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From: firstname.lastname@example.org (Jim H)
Subject: Re: Bypass RS-485 shield--ESD problem??
Date: 2 Oct 2002 11:33:11 -0700
NNTP-Posting-Date: 2 Oct 2002 18:33:11 GMT
"Tom Faloon" wrote in message news:...
> Hi Jim,
> Are you certain that the problem is due to RF getting in via the shield?
> It may well be, but are you sure?
Yes Tom, I could not be more sure.
> What have you used for common between devices.
> A separate 'third wire'?
> Is it grounded anywhere?
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.
> I don't like the shield connected to 'common' via 100 Ohms.
As I understand it, the idea is to limit the shield current, and
thereby reduce any possible interference from "backing into" the Max
> I know that's what the data sheet says, but I'm not impressed.
I am planning to add a ferrite bead to the shield (drain) just inside
the case at the control (transmit) end. This should add a bit (about
200 ohms) of series impedance at the offending freq. If this does'nt
do it I would like to bypass only the RF to earth at the far end of
the bus. I can not terminate both ends of the shield due to earth
potential differences and potential ground loops. The environment is
extremely noisy and has poor grounding ( power generation plant).
> Tell us a bit more.
> Tom Faloon
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