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DS125BR820: SAS-3 Mode

Russell Wetherley1
Expert 4255 points
Part Number: DS125BR820

Hello E2E,

I have a few questions on the DS125BR820.  My customer wants to run this in SAS-3 mode which is looks like we support. However, they would like to have auto-negotiation.  Is auto-negotiation the same thing as Rx detect?

From section 7.2.1 it seems like we support auto-negotiation (or is it truly a linear amplifier)?  Can you confirm?

Table 2 shows that recommended settings for SAS is Manual-Rx detect.  Can you provide some clarity here on what auto-negotiation is vs Rx-detect?  Can we use auto-negotiation in SAS-3 applications?

Thanks!

Russell

  • 0
    TI__Guru* 93670 points

    Hi Russell,

    SAS-3 uses link training. This means the host and the end point negotiate physical link parameters such as output amplitude and, pre or post output amplitude presets/settings. To support this mode, BR820 is like a seamless analog buffer - passing through these signaling information. Further, two devices on both sides of the BR820 would operate at line rate such as 12Gbps and check bit error. Based on the result, they fine tune their optimum output amplitude settings. This is typically called link training or auto-negotiation. Both SAS-3 and SAS-4 operate in this mode. PCIe Gen3 and higher is the same as well.

    Unlike SAS3 or SAS-4, PCIe uses RX detect function. PCIe uses this scheme to determine if there is a termination or an end point before it starts link training. On the other hand, SAS/SATA don't use RX detect function and assume there is a termination - this is why BR820 does not have RX detect feature. SAS or SATA use OOB signaling to determine if there is an end point so it can start link training.

    Regards,, Nasser