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Some RS485 questions (shielding, filtering etc)

cupoftea

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Hi,
Do you know of any common mode chokes for RS485 lines?
I cant find any, but assume a USB2.0 common mode choke would do?
Also, are RS485 cables always shielded?

As per bottom of page 10 of the following, why do RS485 shields (in cables) need to be directly connected to ground at one end, but connected to ground via an RC network at the other end?.....this sort of connection isnt recomended for any other signals/voltage rails in cables?

 

KlausST

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Hi,
As per bottom of page 10 of the following, why do RS485 shields (in cables) need to be directly connected to ground at one end, but connected to ground via an RC network at the other end?.....this sort of connection isnt recomended for any other signals/voltage rails in cables?
all the explanation is given in the according section.

I don´t want to copy and paste it here.

(added) But this is not only true for RS-485. It´s true generally for differential signalling without dedicated DC-blocking and biasing of the receiver side.

Klaus

added:
We use WÜRTH (Wurth / Wuerth) common mode chokes: 744 231 371 for RS-485 interfaces.
 
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cupoftea

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Thanks, thats a nice small common mode choke, though i wonder if the interwinding capacitance may thus be too great or something?...(though admittedly probably not as USB2.0 CM chokes are even smaller)

When i went to Wurth...

..the smallest RS485 CM choke i could find was 6 by 3mm....this one..
 

KlausST

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cupoftea

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