BL> The point I am making is that if you only terminate on one end,
BL> the losses along the cable effectively terminate the other end.

JB> that didn't happen here... cable too short?

 I was wrong. On a mismatched cable, the big problems occur at
quarter-wavelengths along the cable. A short-circuit becomes open, and
a low voltage can tranform into a kilovolt. I know this because I have
ben knocked off a ladder by it.

 Unfortunately like a dill (blush), I also knew 10m was the
wavelength of 30 MHz, but instead of dividing by four for a
quarter-wave, I multiplied and came to the comclusion that problems
would not occur below 120MHz (well outside the range of networking
pulses), when in actual fact it was 7.5MHZ... *right* in the range of
networking pulses. At lengths longer than 10m, losses in the thin
cable would be so large (> 6dB) at 120 MHz that it would effectively
terminate itself.

 I understand transmission line theory, it's just that I can't work
out that a quarter means you divide, not multiply...

 In mitigation for my incompetence,I was led into this farce by the
system itself. Only an utter idiot would design a system where perfect
termination is essential. I forgot that the computer industry is run
by perfect idiots... uniwankers.

BL> The voltage doubles (at worst), but even a Chinese designer
BL> would allow that much tolerance in the system

 You see how I was led astray? Uniwankers have no grip on this real
world at all. They have never climbed a ladder and disconnected a
lo-power transmitter that was still pumping a carrier... the lesson
tends to stick, that way. A lousy 4 watts can still generate a
kilovolt into an open-circuit.

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.

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

JB> they use NRZ coding a 1 is a positive followd by a negative,
JB> and a 0 is is a negatie volatge followd by a positive. (for
JB> 10Mbps each pulse is 50ns, so each bit is 100 ns) a string of
JB> 1s or 0s is results in a 10Mhz square wave, and a string of
JB> alternating 1s ans 0s (10101010) resuilts in a 5MHz square wave
JB> as you see it's a lot like phase modulation applied to a square
JB> wave.

 Yair... clever. I'd ignored to possibility of positives and
negatives.

JB> As for how colliosions are detected (two terminals sending at
JB> the same time) i can't be sure, but it seems to involve the
JB> impedance of the network. 

 No... they must transmit and receive *currents* from a high
impedance. You would get double-voltage on the network if two
transmitted together (triple for three, etc). Detecting double-voltage
would not be hard to do... just use a peak detector.

JB> Ii seems to me that a current sensing circuit in the output
JB> circuit could detect is there was another card sending at the
JB> same time by the output current being not characteristic of a
JB> 25 ohm load... 

 It would be easier to detect double-voltage at the receiving end...

Regards,
Bob




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