Hello Bob.

27 Jun 04 14:22, you wrote to me:

 BL>> That business of terminating the cable has always amused me.
 BL>> There is no reason to terminate a short cable, the reflections
 BL>> at 100MHz will be stuff-all under about 10-metres, and with
 BL>> that silly thin cable they use, the losses are so great that
 BL>> any reflection will be lost at anything *over* that!

 JB>> thin (co-ax) ethernet ran at 10Mbps, with NRZ coding (which
 JB>> means most of the energy in the signal is between 5 and 10Mhz)

 BL>  Ahem. That means reflection along 10-metres will be 10-times less
 BL> (which is why I used the larger number). I could look up the book and
 BL> give you the formaulas, but I can't be bothered. Trust me, I'm an
 BL> engineer.

maybe the impedance mismatch is a problem, the ethernet cards are designed
to drive 25 ohm load when they're sending. (50 on each end of the run)

And while they're sending they also need to be able to detect if another
card is sending... I'd bet that removing the terminators would mess with
that.

 JB>> I'm told that if you pull the terminator off the cable it stops
 JB>> working.

 BL>  ROFL!

 BL>  I don't doubt that you have been told that. Christopher Columbus was
 BL> told he'd sail off the edge of the world.

Those telling Columbus didn't claim to have tried it themselves. I'm not
talking uniwankers either, these were schoolkids...  we never had ethernet
at school while I was there or I'd have tried it myself.

 BL> It's always *nicer* to correctrly terminate a coax, but with only two
 BL> computers on a cable less than 10 metres, it would make bugger-all
 BL> difference, especially when data slices the pulse with aSchmitt
 BL> trigger, anyway.

I'll believe it when I see it.

Jasen

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