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A question about the RS422 interface, is it necessary to have a common mode choke on differential lines?

I ask because in all the guidelines I've read the need a common mode choke is never addressed, but in this application guide, I see that it is added: https://www.we-online.com/components/media/o187971v410%20ANP083b%20Adapter%20PCB%20for%20filtering%20electromagnetic%20interference%20on%20an%20RS-485%20interface.pdf

See pag 11.enter image description here But since it's the first time I've seen a common mode choke on an RS422 interface, I would like to ask if it is really necessary.

I ask for a typical industrial working environment.

If the answer is yes, it is necessary or useful to have a common mode choke, I have a couple of questions. 1 - For RS422 interface it must be added on both differential pairs right? 2 - Should the common mode choke be added to all nodes of the RS422 bus or only to one? 3 - The capacitors C1, C2 and C3 in the figure are also needed or useful. 4 - In the same application I also have a PCA9615DPZ driver, which from an electrical point of view seems quite similar to an RS422, also here a common mode choke could be useful on both differential lines and the capacitors C1, C2 and C3?

Thanks in advance.

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    \$\begingroup\$ The document you linked and the picture you added are for an RS-485 interface and not a 422 interface. Basically, the linked document is trying to generate interest in adding their filters to applications that mostly don't need them <-- a common marketing strategy to drum-up new business (scare the customer into believing something is always needed). Don't be fooled by the pretty pictures and blurb. Having said that some applications (<1%) will benefit from CM chokes but, this methodology is well-known and well-proven long before Würth came along and repeated it. \$\endgroup\$
    – Andy aka
    Commented Jan 11, 2023 at 12:27

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  1. No it is not necessary, but it depends on your application and environment. If your device either emits too much interferece to other devices or environment, or is disturbed by too much interference from other devices and environment, then you need filtering to pass electromagnetic compliance tests. In an industrial application, it might be typical that there are filters.

  2. Both differetial pairs, as in the transmit pair and receive pair? It again depends what problem you are trying to address with the filter, but if you add one, you might as well add one for receive pair and one for transmit pair of RS-422.

  3. That again depends. You can add them to your products, but the other products on bus may or may not have them, and if they work without, you don't need to add an extra filter unless you find out they don't work without in your environment.

  4. Same thing, they are not necessaril needed, but are useful in filtering electromagnetic interference.

  5. From electrical point of view, differential I2C of PCA9615 is very different from RS-422, so same rules may not apply at all. It is more closer to CAN bus. There are very little application notes on it, as it does not seem to be very popular. In general, if you need to use them, you may be in a slituation where it would be better to do something differently than forcibly extend I2C and make it work. But I guess EMI filtering may still be needed to pass EMC compliance tests, the capacitors may be required for that too, but capacitors just add to the bus capacitance and may require slower I2C clocking.

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