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I'm having trouble selecting wire, jack and plug for small (up to 6 nodes, 3-4 m) CAN network. Each node will have 2 jacks wired for pass-through and short patch cables will be used to daisy-chain them together.

Previously, we were using CAT5e hardware with 100 Ohm termination for wiring CAN and 26V power. While not exactly up to 120 Ohm CAN spec, it worked just fine for such a short network and was really on a budget.

Right now we are trying to reduce the dimensions of some components and those RJ45 jacks come out as too big. So, the idea is to use 6p6c RJ25 connectors and 3 pair twisted cables. This gives us the same twisted pair for CAN and two pairs for power as before, but in much more compact package.

The problem:

Unlike CAT5e hardware, there is next to zero impedance data on alternative components. For example, here are some parts I've found on digikey: jack, plug, cable. I might be able to calculate wire impedance using one of the online twisted pair calculators and approximate dielectric constant for polypropylene, but this seems to be rather guesswork. I have no idea how to even approach the same question for jack and plug.

The question(s):

Is there a way to find out impedance for those components? Do such components even exist in the desired impedance range (100-120 Ohm)?

And more generic: how do you approach a task like this? I don't believe every engineer who needs to establish communication between two devices bothers with such low level details. Most likely they would use off-shelf components and focus on the software/device functionality.

Unfortunately, unlike Ethernet, the CAN networking solutions seem to be overpriced and oriented on industrial shoppers buying in bulk. Way out of our price range.

Update:

I've decided to go with CAT3 hardware (cables, jacks and plugs). It is not optimal, but all parts usually have rating clearly indicated and there is no need in guesswork.

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  • \$\begingroup\$ What speed do you need this CAN bus to run at? That is the first question you need to ask when 'spec-ing' busses. \$\endgroup\$ – Voltage Spike Aug 16 '19 at 18:44
  • \$\begingroup\$ @VoltageSpike ATM running at 500 kb/s, but after redesign planning on using 1Mb/s, no FD nodes. \$\endgroup\$ – Maple Aug 16 '19 at 19:42
  • \$\begingroup\$ Not a full answer, sorry, but cables like that tend to have thicker wire insulation than UTP, hence much lower capacitance/length, hence quite an impedance difference. Can you use UTP with smaller connectors? \$\endgroup\$ – Bob Jacobsen Aug 17 '19 at 3:42
  • \$\begingroup\$ @BobJacobsen Sure, I can use UTP, that's what we are using right now. I just did not want to use 8 conductor one, that is how I arrived at that 3-pair STP in the link (it was the cheapest on digikey). 3-pair CAT3 UTP is readily available, they are 100 ohm and good up to 10 Mbps, so I guess that takes care of a cable. But the connectors still a puzzle. \$\endgroup\$ – Maple Aug 17 '19 at 4:46
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The actual connector and wiring impedance is rarely a factor in CANbus installations, the more important ones are that the wiring is either twisted or shielded to reduce outside noise, that any stubs are short, that the termination resistances are placed at the ends, and that your total capacitance is within what all nodes can drive at your data rate.

For 4 meters in length and up to 1Mbit CANFD, you're very free to use just about anything. The industrial gear likes using 3-pin Deutsch connectors because they are water resistant with a strong detent latch, but spade terminals, Molex Micro-Fit, and many other non impedance controlled connectors end up used in quite a lot of trucks and buses, so Ethernet jacks will do the job just fine. Modern Mercedes sprinters running 1Mbit CANFD are using a connector that is practically a 2x9 0.1" header.

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