I'm wondering if it is possible to use a SAS-2 HBA in a "SAS-3" system.
I have a Supermicro X11DPH-T and have attempted to install an LSI 9201-16e. From what I understood (which may be wrong), a PCIe 2.0 card may be installed in a PCIe 3.0 slot, but the slot will negotiate down to 5GT/s. Unfortunately, even setting the PCIe slot to 5GT/s manually did not not allow the card to work in the system. It initializes other PCI devices, but when it gets to the 9201-16e, the screen displays a blinking cursor in the upper left hand corner, then about 30 seconds later, the system reboots. This will continue indefinitely until shutting down and removing the card.
Is it at all possible to use a SAS-2 card in this system or is the issue strictly PCI compatibility?
Likely, that's not a PCIe issue - those are very rare.
Sometimes, multiple RAID controllers in a system won't tolerate each other and the system won't boot at all - try removing/disabling the other controller(s). In rare cases, upgrading to the latest BIOS/firmware might also help. In extreme, you'd have to stick to the vendor's recommended hardware compatibility list.
I haven't seen that problem with multiple HBAs and no or just a single RAID controller.