Electronics > Projects, Designs, and Technical Stuff
Will Can Bus still work with lower impedance cable?
ITman496:
TLDR: Can I use 73 ohm impedance twisted pair cable instead of 120 ohm as long as I match the termination resistors to the new cable or will it not work well?
Long story below:
I am working on a can bus network (designing it right now) in my aircraft that is around I'd say 8 nodes and the total length end to end with them sprinkled along the chain is around 20 feet?
The twisted pair will run along the rest of the wiring harness and thus near some power cables with some amount of amps flowing through them but not terribly much. And pretty steady draw, no real inductive kicks constantly flying around. There will be an aircraft AM radio but not near the wiring, and the engine is pretty far away too.
I have a lot of things with twisted pair, and one of them is a very hardy, very nice cable is this:
https://www.belden.com/hubfs/resources/technical/technical-data/english/76502TS.pdf
Belden 76502TS. I was given a surplus spool of it, it be wonderful.
However, I have a question and have different answers across my googling. The characteristic impedance of this belden cable is 73 ohms. I know that can bus likes 120 ohms. Does it matter that much? Some results from my searching seems to suggest that as long as I use 73 ohm termination resistors on each end, to match the cable, I'll be peachy.
This cable is shielded very well, very robust, and I like it more then regular solid core ethernet cable. The main reason I want to use it is because of its durability and shielding. It will be on plugs that get moved around and used so I am hesitant to use solid core unshielded ethernet due to fatigue and whatnot. Also this nice wire is free and I already have it.
I'd really rather not redo the harness again if it doesn't work, so I figure I'd ask if anyone has any experience using out of spec cable?
My nodes will all be using MCP2515 canbus transceivers powered by arduinos, by the way. My data rate I believe will be around 125-250kbit/s
Worst case scenario I can buy proper cable but... hey, I'd love to just use this fancy nice stuff I have around.
Berni:
Well the CAN transceivers are probably designed to drive those 120 Ohm with the cleanest signal but they won't complain driving lower resistances because CAN works just fine if you have less or more than 2 terminators. The datarate is also not all that high for the distance so it should be fine.
You can always stick a scope on it and see how clean the waveforms look.
ITman496:
So a good test would be to stick a node on each end of the wire on the spool with max length and see how clean it looks? And if it looks good on the spool, it will probably work fine?
Thank you! I'll try that tomorrow.
Niklas:
The CAN bus signals are quite robust and probably ok, perhaps with minor tweaking of the termination networks.
One thing to look out for is common mode conducted emission on the power supply wires at the 125/250 kHz and their harmonics.
Had an issue with common mode emissions from CAN bus in the lab last year. A 2 m long harness for CISPR 25 radiated emissions measurement, made up with 4 wires and no shielding. I forgot to check the material specs of the wire insulation and the thin PTFE dropped the impedance to about 65 ohms due to Er and thickness. Emissions vanished with a new harness made from PVC insulated wire where the impedance was within ohms of 120.
T3sl4co1l:
Probably reduced bitrate-distance, due to the lower voltage into the lower load impedance, and because PVC is pretty lossy stuff, rounding over the edges more than a high-Q cable would.
You can kinda-sorta recover some edge-rounding-over by using a slightly high termination (say 82 or 91 ohms). I suppose it might extend bitrate-distance by a similar amount (i.e., some 10% or so), which really isn't important anyway as you may not have such fine control (i.e. if your choice is steps of 250k, 500k, etc.), and also if you're hitting the bandwidth ceiling that closely, you have a fundamental design problem that shouldn't be solved by cable tweaks.
But if you aren't doing huge runs -- namely, if total bus length is much less than a bit time, it doesn't really matter very much where the terminator is, or if you have one at each end. It's very tolerant.
Yes, CMCs are a good idea. A lot of devices have these, and parts are cheap and common. Typically something like 50uH or 1kohm+ (at 20 or 100MHz) is chosen. Since you have shielded cable, you might also opt to take advantage of that, perhaps instead of CMCs (or as large values) -- mind that grounding a shield at both ends is the only meaningful application of a shield, and that you may encounter galvanic isolation or ground-loop concerns in doing so. (The shield can at least be RF-grounded through capacitors, to keep signal quality acceptable while avoiding those problems.)
Tim
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