[Elecraft] Kio2 cablength

[W3FPR - Don Wilhelm]

Jack, Jim and all,

Adding to that grounding 'magical stuff', what I learned in large system
computer design and testing experience was that shields are only DC grounded
at one end, and that end is the 'driving' device or in the RS-232 world,
that would be the box defined as the DCE - in the DTE devices, the shield
would be grounded through a  capacitor, or alternately left open.  This was
accomplished in the device wiring itself, the cable shield was always
connected to pin 1 at both ends.

Differences in the potential of the chassis (yes - well grounded to the
power ground bus) of a box on one side of a raised floor computer room as
compared with a box on the other side could be substantial enough to cause
current to flow through the shields if both boxes would have had the shield
directly bonded to the chassis.  DTE units that did not follow the shield
grounding rules were not capable of using long communications lines.  But
that was large computer systems with terminal controllers and a bunch of
distributed terminals, the world today with PCs and LAN and WAN connections
is quite different.

Anyway, in my mind, Jim's recommendation to connect chassis ground (pin 1)
on both ends could cause a ground loop noise problem if the computer and K2
chassis are not at the same potential.  Can you elaborate a bit more for us
here Jim?

73,
Don W3FPR

> -----Original Message-----
>
> Jim;
>
> I've learned quite a bit from you, and have a question about what you
> are saying here.
>
> On Jul 7, 2005, at 8:48 AM, Jim Brown wrote:
> >
> > To gain the benefits of the twisting, use one pair (orange and
> > orange/white, for example) to connect pin 2, with orange to pin 2 and
> > orange/white to pin 5. Likewise, green to pin 3, green/white to pin
> > 5. The
> > overall shield should go to the DB9 shell on each end, not to pin 5.
> >
> > If you are using UN-shielded twisted pair, connect the return of
> > each pair
> > to the DB9 shell, NOT to pin 5. This puts a "band-aid" on pin 1
> > problems
> > on each end.
>
> We have gone out of our way in the RS-232 cables to avoid ground
> loops by not connecting the DE9 shell at one end (or in the case of
> the KIO2 cable, both). I have also seen this technique described
> (reference Bob Heil's web page), indicating that it is best to leave
> the computer end of the shield unconnected. So my question is why the
> suggestion to connect both ends here? Why doesn't this create the
> very ground loop problem we are trying to avoid?
>
> Thanks. My engineering career has taught me that grounding, like RF,
> is magical - I continue to learn things about it that change my way
> of thinking.
>
> - Jack Brindle, W6FB
> ------------------------------------------------------------------------
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>
>
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