nuckolls.bob(at)aeroelect
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 Posted: Mon Jul 22, 2013 11:51 am Post subject: Garmin 496 with Trutrak Autopilot, Dynon EFIS D100 and G |
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At 09:31 PM 7/21/2013, you wrote:
<nuckolls.bob(at)aeroelectric.com>
Bob and I differ on where to terminate the screen, but that drills
into some serious minutiae on this project. It's probably just not
necessary at all. (Bob -- I wouldn't mind hearing your thoughts on
the termination location rationale though)
Sure . . . easy . . . flip a coin.
Shielding is a prophylactic against the weakest of all
EMI propagation modes - electrostatic.
Modern serial data signals will deliberately soften
the leading and trailing edges of the data to minimize
the strength of the propagation mode in bundles.
Twisting pairs of a balanced data stream (Like RS422)
drops the coupling mode to a small fraction of and
already weak coupling mode.
Twisting an unbalanced data stream can't exploit
the common mode rejection capability of a balanced
pair receiver . . . but it still reduces electrostatic
coupling by a substantial proportion.
Shielding breaks the electrostatic propagation mode.
The currents flowing in the shield are so tiny as to be
difficult to measure . . . one can terminate the shield
at ANY low impedance node tied to signal or power ground and it's
got all the conductivity needed to do its nearly
insignificant job.
Grounding shields at the listener-end is a convention
we used most places I worked . . . a convention not
driven by significant physics.
P.S. In some cases, the system designer will specify
that the outer conductor of a shielded wire be connected
at both ends.
This is a perfectly valid means by which a 'ground' can
be carried along with the shielded signal wires but
the system designer will also have addressed any
risks for creating a ground-loop on the double terminated
shield. In all other cases, single end grounding is
the legacy convention although in cases where the
shielded wire runs a couple of feet between panel
mounted black boxes, risk of shield induced ground
loop is non-existent.
Bottom line suggests that studying the installation
wiring diagrams for shield handling is a good
practice.
The legacy conventions evolved out of large
aircraft systems wherein small signal wires
run considerable distances through hostile
electrical environments with HIGH risk off
shield induced ground-loops.
Except for legacy magneto p-leads, I suspect
all other shields could be left un-terminated
or even deleted from the wire bundle with
no adverse effects.
Bob . . .
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