Robert Monsen
Guest
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 Posted:
Wed Feb 16, 2005 12:57 am Post subject:
Re: lenght cable tester? |
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Impeller wrote:
| Quote: | I heard about a tester which measure the lenght of the cables, but I
don't know how it work. Does it measure only the resistence cable or
does it measure even inductance and/or capacitance? When I do the
measure do I have to connect end cables or have I to leave ends
opened? Thanks.
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Some ethernet PHY chips have this built in. I believe they measure the
delay of a pulse and it's reflection off of the unterminated end of the
cable. They can use this to report opens, and how far they are away from
the PHY.
--
Regards,
Robert Monsen
"Your Highness, I have no need of this hypothesis."
- Pierre Laplace (1749-1827), to Napoleon,
on why his works on celestial mechanics make no mention of God. |
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Rich Grise
Guest
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| Posted:
Wed Feb 16, 2005 6:10 am Post subject:
Re: lenght cable tester? |
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On Tue, 15 Feb 2005 08:03:01 -0800, Impeller wrote:
| Quote: | I heard about a tester which measure the lenght of the cables, but I
don't know how it work. Does it measure only the resistence cable or
does it measure even inductance and/or capacitance? When I do the
measure do I have to connect end cables or have I to leave ends
opened? Thanks.
|
This is called a "Time-Domain Reflectometer", TDR. In general, what it
does is put a calibrated step voltage into one end of a cable, and
monitors the voltage by displaying it on, essentially, an oscilloscope. So
you see a trace that starts at 0, and at time t0, steps to V. The voltage
reading at that point stays constant as long as the pulse propagates down
the line. When it meets a discontinuity of some kind, some or all of the
signal is reflected back to the source. This reflection shows up on the
display as another edge, which can tell you a remarkable amount of stuff
about the condition of the cable. People with more education than I have
can tell what things like overshoot, undershoot, rise/fall times, and so
on have to do with it - what I was interested in the time I used one was
finding the break in a buried cable. A break (open) shows up as a step to
2 * V, and a short shows up as a step to 0V. Other values mean an
impedance mismatch somewhere between a short (0 ohms) and an open
("infinite" ohms). And the time from t0 to the step gives the length of
cable from the tester to the target. My selective memory wants to tell me
that the unit I used was actually calibrated in feet; I know it got me to
the exact connector where the break was. Unfortunately, where it was
"buried" was almost dead center inside the tail section of an F-4 Phantom
jet, directly above the exhaust nozzles. And the inside surface was lined
with the points of sheet-metal screws. I had to resolder a coax to two
pins of a 144-pin Cannon plug. By flashlight light. =:-O
I think that kind of work was paying about $187.00/week in those days. But
the food was provided, all you can eat, and was surprisingly good! Maybe
not surprising - the cooks have to eat their own cooking, after all. ;-)
Hope This Helps!
Rich
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