CAN bus over RJ45 - shielding

[StephanR]

Hi there,

what is the best shielding technique for CAN when using RJ45 connectors?


According to several sources like e.g. CiA 303-1 V1.3 the 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 design TIDA-01255 from TI.

[KrudyZ]

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.

[MosherIV]

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.

[StephanR]

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.

[BreakingOhmsLaw]

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.

[ConKbot]

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.