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Thread: Splicing into CAN bus wiring

  1. #1
    Problem? sssgtr's Avatar
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    Splicing into CAN bus wiring

    Is it acceptable to splice into CAN bus wiring using open barrel crimps (then heatshrinked) with no negative effects?

    Main CAN bus length is short, less than 2m and will be using twisted pair 24ga. The stub it need to splice in is about 30cm or so.

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    Registered User BLO 767's Avatar
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    Define acceptable...

    Jokes. Course it'll work. You could cut into it with a pair of kitchen scissors and sticky tape your trunk in and it'll work, but some ways are more elegant than others. And not all of us have the tools (or setup pre-modifying) to do the most ideal way.

    It'd be fine, make sure you're not interfering in any pre-existing terminating resistors and that the BUS is still complete. And just dry test it of course, first, before you shrink.

    EFI Hardware make a nice coated 22 gauge or so pre-twisted Tefzel comm wire for CANBUS applications if you get stuck with the 24 wire.
    Last edited by BLO 767; 17-10-18 at 03:15 PM.

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    \_(ツ)_/ burn is weird's Avatar
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    make your splice length as short as possible and it'll work fine as you won't be using a termination resistor here.

    Can being a differential signal is very robust as long as your impedance is right and bus reflections are either adequately damped, or at a frequency well above double the bus data rate.
    Last edited by burn is weird; 17-10-18 at 04:02 PM.

    Oo___oO


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    Problem? sssgtr's Avatar
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    Thanks fellas, appreciate it.

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    Quote Originally Posted by burn is weird View Post
    make your splice length as short as possible and it'll work fine as you won't be using a termination resistor here.

    Can being a differential signal is very robust as long as your impedance is right and bus reflections are either adequately damped, or at a frequency well above double the bus data rate.
    About to say some of the brutal things I've done with 2-wire RS485 buses and it's still worked I doubt it would be any issue.

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    Problem? sssgtr's Avatar
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    Excellent, twist and tape it is.

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    Purist, whats that? Jason Broadhurst's Avatar
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    Quote Originally Posted by The Pupat View Post
    About to say some of the brutal things I've done with 2-wire RS485 buses and it's still worked I doubt it would be any issue.
    Fuck do I have some stories for you LOOOL

    80 m starred runs of twisted pair, about 15 terminating resistors on the line (one at each star) and the little network that could was still online.

    It looked like a rolling meadow on the scope hahhaha.

    9600 baud though.
    Jason Broadhurst

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    Quote Originally Posted by Jason Broadhurst View Post
    Fuck do I have some stories for you LOOOL

    80 m starred runs of twisted pair, about 15 terminating resistors on the line (one at each star) and the little network that could was still online.

    It looked like a rolling meadow on the scope hahhaha.

    9600 baud though.
    Yeah loves it, normally found the resistors are purely optional though.

  9. #9
    Purist, whats that? Jason Broadhurst's Avatar
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    What resistors hahaha
    Jason Broadhurst

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    Registered User BLO 767's Avatar
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    My MoTeC bus wouldn't work a damn without resistors LOL Really short, too! Soon as I added them in it powered straight up.

  11. #11
    Purist, whats that? Jason Broadhurst's Avatar
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    It's often to do with the drivers capability too. While I'm not saying the MOTEC ones would be budget by any stretch, an industrial field bus used on nuclear power stations is probably a cut above.

    It's a very interesting thing the terminating resistors. Lots of theory involved to get the 127, 250 etc etc ohm resistor.
    Jason Broadhurst

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    Registered User BLO 767's Avatar
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    At the very least they seem temperamental, as if anything I'm underselling how short this thing was. The theory goes right over my head, I'm just capable enough to comprehend the "insert X-rated resistor in Y and Z position".

    Giggity. X-rated resistor.

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    Quote Originally Posted by Jason Broadhurst View Post
    It's often to do with the drivers capability too. While I'm not saying the MOTEC ones would be budget by any stretch, an industrial field bus used on nuclear power stations is probably a cut above.
    Yep. Tolerance to shit-ness is very low in industrial gear.

    That sounds like they were missing the measuring resistor in the Motec then. Think Ive dealt with a cheapo device like that before.

  14. #14
    \_(ツ)_/ burn is weird's Avatar
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    if you're doing it properly in high availability environments you do it this way with split termination:



    improves EMC a lot.

    matching the characteristic impedance of the signal lines with the terminating resistors is just as much about the transceiver driver design as it is about wave propagation on the cable, but the cable properties are defined by ISO 11898 and the transceivers are also designed to this standard. yes you can use any old wire up to a point, but it wont work nearly as well as cable with the proper 120ohm characteristic impedance. a bit like using speaker cable for gigabit cat 6 wiring. it'll work, but not optimally.
    Last edited by burn is weird; 29-10-18 at 01:48 PM.

    Oo___oO


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