On the "ASRock EPYC3251D4I-2T" board there is a "OCuLink x4 Connector" port with included cable, but this cable is without a sideband - this port doesn't support it. So I can connect disks directly, but not throught the backlpate controller that is built into the SuperMicro case.
Hey, I'm building a network appliance with the same motherboard and case/enclosure and wonder about the same exact thing. I'm still waiting for the drives to arrive, but from what I understand about how everything should work, I believe I won't get activity and failure LEDs working properly without some sort of sideband connection. I don't think I want to buy and plug a PCIe controller just for this - my rationale is that motherboard has its own controller and I can find some better uses for the slot.
So... There is a SATA_SGPIO1 connector on the motherboard. It looks like it matches with SC721's backplate's (I think it has two versions, mine is with BPN-SAS-733TQ) JP51 sideband header. I've also compared it to Supermicro's own MBD-X11SCL-F motherboard I-SGPIO1 pinout and looks like it's either the same or mirrored across the long axis - a bit hard to tell because they don't not draw the pins but lists them by numbers and seem to mix numbering schemes.
I wonder switching backplate to SGPIO mode and connecting JP51 with SATA_SGPIO1 would get the LEDs working.
There are CBL-CDAT-0661 or CBL-0157L cables from Supermicro, I believe one is 1:1 and another is mirrored. But I cannot find pinouts for those cables, so I'd probably go with a DIY solution out of individual DuPont wires. This looks like a fairly simple serial bus, so I guess if I'd connect the pins according to their signals (clock to clock, data to data, grounds to grounds, etc), everything should be fine.
If someone had already tried something like this, has some success or failure stories, or knows a readily available cable that works in this scenario - please tell
Thanks!
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Edit 2021-11-20: Okay, my drives had arrived, and it looks like at least the activity LEDs work even without the SGPIO cable. I wonder if there's a way to test a failure LED (fortunately or not, I don't have any faulty drives, hah)... But anyway, now I lean towards that maybe I was overthinking it and it's not needed at all.