NIC not recognized in HP DeskPro 400 G3 PCIe x16 slot

[tpfeffer]

I have an HP DeskPro 400 G3 SFF Business PC bought used from a computer recycling center that I use to host a pfSense firewall.  This PC was first warranted in 2017.  It's outfitted with an Intel i5 6500 cpu, 8 gb RAM, and 250 GB SSD.  

The computer has one PCI Express x1 expansion socket and one PCI Express x16 expansion socket.  The maintenance and setup manual indicates you can install a PCI Express x1, x4, x8, or x16 expansion card in the PCI Express x16 socket.

I'm successfully hosting the pfSense firewall using a dual Broadcom NIC in the x1 expansion socket but I'd like to add additional ports. I've been unsuccessful getting any of three following dual port NICs to be recognized in the x16 socket: the x1 Broadcom NIC that works in the x1 slot, a x1 RealTek, and a x8 Intel PRO/1000 PT.  All three NICs are recognized by pfSense and usable in an older Del desktop.  

I've updated to the latest BIOS (N03 Ver. 02.47) released 4/28/2020.  I've configured the BIOS to "Legacy Support Enable and Secure Boot Disable."  I've enabled both the PCI Express x16 Slot 1 and PCI Express x1 Slot 1 in the BIOS.

I found a helpful topic on Spiceworks forum from July 2017 "Installation of Discrete Graphics Card into Older HP PC" that I've followed without success.

Questions: 

  -  Can I use the x16 slot to host a dual or quad port NIC?

  -  Any suggestions on further steps?

Thanks

[banhien]

@tpfeffer 

Normally we use PCIe x16 slot for a video graphics card/adapter. Please use the following Guide for network card

             https://www.youtube.com/watch?v=p6aaRYedplA

Regards,

[tpfeffer]

Thank you for the link, but frankly the link provided no information as to why the PCIe x16 slot on the HP DeskPro 400 G3 computer does not recognize the NIC cards.   

The NIC cards have been used successfully with pfSense on another desktop computers.  Therefore, the problem is not with the NIC cards, FreeBSD, or pfSense and must be with the HP DeskPro 400 G3 computer.  Apparently, it is a BIOS setting, a missing device driver, or an incompatible chip set that is preventing the x16 slot to recognize the NIC cards.

Can you recommend a solution?