RE: [SI-LIST] : Cable Shielding: How do you connect the drains?

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From: Michael Nudelman (mnudelman@tellium.com)
Date: Mon Apr 02 2001 - 10:25:03 PDT


You really have to talk to the cable manufacturers. But most of the shields
are non-signal-carrying shields (it is not their purpose to provide the
return current paths), so they are connected to structure (shielding)
Grounds on both ends. A shield connected on one end, especially a long one,
will provide worse protection.
 
I always imagine any elongated plane or shield as a flexible rod; if
connected by one end, the other end will wabble, picking up noise and
resonating at its own frequency.
 
At the output side the best way is to connect the shield to GND right by the
ouptuts. At the input side the best way to connect cables for weak signals
is at the GND on the board right by the input connector, and then to the
Structure GND. However most of the time the shield gets connected by the
Structure GND (for mechanical reasons it is always close to respective
signals' entrance/exit anyway).

-----Original Message-----
From: Farrokh Mottahedin [mailto:farrokh@cisco.com]
Sent: Monday, April 02, 2001 12:52
To: Stacy_L_Gore@raytheon.com
Cc: si-list@silab.eng.sun.com
Subject: Re: [SI-LIST] : Cable Shielding: How do you connect the drains?

If terminated only at one end, your shield will provide no magnetic
shielding effect. You can hard wire both ends to the chassis providing both
racks are serviced by the same green wire ground. If both racks are not
serviced by the same green wire ground, you run serious safety risks due to
potential difference in different ground wires.

   

At 05:46 AM 4/2/01, Stacy_L_Gore@raytheon.com wrote:

If you are running differential signals between to racks through a shielded
cable, what should you do with the cable drains?

        
* Hard wire both-ends to the chassis?

* Hard wire one end and let the other float?

* Float both ends?

* Connect through a series resistor, or series cap, or series cap and
resistor in parallel?

* Is there a rule of thumb or formula based on something to help you
determine how to connect the shielding?

* I have seen it done several different ways but never have been real
sure which way is correct.

* Thanks,

* Stacy

==============================================
Farrokh Mottahedin, Cisco Systems, Inc.
Gigabit Systems Business Unit GSBU
(408) 525-9185
farrokh@cisco.com

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