BL> You have to terminate it at *one* end, or the system will be
BL> running into an open-circuit generating 20V! But that's got
BL> nothing to do with the cable.

JB> if you used a standard terminator on one end wouldn't you get
JB> only half the expected impedance, how many volts would you get
JB> then? 13.3 ? and what's it supposed to be? 10v ? 

 Actually, half. Now you\re startign to think like an engineer,
rather than a tech who only sees black and white. The next thing you
have ot ask youself is *what difference would it make?"

 Hint: two words: first word starts with "F", second word "all." 

 Imagine... you'\re an engineer designign a system, where one
computer might be right alongside, or 50-metres away, where the
level drops 20dB (10-times). What tolerance would you allow in levels?

 In the sixties, I would have set up a cable and tried a few things,
but being a uniwanker in the 80's I would write a *really* complicated
specification instead, and I wouldn't even call it that... I'd call it
a "recommendation for comment,"  and RFC, so it would never be traced
back to me and would only row in compication if there were in fact any
comments.

BL> The other thing I mentioned was the length. A long lossy cable
BL> (thin cable) ends up looking like 50-ohms anyway. There are
BL> reflections, but they are lost on the way back. For that thin
BL> cable, 10-metres will be as good as infinity.

JB> at 10Mhz I don't think there'd be be much loss over 10m

 Ahh, Grasshopper... have you ever heard of resistance besides
dialectric losses? Both of which increase, btw, with thin cable.
Dialectric loss hardly matters below 200 MHz.

JB> yeah, but I get the feeling that the ethernet cards refuse to
JB> work unless they see a 25 ohm load.

 "Feelings," JB? Electronics is not a touchy-feely subject. Perhaps
you should try social work, or work with animals.

JB> as you say I seen no reason why it wouldn't work with a single
JB> terminator but I feel it'd need to be a 25 ohm resistor. 22
JB> ohms would probably be close enough... Hmm I'll have to score
JB> some T connectors and liberate one of my BNC cables from the
JB> video lash-up and try it myself. 

 It'd still work, but don't do it that way! A long-lossy cable
already looks like 50 ohms. Nearby computers woudl see a mismatched
cable. The ones further away wouldn't matter...

Regards,
Bob
  


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