| Electronics > Projects, Designs, and Technical Stuff |
| Will Can Bus still work with lower impedance cable? |
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| T3sl4co1l:
Stubs are acceptable given restrictions on length and bitrate. Again, electrical length versus bit time. If you don't need much bandwidth, it may be worth the weight savings alone (not to mention ditching the heavy cable you have, free though it was!). You can absolutely use two-pair cable for a loop route; wire the pairs together at the dead end. Carefully keep track of which pairs are going where! BTW, are there any restrictions on wiring used in GA craft -- I'm assuming this is the situation, you're working on your own plane? Commercial craft wiring is skookum as heck, but I'm guessing it's relaxed for GA, as many other things also are. I've never read up on it, just putting the thought out there. Tim |
| ITman496:
Things are extremely restrictive for GA, which is why I'm not doing that. I've got an ultralight aircraft, which is the most experimental class of experimental aircraft, with... well, no restrictions what-so-ever. So I'm putting fancy electronics in because its fun. It's a minimax 1030R if you are curious! I consulted my friend who has lots of wire and he offered this up as an alternative: https://www.alliedelec.com/m/d/c9b490c9eef9b2520977afc5146c0284.pdf Belden 88641 It does not list its impedance which is a bit worrying, but it sure looks nice and much thinner. I'm trying to figure out how to calculate that now to see if this will work for me. |
| ITman496:
I ran it through this calculator: https://www.allaboutcircuits.com/tools/twisted-pair-impedance-calculator/ And got a shocking.. 14.6 ohms impedance. |O Is it because the FEP insulation is so thin that it brings the conductors too close together? Parameters used: .02011in wire diameter .012in wire separation (due to wall thickness of 0.006 FEP) 2.1 dielectric constant (looked it up and it said that it was the same as PTFE/teflon, which I googled at 2.1. Am I messing anything up or this this wire despite being twisted pair and shielded completely unsuitable for can bus? |
| T3sl4co1l:
Ah yeah, ultralight, that makes more sense! You shorted em together. :-DD Separation is center to center, it looks like. I get... 75 ohms or thereabouts? Quite typical and reasonable. Note that STP is lower on account of the shield, making it act more like a pair of coax cables sorta merged. The above calculation assumes ground at infinite distance, I believe. Tim |
| ITman496:
Oh dear. I feel like a fool. Thank you! :-[ :palm: Do you know of a calculator I can use for STP? I'm just trying to figure out what resistor I'll need... I guess I can just play with it until I stop seeing reflections, though that is an annoying approach. Also I meant to clarify, so I am going to hard ground the shield on one end, and the other, capacitive ground to avoid ground loops. Several capacitors surrounding the ground, what values are you imagining? 0.1uf little smd caps? Thanks again!! |
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