Re: Recording digital data to analog tape... revisited

From: Mark Borgerson (m-a-r-k_at_oes.to)
Date: 10/18/04 

Date: Mon, 18 Oct 2004 14:52:53 GMT

In article <4173370B.7ED274E2@yahoo.com>, cbfalconer@yahoo.com says...
> Mark Borgerson wrote:
> >
> ... snip ...
> >
> > My application was for an oceanographic cable, which can be quite
> > different from a telephone cable. In particular, I had to cope
> > with the fact that one of the wires might have about 300V DC
> > to ground (or the other wire). I've also heard, but not been
> > able to verify, that the cable characteristics change as the cable
> > is reeled out into the ocean. I do know that we had be be able
> > to cope with slip ring noise also--which was handled with ACK/NAK
> > and packet repeats.
>
> That sounds like moving power on the same copper. My approach
> might be to use the common mode of a twisted pair to supply one of
> the power potentials (including ground). Thus, if a 100 ohm line
> is terminated by two 50 ohm resistors at each end, they look like
> 25 ohms per end to the common mode supply. The remote end can use
> DC/DC converters to trade volts for amps. Redundant signal paths
> mean redundant power paths with means of detecting partial
> failures.

Yes, power is often moved along the same copper. When towing
multiple instrument packages at the end of a kilometer of
cable, you can quickly run out of conductors. The package I
worked up was designed to allow 4 or 5 instruments with modest
data rates to mux their data together over the same pair of
conductors.
>
> Changes in line characteristics look like taps on the line. The
> system should be fairly immune to these when both ends are
> terminated with a reasonable match, provided the taps are
> reasonably high impedance. Obviously a short circuit doesn't let
> anything past it.
>
> You can even apply TDR (time domain reflectrometry) to the
> installed line, and decide exactly what you have. You can then
> compensate for its deficiencies or tell the maintenance people
> where to look.

"installed line" is probably not the proper term for an armored cable
that gets winched in and out several times a day. They do use TDR
to locate breaks and pinches on the line, though. However repairing
a problem is extremely difficult---particularly 400 miles off the
coast of nowhere! Oceanographic cables often grow shorter over their
lifetime as new terminations are made or you get kinks over the
sheaves---particularly as you bring the package aboard. The worst---
and very expensive case---is a problem in the middle of a 1 or 2KM
cable. You will often see large spools of cable sitting in the
shore facilities. I sometimes wonder whether they are too short,
too few conductors, or simply too expensive to repair.

Mark Borgerson

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