[sdiy] preferred shielded cable, connectors

[Cornutt, David K]

From: WeAreAs1 at aol.com [mailto:WeAreAs1 at aol.com]
> 
> In a message dated 4/22/04 12:07:27 PM, jay at denonville.com writes:
> 
> << Every single commercial cable I've ever seen has both ends 
> of the shield
> terminated.
>  >>
> 

Well, the obvious reason is that, in many audio applications,
the equipment expects to be able to use the shield as a signal
return conductor.  If it weren't connected on both ends, you
wouldn't have a circuit.  

> This makes sense.  If they wanted to make a cable that had 
> only one ground 
> connection, they'd have to label it so you'd know which end 
> was grounded, and in 
> order to take really advantage of the benifit of such a 
> cable, the user would 
> need to be careful to plug it in with the ungrounded end 
> connected to sources 
> (outputs) and the grounded end connected to destinations 
> (inputs).  

It doesn't really have to do with which end is the input
or output; it has to do with which equipment can provide
the shield with a path to ground (without which the shield
can't do its job).  Some audio equipment these days is
built to be operated ungrounded.  And, let's face it, there
are a significant number of people in the music industry
(hopefully none are on this list) who "solve" ground loop
problems by arbitrarily lifting grounds until the problem
goes away.  (How many of you have seen guitar amps where
the owner "improved" it by cutting the ground pin off the
plug?  Come on, raise your hands... I thought so.)
A two-end-connected shield has a better chance of finding 
at least one path to ground.  And, in a pinch it might 
provide a safe way to dispose of a fault current in an
ungrounded piece of equipment by carrying it through the
cable to another piece of equipment which is grounded.  So it's 
both a functionality thing and a safety thing.

> The one-end-grounded cable, however useful, 
> probably doesn't have a lot of 
> commercial appeal.

Well, I can think of one kind of one-end-grounded cable that
just about everyone on this list has at least a few of:
MIDI cables.  The spec specifically says that they are to
have their shields grounded at one end only.  That's why
you seldom have ground loop problems that involved your
MIDI wiring.  (In the case of the MIDI cable, it contains
two internal conductors for signal supply and return, so
the sheild isn't needed for carrying a signal.)  And, the
moral equivalent of this is the patch cord used by modular
synths that employ banana jacks (Serge, Modcan, Cynthia, et al).
The jacks and plugs have no provision for a shield, and so
typically patching on these synths is done with cords that
have only the one center conductor and no shield at all.
(The synth hardware makes some other provision for tying all
the signal returns together so that the patch cord can make
a circuit; this is usually done either through the power
supply wiring, or through being mounted to a common metal
front panel or set of rails which serves as a ground plane.)

I can tell you that in spacecraft wiring, cable design is
nearly always done with a wire for the return conductor, and
the shield is connected only at one end.  In spacecraft, it's
notoriously difficult to maintain a consistent ground reference
(think about it; what constitutes "ground" when the ground
is hundreds of miles away?), and so equipment is nearly always
star grounded.  If the cable shields were connected at both
ends, it would defeat the single-point grounding.  The only
bad thing about this is what happens when a signal conductor
shorts to the shield; it makes the cable, the equipment 
connected to it, and sometimes the whole spacecraft turn into
one big antenna, and then the EMI gods go on the rampage.
I had to deal with a situation like this a couple of years 
ago and it took weeks to figure it all out.