8 Replies Latest reply on Jun 3, 2014 2:46 PM by mjoelner

    DX79SI downshift to PCIe 2.0 ?


      Hi there, does anybody know how to downshift the DX79SI motherboard permanently to PCIe 2.0 speed or less ? Considering that PCIe 3.0 isn't really supported by the Sandy Bridge CPU (the only one supported by the board) I don't understand why the new generation of RAID controllers try to run at the higher speed ? Both LSI's, Intels and Adaptecs newest controllers all refuse to work with the DX79SI board. At least the Intel RS3WC080 boots up but since its running at 8 GTs its unstable. The Adaptec series 7 controllers refuse to work at all. The board doesn't boot with a POST code 51 error.

        • 1. Re: DX79SI downshift to PCIe 2.0 ?

          Hello mjoelner,


          Let me help you with this.


          The motherboard supports PCIe 3.0 devices but as you mentioned, the processors for this chipset support up to PCIe 2.0 meaning that the cards you connect will run at PCIe 2.0.


          Now If the Controller cards you connect are PCIe 3.0 it is normal that will try to run at that speed but it is important to confirm with the manufacture of those devices if are compatible with the motherboard.

          • 2. Re: DX79SI downshift to PCIe 2.0 ?

            Hello Kevin, as I understand the nvidia GPU's do exactly as you mention. Downshift automatically to PCIe 2.0 speed, when they see a Sandy Bridge-E CPU. Unfortunately some RAID Controllers don't. At least not the models I have tested. I'm sure older PCIe 2.0 models work fine, but I don't have any to test with.


            The controllers are not listed as compatible with the motherboard. I didn't know I should check that first. You see, the DX79SI uses more or less the same (intel) technology as Asus P9X79WS, Supermicro X9SRA and GigabyteGA-X79-UP4 (that I have tested so far). These boards allow a manual downchange of the PCIe express speed in the BIOS. Intels servers seem to all have this option as well, but the DX79SI was apparently forgotten. Perhaps because of marked segmentation ? But until the Haswell-E models come out, I'm stuck with what I have. And yes I'll buy a workstation board the next time. The DX79SI supports ECC memory and Xeon CPU's, so alas I thought it was kind of workstation'ish.

            • 3. Re: DX79SI downshift to PCIe 2.0 ?

              Well I understand and I know the DX79SI seems like a workstation because it supports Xeon processors but in fact it is not so that can limit features on the board. Servers or workstations do have more or different capabilities than desktop units so this may be the reason.

              • 4. Re: DX79SI downshift to PCIe 2.0 ?

                Hello again, I have once more tested Adaptec series 7 controllers with a SuperMicro X9SRA motherboard. You see, I'm quite desperate to get something working here and it seems I'm on my own on this one, and now It seems I have found the bug. It's in the BIOS of the DX79SI.

                Apparently the board refuses to boot at all if the "BASE MMCFG" is set to D0000000 in the BIOS (UEFI) setup of the X9SRA motherboard. This is the same "Base MMCFG" setting that the DX79SI motherboard uses. Apparently the DX79SI is tuned to maximize the available memory for 32 bit systems. Unfortunately there is no setting in BIOS to change this, like there is on the X9SRA.

                • 5. Re: DX79SI downshift to PCIe 2.0 ?

                  Thanks for the information and feedback. What happens if you reset the BIOS default settings? Is this still happening?

                  • 6. Re: DX79SI downshift to PCIe 2.0 ?

                    Hello Kevin, yes this happens with the default settings. I have tried the three latest BIOS versions for the DX79SI motherboard. They all use the same "Base MMCFG" value of D0000000.

                    The Supermicro X9SRA board uses a lower value of 80000000. This apparently leaves more PCIe address space for adapters like Adaptecs. By the way, 64 bit operating systems just reclaim the lost memory and puts it above the 4GB boundary.

                    The problem is, that the "Base MMCFG" value cannot be changed in the BIOS of the DX79SI. I guess this is one of the differences between workstation and desktop hardware?. Perhaps it's not a bug but a tradeoff ?

                    Nevertheless it would be nice with a "Base MMCFG" option in the BIOS. Especially since the board only features 6 SATA ports and only two are 6Gbit capable. With a RAID adapter you can  have 8 disks with 6 Gbit connections!

                    Oh and the Intel RS3WC080 RAID controller do actually supprt TRIM for SSD's in JBOD mode. Nice! (I tested it with trimcheck-0.6).

                    • 7. Re: DX79SI downshift to PCIe 2.0 ?

                      Hi mjoelner,


                      I was checking this and as you say, it seems that this is a limitation for desktop units.

                      • 8. Re: DX79SI downshift to PCIe 2.0 ?

                        OK, I'll be using Supermicro or intel workstation boards in the future. The models I have tested work with all the RAID controllers I have thrown at them. They support "above 4G PCI decoding", configurable "base mmcfg" value and downshift of PCI-e speed. This also makes them compatible with Quadro GPU's and Xeon Phi boards. They also tend to not have lane-switching IC's that seems to create noise on the PCIe busses. They have high quality power phases and they don't overclock the CPU(s) behind you back. Also the built in LAN adapters tend to be high quality.

                        Guess I won't be buying any Haswell-E cpu's or X99 boards. I'll just have to wait for the Xeon models to arrive.