SAS disks vs. SATA connectors. How does it work?

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tcpluess

Member
Jan 22, 2024
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Hi,
I made the following observation, and I cannot understand how this works.

For example:

a) the Supermicro backplane BPN-SAS-825TQ can hold 8 SATA or SAS disks. On the rear, it has one individual connector for each disk. However, these connectors are SATA (male). How can this work? the SATA connector has less signals on it than SAS. But according to the datasheet, the backplane can work with SAS disks (provided the correct HBA is used).

b) I was looking for cables to install one SAS disk in my PC. My HBA has one SFF-8643 connector. However, all the cables I found so far that have an SFF-8643 on one end, have either a SFF-8643 on the other end too, or SATA connectors (female). Again, why SATA connectors? aren't there signals missing on that connector?
and then, to connect to the disks, I found an adapter piece (it seems to be really only a mechanical adaptor) that plugs into the SAS disk, and at the other end has a SATA connector (male) and SATA power connector.

So apparently, if I want to use a SAS disk, I need a csble for my HBA that has SFF-8643 on one end and a female SATA connector on the other end. And I need this mechanical adapter piece that plugs into my disk and provides a male SATA connector on the other end.

I know that somehow SATA and SAS are similar, as a SATA disk can be run on a SAS controller (but not the other way round) but I don't understand why the backplane and practically all cables I found have just SATA connectors. I could not find one single cable with a SAS connector for the disk and I wonder why this is so. Can somebody explain?

thanks!
 

kpfleming

Active Member
Dec 28, 2021
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Pelham NY USA
I wonder where you got the information that SAS connectors have 'more signals'. The connectors on SAS and SATA disks are identical except for a blocking bar to keep you from plugging a SAS drive into a SATA-only backplane/cable.

I have that exact backplane in my server and have easily swapped between SAS and SATA drives in all of the bays with no issues at all, since I have a SAS/SATA controller connected to the backplane. If I'd connected a SATA-only controller to the backplane (which would use the same cables between the controller and backplane), I'd still be able to plug in SAS drives but they would not be functional.

TL:DR; there's no issue here, you can connect a SAS/SATA controller to the backplane and use whichever type of disks you like.
 

tcpluess

Member
Jan 22, 2024
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SAS disks have additional contacts on the connector at the place where SATA disks have this little "gap". I found that the SATA connector has the signals

TX+, TX-
RX+, RX-
GND

on the connector, whereas SAS disks have the signals

TXA+, TXA-
TXB+, TXB-
RXA+, RXA-
RXB+, RXB-
GND

so clearly, a SAS disk has more signals, the B signals are on that spot where SATA connectors have this little gap. So I wonder, when I use for example this SAS825 backplane, where do the B signals go? obviously they are not connected to the SATA male connector, as this connector only has pins for the A signals. No?
 

kpfleming

Active Member
Dec 28, 2021
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Pelham NY USA
Which flavor of SAS has those additional signals? It's possible that speeds above 6Gb/s use them, but this backplane does not support those speeds.
 

tcpluess

Member
Jan 22, 2024
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Here is what I mean. Pictures from a SAS2 SSD and a SATA HDD. Both 6Gbit/s. We can see that the SATA connector has this gap. And the SAS SSD doesn't have the gap, but instead has additional contacts there.
I wonder where these additional contacts are connected to, when a SATA connector is used on the backplane.
 

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kpfleming

Active Member
Dec 28, 2021
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Pelham NY USA
I believe the answer is they don't connect to anything, and the drive is able to tolerate that. I just checked a Seagate EXOS SAS HDD I pulled out from my backplane (again, the same one you listed), and it also has those additional contacts on the top side of the connector but works perfectly in the machine.
 

tcpluess

Member
Jan 22, 2024
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That a dual port sas device
this is probably the solution. So on a backplane that supports dual channel, the extra pins are simply connected to the other channel, and are simply ignored in case of single channel, right? with the additional channel being for added redundancy?
 

tcpluess

Member
Jan 22, 2024
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Yes, and for some sas3 ssds to get around the 12GBit/s limit (throughput performance)
ok but with just one port, and a proper HBA, I can still have the 12 GBit/s performance, right?

I have a 9300-8i HBA and a couple HGST HUSMM1640 SSDs, which should be SAS3 12Gb/s.
 

ericloewe

Active Member
Apr 24, 2017
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Again, why SATA connectors?
Because why not? They're cheap and cheerful and easily handle 3 Gb/s and 6 Gb/s per their spec. 12 Gb/s would get dodgy, though. Of course, single lane only, but you would never connect both lanes to the same host port, so there's no point in having a dual-lane SAS cable.
o apparently, if I want to use a SAS disk, I need a csble for my HBA that has SFF-8643 on one end and a female SATA connector on the other end. And I need this mechanical adapter piece that plugs into my disk and provides a male SATA connector on the other end.
As you've discovered, the gap between SATA power and data is missing on SAS (and U.2). The practical result for the direct cabling scenario is that you need a single connector for power and data, hence all the weird adapters.
You would need a BPN-SAS3-825TQ
Or -825A, preferably. Cabling a TQ backplane is the stuff of nightmares.
 

kryten

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Apr 10, 2023
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one thing im curious on is the sas ssd's that also have the u.2 connector, but dont require a u.2 connection to work.

Like the "SDLL1MLR-038T-CDA1"
 

tcpluess

Member
Jan 22, 2024
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what's then the difference between u.2 and SAS? I think u.2 specifies the form factor and the connector shape, no?
 

kpfleming

Active Member
Dec 28, 2021
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Pelham NY USA
But the U.2 connector is yet-another-variation on the original SATA connector; as noted in the link above, a backplane receptacle can support any or all of SATA/SAS/U.2 if it has the proper electrical connectivity.
 

alaricljs

Active Member
Jun 16, 2023
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SATA and SAS use the "bottom" side of the connector for electrical contacts (drive perspective). The void in SATA between power and signal is the 2nd channel for SAS. U2 uses the top side of the same physical connector which allows for a disk to have SAS and U2 the hard part being how to handle being plugged into a back plane capable of both.