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I need to interface two PCBs via RS422 (250kbps) in full-duplex mode (4-wire, 2-twisted pairs 100ft cable). This system will operate in a noisy (CNC) industrial environment, ie, industrial controls and automation.

I realize that RS422 is fully balanced (differential pairs) but the more research I do I seem I get conflicting recommendations about shielding individual pairs or just the (multi-pair) cable as a whole.

In terms of cable selection should I use a cable with each pair individually shielded or go with both pairs shielded as a whole?

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  • \$\begingroup\$ I think there is large diminishing returns going from individually shielded to shielded pair. The only thing I can see individually shielded pair making a difference is if the induced mode noise was somehow not common mode and so high that it actually exceeded the voltage limits of what could be handled by the transceivers and would therefore need to be reduced. Other than that, shielding them as a pair should do the exact same thing as shielding them individually. Both are pretty excessive though. \$\endgroup\$ – DKNguyen Feb 18 at 21:50
  • \$\begingroup\$ Where's the ground? \$\endgroup\$ – CL. Feb 18 at 22:15
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The cables are twisted to make the loop area smaller and to cancel out magnetic interference. They are shielded to block noise from electric fields.

enter image description here Source: https://instrumentationtools.com/rs485-serial-communication/

There are a few ways noise can enter a shielded cable and create noisy currents. The main one that you need to worry about is called mutual inductance. When two parallel conductors are next to each other, the magnetic field from one can couple to the other conductor and create noise. This is going to be less of a problem with twisted pair (because of the magnetic cancellation of twisted pair, but it doesn't cancel everything)

enter image description here

In terms of cable selection should I use a cable with each pair individually shielded or go with both pairs shielded as a whole?

It depends... there will be crosstalk between the cables if they are all in the same shielded cable. There isn't a lot of information on how much crosstalk, and it would be easier to get some cable and measure the cross talk than to calculate it (IMO). It depends on how worried you are about noise. If you are designing for cost, I would go with one cable with four conductors, if your designing for low noise, I would go for four shielded cables.

It looks like your not pushing any boundaries with length\data rates...

Source: http://www.chipkin.com/articles/wp-content/uploads/2008/03/cable_length.GIF

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  • \$\begingroup\$ I am enlightened:-) \$\endgroup\$ – Steve Feb 18 at 23:37
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Very briefly:

If you're using Cat5 or above (and especially Cat6 cable with the plastic + shaped core that keeps the 4 pairs equally spaced), or something similar with well quantified coupling between the pairs. then particularly in your low-speed application there's no need to shield each pair from each other.

At that slow speed I'd make sure I use a very slew-rate-limited transceiver (transmitter in your full-duplex case), and there's lots available that are limited to 250k/500k/1M these days, which will further reduce crosstalk (due to rising/falling edges) to negligible.

And of course make sure you get your termination right (i.e. 100 ohms for Cat5/Cat6). As this is 1 point to 1 point full-duplex, there's no need to bother with pull-up & pull-down biasing, the diff-pair is always being driven by 1 end.

If you do all that, I'd be tempted to not even bother with overall-shielded cable either (i.e. Cat5/Cat6).

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  • \$\begingroup\$ @ Techydude. Yes, I am using a LTC2863-2 that has Low EMI slew rate limited data transmission. \$\endgroup\$ – Steve Feb 18 at 21:35
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https://www.cpii.com/docs/library/4/485appnote.pdf looking at figure 3.1 it appears you don't need a shield at all for these data rates (but if you're connecting in a noisy environment or to anything critical you should use one anyway.)

The way the pairs are twisted should prevent them from coupling too strongly.

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