there is such an option, it could be set to automatically detect if there is a graphics card present and be making the wrong choice. – Andrew Morton Nov 16 '20 at 20:55
  • @RémiBaudoux If [this](http://h10032.www1.hp.com/ctg/Manual/c06696065) is the correct BIOS (UEFI) manual, then perhaps the "Sure Start Security Event Policy" setting is preventing it from booting when it detects new hardware has been added. HP support should be able to clarify that for you. – Andrew Morton Nov 16 '20 at 21:05

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We contacted HP, they answered that they did not know why. they said that they did not test this PCIeX4 in particular. They did not know how to help, neither where the problem could come from.

This extension PCIEX1 to PCIEX4 is making the job, probably 4 times slower, but luckily, the speed is not critical in this application

https://www.amazon.com/GLOTRENDS-Extension-Limited-Installation-UEX105/dp/B07N39KWHZ/ref=pd_di_sccai_2/143-3411009-1910802?_encoding=UTF8&pd_rd_i=B07N39KWHZ&pd_rd_r=1d43f406-facf-4ba2-9ca1-6c74184cb052&pd_rd_w=VLA82&pd_rd_wg=42p3E&pf_rd_p=c9443270-b914-4430-a90b-72e3e7e784e0&pf_rd_r=4C2E1HW0J48FQ9A0CZPK&psc=1&refRID=4C2E1HW0J48FQ9A0CZPK

Rémi Baudoux
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