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J Collins
Asked: 2020-02-13 06:45:43 +0800 CST

Why do internal <> external SAS expanders require a PCIe slot?

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I have seen examples of home-brew NAS units that use SAS expanders plugged into a host motherboard that has neither a CPU or memory, literally for the purpose of powering the card. If the card doesn't need the PCIe slot, why are they all designed to consume one? HighPoint make a SAS expander that uses a floppy drive power connector and the SCSI bracket mount, though I'm learning it is proprietary in the communication so unsuitable for my needs.

Some example cards with internal/external ports. Interestingly the second card seems to have no pin contacts at all for the slot shape, how it is powered is ambiguous:

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Would this board be effective at powering such cards? Would there be any missing features or functions?

PCIe power board

pci-express sas expansion

2 Answers

  1. A PCIe card is a convenient form factor for a SAS expander. Most often, the number of internal ports is insufficient and it's easily expanded internally.

    Would this board be effective at powering such a card?

    Possibly. You're not providing details to that card nor a link to its datasheet.

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  2. https://www.intel.com/content/dam/www/public/us/en/documents/product-briefs/raid-expander-res2sv240-brief.pdf

    Intel made this SAS expander card with an RA 4pin power connector to power from cable instead of the PCIe slot, the card has alternate mounting holes so you could stick it pretty much anywhere.

    I've used powered riser boards like you pictured for GPUs and the only tricky bits are getting a solid mounting between the board and the GPU to make a reliable connection, and some issues with QA. If you power the riser board directly from the PSU it should manage 15W for the expander card easy. I've used one for a 50W GPU continuous load successfully (ie. Without causing a fire)

    Due to the unknown providence of the riser board I'd recommend stress testing it in the open until confident it's not going to fail catastrophically.

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