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DS100DF410 optimum settings for SFP+ optical versus Direct Attach Copper (DAC)

Casey Morrison
Intellectual 1985 points

Other Parts Discussed in Thread: DS100DF410

This question was paraphrased and posted on behalf of another user.

Question: My application will use the DS100DF410 Retimer to support both SFP+ optical and Direct Attach Copper (DAC) cables. A variety of copper cable lengths will be used, including some passive and some active cables. How should the TX/RX settings change for optical versus copper, and will the device auto-detect the cable length and adjust its settings accordingly?

Casey Morrison

  • 0
    TI__Intellectual 1985 points

    There are two separate parts to this answer, the DS100DF410 Retimer's TX settings and its RX settings:

    TX settings for 10G optical/DAC applications

    The optimum TX settings for an SFF-8431-compliant 10G optical and direct attach copper (DAC) application are solely dependent on the output eye opening and jitter measured at compliance point B, which is the output a host compliance board (HCB). Refer to Table 12 in SFF-8431.

    Normally, in these types of applications, the DS100DF410 Retimer TX is configured with 600-800 mVppd VOD and 0 dB to -4.5 dB de-emphasis, and this is more than sufficient to meet the jitter and eye mask requirements at compliance point B. The optimum values for VOD and de-emphasis will depend on the specific application (board loss between Retimer's output and SFP+ cage).

    SFF-8431 does not have a protocol mechanism nor a requirement for adjusting the TX output settings for different cable lengths. It is the burden of the receiving device to equalize the different lengths of copper cable which may be plugged in to the SFP+ port. Strictly speaking, the output of the SFP+ port must always comply with the eye mask and jitter specifications, regardless of the length of copper cable being used. That said, it is not uncommon for some systems to utilize cable length information read from the I2C interface to increase VOD and de-emphasis for longer cable lengths. However, this is outside the scope of the SFF-8431 interface standard, and therefore it is a non-standard technique.

    Summary: TX settings should be selected to meet the SFF-8431 Table 12 eye opening and jitter requirements, regardless of optical versus copper.

    RX settings for 10G optical/DAC applications

    The DS100DF410 Retimer's RX equalization is auto-adaptive and consists of a CTLE and DFE. The device will automatically adjust its CTLE and DFE to compensate for input channel impairments such as ISI, reflections, and crosstalk. Therefore, if a longer copper cable is inserted, the Retimer will automatically adjust its settings accordingly to compensate for the additional loss.

    Summary: The SMBus master does not need to re-configure the Retimer for different types of cables, as the equalization will automatically be adjusted.

    Regards,

    Casey