CAN bus over RJ45 - shielding - Page 1
Forum section: electronics / projects

author: StephanR
author_rank: Newbie
timestamp: June 10, 2020, 05:55:13 pm
content: Hi there,what is the best shielding technique for CAN when using RJ45 connectors?According to several sources like e.g.CiA 303-1 V1.3the pinout should look like the attachment shows (optional shield on pin 6).But why does no one use the actual shield (the casing of the connector)? I know that a lot of cables do have a plastic jack, but is a single wire even useful as a shield?And given that I have a metal connector and cable wouldn't it make sense to use pin 6 for compatibility and the casing for actual shielding as shield?I am also happy about suggestions on how to connect the shield to my (isolated) interface part of the PCB. Some recommend a resistor, others a capacitor.My CAN interface is designed according to the reference designTIDA-01255from TI.

author: KrudyZ
author_rank: Frequent Contributor
timestamp: June 10, 2020, 08:13:17 pm
content: My guess as to why they picked pin 6 for the shield connection is that it would be twisted with the wire on pin 3, which is a ground.That would give you isolation, but maximize capacitive coupling to circuit ground. This helps shunting common mode AC currents to chassis ground, reducing emissions.Using a proper shield as you described would be better, but CAN is fairly robust and, being differential, usually isn't too bad as far as emissions is concerned, assuming you use some common mode suppression on the nodes.

author: MosherIV
author_rank: Super Contributor
timestamp: June 10, 2020, 10:01:48 pm
content: As far as I know, one reason the shield is not used on CAN network is ground loop problems.Many CAN networks are complex and figuring out where all the ground/earth points are may not be easy, so they avoid problems with ground loops by avoiding joining the grounds together by not using the cable sheild.CAN is twisted pair and differential so fairly immune to noise as krudyz pointed out.

author: StephanR
author_rank: Newbie
timestamp: June 23, 2020, 07:00:35 pm
content: Thank you for the very quick response.I will not connect the shield since I don't have an earth ground to connect it to (at least not on the circuit board). This means I can only connect it to the isolated ground which is connected anyway.Since this device will get tested for EMC I will share the results and the final schematic in case it is helpful.I think it is quite hard to decide what is overkill and what isn't (choke for can lines?, EMI filter for isolated DCDC?, ...) in this area.

author: BreakingOhmsLaw
author_rank: Frequent Contributor
timestamp: June 23, 2020, 07:45:38 pm
content: In every vehicle wiring harness i've ever seen, not once was a CAN bus shielded.Up to 1Mbaud works fine w/o shielding with less than 1 ppm error frames.I would guess that adding shielding is bad for the rise time as you add capacitance.

author: ConKbot
author_rank: Super Contributor
timestamp: June 24, 2020, 02:12:44 pm
content: Is this shield intended to be more of a "case ground" than a wire harness shield braid? I.e. Power is on this connector, so going to a widget with a single cable for power and coms would be feasible. A separate signal ground and case ground could be more understandable than trying to make a harness with rj45/cat x cable that has the shield connected to pin 6.

author: Smokey
author_rank: Super Contributor
timestamp: March 30, 2025, 08:26:41 pm
content: Good demonstration of shielding.https://youtu.be/Hj-HVve1QC4?si=BGistM7UM-Zrr1Z7

author: uer166
author_rank: Super Contributor
timestamp: March 30, 2025, 10:23:21 pm
content: Quote from: MosherIV on June 10, 2020, 10:01:48 pmComplete nonsense, CAN is mainly used between non-isolated controllers which *requires* a hard ground connection between the 2 transceivers. CAN is a 3-terminal bus, not 2. If you only this then the common mode voltages will not be well defined and it will be a shitty non-reliable network.Shit like this advice is what causes untold number of systems in the field to fail spectacularly and cause $$$$ in damages because some kid was worried about "ground loops" and created something unreliable.In automotive you'll find non-shielded twisted pairs with a 3rd "ground" reference connection between ECUs, or a hard chassis connection between them. They're usually not super well impedance controlled, and are regular PVC wire.Now if your interface is isolated, you STILL need a 3rd "reference" signal from one transceiver to the other and not let the diff pair define the CM voltage. In a RJ45 connector it would be much easier to use an extra pair of conductors for that than trying to use the shield. These connectors are not well suited for shielding anyway.

author: Siwastaja
author_rank: Super Contributor
timestamp: March 31, 2025, 08:53:54 am
content: Take a look at how twisted pairs operate. Simply put, it's the geometry: add a source of interference outside of the cable, and it couples equally to the two wires; even if one of the wires is closer to the interference source at one location, wires twisting means that at the next point, the other wire receives the interference. Thus, interference becomes common to the two wires, and is cancelled by subtraction in the receiver.Alternative way to reject interference is co-axial design, where one of the wires acts as a shield.But you usually do not need both. Because twisted pair geometry cannot be perfect (nothing is), additional shield can be used, but that would be quite rare. (Notextremelyrare, butquite)The idea of "ground loop problems" is of course false, a shield won't make any difference because CAN already has a mandatory ground wire anyway. Maybe some are mixing CAN and Ethernet up. Ethernet is conveniently transformer-isolated. If you choose to add a shield (so STP instead of UTP) - also usually not recommended - then there is a new question how this shield will be connected (no one single right answer there), and if hard connection to local circuit ground (instead of capacitor coupling) is chosen, then sure, it bypasses the benefit of transformer isolation and adds a possibility of ground loops.But that is irrelevant to CAN. CAN has very small common mode range (one hundredth of Ethernet or so), and the standard requires common ground anyway (see e.g. ISO 11898-2:2024: "an electric two-wire cable, relative to a common ground", also see figures)I thought one of the benefits of CAN over RS485 is the cultural one, absence of the "no ground wire" cult bullshit, but lately I have stepped into this groundless CAN bullshit more than once, which makes me wonder if it's lost cause too.Having to deal with non-compliant, non-standard, broken modbus RTU devices is constant pain, seeing the same in the world of CAN is, I think, still a bit more rare but clearly not non-existent.I wonder what is so ****ing difficult in RTFS (read the fucking standard). RS485 standard itself is vague enough not to say much anything about anything, but Modbus RTU standard luckily fills in the blanks and clearly and totally unambiguously mandates usage of a ground wire and biasing resistors as non-optional requirement, yet 50% of products lack the ground wire and 95% of the installations lack the resistors, making RS485 one of the least robust and most problematic buses in the history of wired electronic communication.CAN folks managed, by a clever trick over RS485, to combine biasing and termination in one component and, as such, less likely to forget it. Also with CAN it was possible to clear the legacy of incorrect copy-pasting of appnote after appnote and start from scratch again, and the situation with appnotes is much better.Yet somehow there are always idiots who will do random stuff like leave one of the three mandatory wires out, but most surprisingly it also happens in design, not only on the field. And it's nearly impossible to fix a product after it left factory. We could of course try to report this crap to authorities to see if they pass EMC (very likely not), but that would be counterproductive. We have to live with broken crap.

author: PGPG
author_rank: Frequent Contributor
timestamp: March 31, 2025, 02:46:49 pm
content: Quote from: Siwastaja on March 31, 2025, 08:53:54 amVery likely not but it is (in my opinion) possible to design RS485 connection using only 2 wires and being EMC compatible.Third wire reduces CM disturbance (both - in emission and sensitivity) by reducing the CM current circuit area.But the other way of reducing it is by reducing CM currents by inserting high impedance in CM circuit. It can be done by galvanic isolation and by CM chokes.The specialized RS485 cables are more expensive than widely used Ethernet cables so tendency to everywhere use Ethernet cable. When in 2004 I designed devices using RS485 I decided to use two Ethernet cable pairs - in one wire A twisted with GND and in second wire B twisted with GND. GND cables shorted together at both ends. That way I assumed disturbance at each wire is reduced against GND so also reduced between them and I am avoiding people not using third wire. I have measured impedance between A and B in such configuration and got about 130 ohm (if remember well) so I have used terminal resistors of 130 ohm. Devices passed EMC tests.Installers using our devices were surprised why for simple differential communication we ask them to use two pairs.Now I use single pair (so 100 ohm termination) and write that devices have to have their GND connected but design devices such that even without these connection they work.