Applying short cable fix



  • Running 2.0.1 on a single core 500 Mhz machine with three NIC (two realtek one 3com) and have noticed upon boot and multiple times throughout the day the message:
    applying short cable fix (f7) or (f5). This is always attributable to the realtek connected lines sis1 or sis0.

    When using 1 meter cat 5 length lines the message is most ofter seen. If I extend the lines to 20 meters in length the message is rarely seen. I have not noticed a performance problem that can be linked to the line length.

    Would like to know if the problem is related to:

    a- the cable itself
    b- some combination of the end plugs/cable
    c- the NIC cards themselves
    d- the NIC cards receptacles ( molded plugs receivers on the card)

    Would appreciate any observations that people might have to shed some light on the nature of the problem.


  • Netgate Administrator

    Ethernet requires a minimum length of cable in order for collision detection to work correctly. However pinning down exactly what that length is seems tricky.  ::) It's probably around 2m.
    It should only ever be an issue if you are using half duplex/hubs so in most modern networks not a problem, collisions don't happen with switches.
    I would guess that the sis(4) driver (not a realtek driver) which is using hardware collision detection in some way.
    Almost certainly nothing to worry about.

    Steve

    Edit: The source is revealing:
    @/usr/src/sys/pci/if_sis.c:

    /*

    • Some DP83815s experience problems when used with short
    • (< 30m/100ft) Ethernet cables in 100BaseTX mode. This
    • sequence adjusts the DSP's signal attenuation to fix the
    • problem.
      */


  • Very interesting.  Up till this I've mainly only heard anecdotal evidence (usually circumstantial evidence, even) that minimum cable lengths could be an issue.

    Filed under: Interesting stuff that probably won't actually come up, but could add more anecdotal evidence if asked again in the future.



  • Thanks for your comment and insight stephenw10. Maybe the best place to start is 3 meters.

    My reason is that the signals are most likely close to 100 Mhz between the realtek NIC's and the other end of each respective line. Based on what I learned in Networks, Lines and Fields more years ago than might be expected, it's always best to set line length to an integral wavelength which would be 3, 6, 9, 12 etc if the line is operating at 100 MHz. This would minimize impedance match problems between the lines and terminations.

    Let you know how it works.


  • Netgate Administrator

    I think my point is that you don't have a problem.
    The messages you are seeing is just the driver reporting what it's doing as part of it's normal operation.
    Unless you are seeing a huge number of collisions, which might indicate that collision detection is not working correctly, then I wouldn't worry about it.

    Are they Realtek cards? The sis(4) driver which is reporting this is for SIS chipsets.

    Steve



  • Stephenw10

    The NIC cards are Netgear FA 331 rev C1 and Netgear FA331 rev A1. My mistake.



  • If I recall the details (it's been years since I've seen/looked into that), some NIC chipsets have bugs with short cables, and the drivers apply a work around to fix that. The snippet from if_sis.c seems to confirm that. It's safe to ignore as long as it's not causing problems. If it does, replace the NICs.


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