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Visitor yazar
Registered: ‎07-02-2018

XLAUI on Passive DAC for 40G (40GBASE-CR4 ?)



Almost all boards with QSFP+ connector and 40G IP cores from Xilinx or other third party vendors provide XLAUI interface. According to IEEE 802.3-2018, Annex 83A XLAUI is an interface between PMA layers and there is PMD layer between XLAUI and MDI. I am planning to connect an FPGA board to a 40G switch using a passive cooper cable which supports 40GBASE-CR4. However I can't figure whether this connection will work or not. Can I connect an FPGA board which provides XLAUI interface via QSFP+ port using a passive copper cable? Should I use active copper or optical cable which includes circuits for signal conditioning (amplitude or timing)? I searched IEEE 802.3, SFF8436 standard and read cable datasheets but couldn't find any clear information.




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Scholar drjohnsmith
Registered: ‎07-09-2009

Re: XLAUI on Passive DAC for 40G (40GBASE-CR4 ?)

I think of the QSFP as just a connector.


at short distances, I can just wire the pins of one qsfp to the other,

    direct cable.


As length increase, I'd need to put some extra buffers / receivers in the QSFP, 

     active cable.


As length increases more, I need to go at the speed of light,

      fibre optic leds.


As the distance goes even further, I need more opticlal power

          laser based fiber optics.



QSFP is designed that the same connector can support any of these by different plug in modules.

   so one board can support cable to cable , or laser to laser.


the elephant in the room, 

    some switch manufacturers, only support their modules

        an example would be you plug a HP into a cisco and it is not "turned on"  as its not recognized


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Visitor yazar
Registered: ‎07-02-2018

Re: XLAUI on Passive DAC for 40G (40GBASE-CR4 ?)

I agree that it looks like "the elephant situation". Fortunately, I asked manufacturer about XLAUI compatibility for specific cable and switch model and they were kind enough to test the setup and verified that equipment was compatible. I was lucky that time.