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
Best Answer
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