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From: Fred Bloggs
User-Agent: Mozilla/5.0 (Windows; U; Win 9x 4.90; en-US; rv:1.0.1) Gecko/20020823 Netscape/7.0
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Subject: Re: Bypass RS-485 shield--ESD problem??
Date: Fri, 04 Oct 2002 14:38:02 GMT
NNTP-Posting-Date: Fri, 04 Oct 2002 07:38:02 PDT
Organization: EarthLink Inc. -- http://www.EarthLink.net
> I am using the "drain" wire, which is under the foil shield. Drain is
> connected to chassis ground at the control unit; which is connected to
> earth ground. The 100 ohm resistor to common is in the isolated
> output of the Max 1480 chip.
It is pointless to use the isolated MAX1480 at the end with shield GND'd
to chassis. And if this is the side receiving the 100MHz interference at
the relatively weak 10V/m external field then you obviously have a
layout problem with your grounding with the RF coupling onto the signal
twisted pair through parasitic capacitance-now that you have established
the GND return. The way around this is good RF filtering in *series*
with each twisted pair lead. See the "data line" filters from Panasonic
as advertized in Digi-Key. These will completely eliminate the RF at
this frequency. The 100 ohm shield to ISO COM resistor as shown in the
data sheet is to limit driver transient current between the twisted pair
and the shield in the event the shield charges to a large potential
relative to ISO COM. This only applies to the isolated circuits and
serves to discharge this potential and prevent its buildup.
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