Hello John.

07 Jul 04 19:51, you wrote to all:

 JT> From: John Tserkezis 
 JT> Reply-To: Fidonet AVtech Echo 

 JT> Jasen Betts wrote:

 >> 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)

 JT>   As I understand it, it's still 50 ohm.  The terminators make the
 JT> ends look like an infinite length cable so there are no reflections.

It is 50 ohm cable but the card is driving two stretches of cable in
connected to it in parallel (by the Tee connector), so it sees a 25 ohm
load.

(driving or one stretch of cable and one terminator, still two 50 ohm loads
 so still a 25 ohm load when taken in parallel.)

 JT>   Reflections garble the data enough to make it useless for everyone
 JT> let alone the transmitter trying to detect a collision or not.

IIRC Signals travel in coax at arround 200 Mm/s (2/3 lightspeed)
so a 50 ns pulse (10Mbps ethernet) is about 10m long

That short.

Hmm, maybe I should repeat that experiment with a shorter cable.

I lugged machine "B" up to this end of the room and borrowed a 1m
BNC cable from my video lash-up

results:

it works configured nornally
with 0 or 1 terminators still no success
but with both terminators (and both tees) at one end it works.

conclusions
a 25 ohm load is needed
a 10m cable is long enough for reflections (or its impedance when
unterminated) to be a problem at 10Mbps
a 1m cable is isn't.

 JT>   In theory, cable length should not be an issue (aside perhaps from
 JT> degraded signal strength by the time you get to the end of that 300
 JT> metre stretch), although I've seen some cards have a _minimum_ cable
 JT> specification (about two
 JT> metres if I recall).

hmm... I wonder what that minimum does.

 JT>   Again in theory, if you had a long enough cable length on one (or
 JT> both) ends, the system should still work, because there are no
 JT> reflections, thus no corruption of signal. What you would need is a
 JT> cable length long enough at the ends that the inevitable reflections
 JT> are degraded enough by the time they come back, that they offer little
 JT> or no effect on the closer valid signals. --

or that they take long enough to come back that the original has already
reached it's destination :)

I wonder how much loss you get in 100m of that cable at 10Mhz.

Jasen

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