Groundless RS-485 differential driver as a RFI receiver? - Page 1
Forum section: electronics / beginners

author: e100
author_rank: Frequent Contributor
timestamp: October 08, 2023, 03:58:36 am
content: In this "AN-1057 Ten Ways to Bulletproof RS-485 Interfaces" app note from Texas instrumentshttps://www.ti.com/lit/an/snla049b/snla049b.pdfon page 10 it says, "A typical mistake is to connect two nodes with only two wires. If you do this, the system may radiate high levels of EMI,...".So if something is a good RF transmitter, does that also mean that it's a good receiver of RFI, or are the behaviors totally unrelated?

author: JustMeHere
author_rank: Frequent Contributor
timestamp: October 08, 2023, 08:35:57 pm
content: No. A circuit with large ground loops will radiate EMI. And that's what the article is talking about. If there is no ground wire then the ground loop will be huge. Look up displacement current.For example in a ribbon connector the wiring will go like this:Signal = a,b,c,d,e,fGound = GaGbcGdeGfThis way there is a ground next to every signal.If it goesaGbcdGefThen c and F will still need to couple with G, but in this case b,d,e are between a signal an G. All three of these will recceive a lot of EMI and have very noisy signals. Obviously b,d,e will not be able to receive RF signals either. This is because they are inside of a noisy magnetic field.I could help to even goGabGcdGefGSo the outside Gs help protect from EMI, and even stop a and f from radiating EMI.If you look at BGAs you will see alot of G pins. This is to help with the pins around them and the ground loops.

author: TimFox
author_rank: Super Contributor
timestamp: October 08, 2023, 09:05:34 pm
content: That's the important details for RS-485 cabling.Simpler: Think of your local broadcast transmitter for MW AM or VHF FM -- do you think it works as a receiver?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 08, 2023, 09:29:05 pm
content: Well, yes and no. Define in what ways you mean, first.In general, EMI is reciprocal. A good transmitting antenna is also a good receiving antenna.What the EMI does, once it's in the circuit, is another matter.The circuits need not be reciprocal, and a circuit radiating 1V needn't do anything special when subjected to 1V from an external source. With undefined circuits, it's not even meaningful anyway, what it means to "receive".There is somewhat implied that it might receive erroneous data, being that RS-485 is bidirectional, but this depends on the application, and you could well have a 485 application where a given node is transmit-only. Or a burst of EMI occurs during a transmit packet and therefore the transmitting node doesn't give a crap.Note that RS-485, like any differential standard, is only differential within the given assumptions. Namely the common mode input range. Exceed that, and data is corrupted, or erroneous data is received. This should be pretty obvious, it's just a diff pair receiver, a comparator or amplifier, and it stops working outside of nominal range, or the inputs become clamped by diodes. But it's ashockingly common misconceptionthat -- oh yeah it's differential, that means I can turn my brain off and never have to think about it again -- no, that's not how reality works, that's not how it works at all.Even a fully isolated circuit is only isolated until the barrier breaks down, or within its CMRR, which varies with frequency, particularly degrading at high frequency.So, please ask, and please think in terms of, what it means to receive RFI or EMI at a given device.As for ungrounded RS-485, it works often enough, in spite of such sabotage, because the CM range is +12/-8V, and it takes a hell of a lot of RF (e.g. commercial levels are typically 3V, 3V/m; industrial, 10V, 10V/m) to exceed that range, at least if it's well damped (peaking is avoided -- that is, extra voltage gain, from fields to the interface, at "unlucky" frequencies, usually cable resonances). It's still better with ground, particularly if a shielded cable is provided --and grounded at both ends-- then the CMRR is extended, by whatever the shielding effectiveness of the cable is: maybe 10s of dB for a shitty connection, maybe 80dB or more for a solid (shield) connection, and now you can survive abuse of 100s V or V/m, even endure ESD and EFT pulses without any disruption of communication.Tim

author: Doctorandus_P
author_rank: Super Contributor
timestamp: October 09, 2023, 11:17:57 am
content: As T3sl4co1l already mentioned. Antenna's work both ways.I've been using a self designed home network for nearly 20 years now and it uses two pairs of CAT-5 cable. One pair for 24V power (So it also has GND) and one pair for RS485 data, and I have had plenty of trouble with the long cables picking up excessive noise. This usually results in microcontrollers resetting when I do things like turning on CFL or Halogen lights. Slapping on some of those splittable ferrite cores over the cables helped but did not completely eliminate it. Putting on two or three in a row worked even better. This was the first time I got slapped in the face with EMC troubles and a good learning experience. In a "real" design such things are of course handled by integrating filters on the PCB. Any cable entering or leaving a PCB needs to be considered as an antenna, and often extra circuitry is needed to mitigate this effect.

author: e100
author_rank: Frequent Contributor
timestamp: October 09, 2023, 01:24:46 pm
content: It seems intuitive that a good RF transmitter would also behave as a good RF receiver, however I don't remember anything in the application note that talked about this specifically.

author: wasedadoc
author_rank: Super Contributor
timestamp: October 09, 2023, 01:36:42 pm
content: Quote from: e100 on October 09, 2023, 01:24:46 pmNo. A good transmitteraerial/antenna(either intentiona or unintentional) can be a good receivingaerial/antennaat the same frequency. A transmitter - a complete device that delivers RF power into the ether - is not designed to receive.