AUI cables needed
Paul_Koning at Dell.com
Thu Apr 2 09:15:44 CDT 2009
>>>>> "Pete" == Pete Turnbull <pete at dunnington.plus.com> writes:
Pete> Richard wrote:
>> What's the significance of the 2.5m interval?
Pete> It's some prime-number-fraction (17 IIRC but it's been a while)
Pete> of the wavelength of the data bits (and it's too late tonight
Pete> to work it out) The idea is to ensure that the interference
Pete> effects introduced by the discontinuities of the taps are not
Essentially, yes. That is the purpose. It's not actually a round
number multiple, or round fraction multiple, of the wavelength. Using
the minimum velocity factor given in the 802.3 and DIX specs (0.77),
2.5 meters translates to 0.10823 wavelengths, or 1/9.24 wavelengths.
That makes sense, it's not a round number or 1/round number, so you
don't get constructive interference even if you have transceivers
separated by several marks.
Then again, it's all somewhat approximate because the spacing is fixed
while the velocity factor only has a minimum spec. But you don't get
in trouble (with a round number) until you have a VF of 0.83, which is
pretty high for small coax so the designers may have figured that
practical cable would be 0.77 or above, but below 0.83.
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