We have a long run (out and back) of CAN wire and are wondering if we can use CAT5 wire as a substitute? Is it legal? Is it wise? Is it more robust?
R77, R78, and R79 govern the CAN bus and CAN-controlled devices. There’s no rule specifying what kind of wire you have to use, so using CAT5 wire would be legal.
That being said, I wouldn’t recommend it. First of all, the CAN bus only uses two wires, whereas CAT5 wire uses 8. So the wires will be smaller than otherwise, and 6 of them will go unused. More importantly though, the CAN bus is designed to used twisted pair wire. This keeps the wires from being bothered by induction, keeping down noise.
Instead of using CAT5 wire, I would suggest using two single wires and twisting them together with a hand drill. You can easily create feet of twisted wire in a few minutes. One twist per inch is recommended. The other thing I would recommend is removing the “back” run of the CAN bus. If you have one CAN-controlled device far away from the others, you can have that device at the end of the chain and terminate the bus there with a 120Ω resistor. That should save you a significant length of wire.
Cat5 is twisted.
I don’t know if op is suggesting this, but I would open up the CAT5 and use one of the twisted pairs (there are 4 pairs) as the can wire. They are conveniently colored as white and another color in most CAT 5 cables.
Ah, missed that. So you can use CAT5 for the CAN bus. I still don’t see the advantages over twisting your own wire though. It’s pretty easy and saves you weight over using CAT5.
Edit: CAT5 also uses AWG 24 wire, which is the minimum size for the Weidmuller connectors. It should work, but you may have problems getting it to stay in the connectors. If you twist your own wire, you can use a size of wire better suited for the application.
One thing that I think is important to note is that the CAT5 used to interconnect patch panels is solid and the CAT5 used to make patch-cords is typically stranded.
I would not recommend solid wire in a vibration environment (robot). So if you’re going to go ahead with this plan, then you probably want to sacrifice a patch cord instead of using the 1000’ box of CAT5 that your mentor probably brought in from his/her garage.
Fun Fact: there are different RJ45 crimp connectors for solid vs stranded.
Generally poor practice, as this unnecessarily twists and stresses the conductors, which can have them breaking more easily. The correct way to twist wire pairs is the same way the manufacturer does it: Turn the spools as they unwind to the twisted pair, as seen here.
Easily done by hand for short (<25’) lengths. The number of twists per inch depends on the desired characteristic impedance.