Boris,
Whether it is okay to connect the shields of cables going between boxes, or
for that matter, the internal wiring, depends on whether or not the cases of
the two boxes are at the same potential. I cited earlier a situation where
the green wires between the first floor and the fifth of a building were
different by 6.5 VAC. Imagine if the shields of wires between boxes on
those two floors were connected to the boxes! They would probably fry.
That is why the original 10B2 Ethernet protocol which was coaxial cable not
twisted pairs (the cables have transformer coupling at both ends) has a
requirement that the shields not make a DC connection to the boxes on either
end. This caused great pain with respect to EMI problems until engineers
discovered how to make a good AC connection between the shields and the
boxes.
Hope this helps.
Lee
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Boris Bakshan
Sent: Tuesday, January 10, 2017 4:28 AM
To: Lee Ritchey <leeritchey@xxxxxxxxxxxxx>
Cc: si-list <si-list@xxxxxxxxxxxxx>
Subject: [SI-LIST] Re: An Article on what ground is
Hi Lee,
This is rather interesting because today, the most popular interconnect in
datacenter is DAC (Direct Attach Copper) cables (QSFP/SFP ) which is used to
connect switches and servers within a rack.
In a QSFP/SFP copper cable the plug's diecast(shell) is terminating the
external shield of the copper cable and there is no connection between logic
Ground (return path for high/low speed signals in the cables) and the
diacast itself.
These cables are plugged into a QSFP/SFP cage which makes contact with the
front panel and eventually with the card cage.
If I understand correctly, you recommend having NO connection points on the
front panel between logic Ground and card cage/Front panel/QSFP Cage.
1. Is that correct?
2. The front panel is connected to the PCB with screws. I assume that the
same applies here and the PCB holes of these screws should not be plated and
connected to logic Ground, correct?
Thank you.
QSFP Plug:
[image: Inline image 1]
Shield 2 is connecting to the diecast:
[image: Inline image 3]
Point of contacts:
[image: Inline image 5]
On Sat, Jan 7, 2017 at 9:30 PM, Lee Ritchey <leeritchey@xxxxxxxxxxxxx>
wrote:
If the ground plane in the backplane is connected to sides of the cardthe Faraday cage.
cage then logic ground in the plug in modules must not be connected to
the card cage.
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]
On
Behalf Of Boris Bakshan
Sent: Saturday, January 7, 2017 10:16 AM
To: Lee Ritchey <leeritchey@xxxxxxxxxxxxx>
Cc: si-list <si-list@xxxxxxxxxxxxx>
Subject: [SI-LIST] Re: An Article on what ground is
Hi Lee,
Thank you for prompt reply.
So, in fact, you are making a direct connection between the logic
ground of the cards and the metalic chassis enclosure so that
backplane ground (return path of signals) , cards logic ground and
metalic chassis enclosure are all connected, did I get it right?
On Jan 7, 2017 7:57 PM, "Lee Ritchey" <leeritchey@xxxxxxxxxxxxx> wrote:
Thanks for the nice feedback.
Aha! You have asked an EMI question! I use the ground plane in a
backplane as one side of my Faraday cage. Where that backplane
meets the flanges on the card cage, I leave the copper on the top
layer and connect that with vias to the ground plane in the
backplane. Now, there is the one connection between logic ground and
cage.There can be no other connections such as on face places if one is
to avoid "leaking" at the cracks which is a common complaint when
more than one connection is made between logic ground and the Faraday
Notice, I did not use the phrase "chassis ground" in this discussion.
I have used this method in dozens of systems with very good EMI results.
Hope this helps.
Lee
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]
On
Behalf Of Boris Bakshan
Sent: Saturday, January 7, 2017 5:24 AM
To: Lee Ritchey <leeritchey@xxxxxxxxxxxxx>
Cc: si-list <si-list@xxxxxxxxxxxxx>
Subject: [SI-LIST] Re: An Article on what ground is
Hi Lee,
Very good, comprehensive article, thank you.
What is your recommendation when it comes to backplanes? a backplane
is attached to the chassis with bolts/other conductive mechanical
parts. Do you recommend plating these PCB holes in the backplane? If
you do, should these holes be connected to (any) electrical net?
Thanks Lee.
On Fri, Jan 6, 2017 at 10:43 PM, Lee Ritchey
<leeritchey@xxxxxxxxxxxxx>
wrote:
I have written an article on ground following the brisk exchange
on this subject last month. I hope it clears up some of the
confusion around this subject.
Here is where it is on the web.
http://ubm.io/2ikEDat
Lee Ritchey
Speeding Edge
P.O. Box 2194
Glen Ellen, CA
95442
707-568-3983
I just used the energy it takes
To get mad and wrote some blues.
Count Basie
Or:
Worry is like a rocking chair,
It keeps you busy but.
It doesn't get you anywhere.
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