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

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

 That's not how it works.

 If you hook a string of computers to a 50-ohm cable, each able to
send and recieve, then you have to send using a constant courrent
source (>> 50 ohms) and receive with a high impedance (>> 50 ohms).
That's a recipe for disaster if you don't terminate, becasue the
*voltage* can rise to high levels (above the actual sending voltage).

 The *ideal* situation, is to terminate both ends of the cable with
50 ohms. All the computers see a 25-ohm load and source, and
reflections along the cable are absorbed by the matched load at
either end. Of course, what happens *along* the cable at each site
is another matter. You then design the transmitter to provide enough
*current* into the 25-ohms (as you said) to generate enough *voltage*
to drive the receiver. The sytem is a compromise that just doesn't
work, which is why they have changed to the hub method and individual
cables. 

 The point I am making is that if you only terminate on one end, the
losses along the cable effectively terminate the other end. The
voltage doubles (at worst), but even a Chinese designer would allow
that much tolerance in the system, and in any case, losses in those
*really* skinny cables will make sure it never happens. With only
*two* computers at distances over 3m, it is actually better to only
terminate one end, and it doesn't matter which end.

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

 How do they do that? They *have* to pause, because two
transmitting simultaneously would mean at best a 2:1 shift in level,
and 2:1 is not nearly enough. Do they use carriers, or something? With
a carrier, levels wouldn't matter a rat's arse.

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

BL> ROFL!

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

JB> Those telling Columbus didn't claim to have tried it
JB> themselves.

 The key word in that statement is "claim,"

JB> I'm not talking uniwankers either, these were schoolkids... we
JB> never had ethernet at school while I was there or I'd have
JB> tried it myself. 

 Schoolkids! Jesus...

 ROFL!! (and slapping my thighs, choking, and coughing chardonnay on
the keyboard)

 When I was a schoolkid, my favourite legend was the brother who got
his sister pregnant by masturbating in the bath. I always wanted to
try that... (getting a sister pregnant, that is).

Regards,
Bob



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