I got some Microchip CAN transceivers and some Stellaris Launchpads from TI. I would like to learn how CAN works. It looks very nice for what I want to do (one controller per room, controls lights, vents, etc.)

The first thought was to use Cat 6 for the data line. But I don't understand how I am supposed to tie the connector to the twisted pair (unless I just do a pass through, two RJ45s for each node).

Anyway, my real question is, while 60 ohms is the impedance recommended for CAN, can I use 50 or 75 ohm coax cable? Googling it didn't yield anything much for me, nothing definite.

So I can just run a coax cable and T it off instead of passing through (or splicing the cable I guess).

Coax seems a nice clean way to do it, but since it's not talked about I am missing something, I imagine.

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    \$\begingroup\$ 120 Ohms not 60 ohms termination resistance is required for CAN! \$\endgroup\$
    – Swanand
    Jan 28 '13 at 7:34
  • \$\begingroup\$ I thought it was 2 paralleled 120 ohm resistors \$\endgroup\$ Jan 28 '13 at 7:51
  • \$\begingroup\$ @MichaelStyln- It's two 60 resistors in series, with the center often tied to Vcc/2 \$\endgroup\$ Jan 28 '13 at 8:01
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    \$\begingroup\$ @FakeName, two 60 ohm resistors in series, possibly with the center tied to Vcc/2, with such a termination at each end of the cable yields a net 60 ohm differential impedance on the bus. An ohmmeter test on a powered-down bus is the first test I do when things aren't working right to assess under- or over-termination. \$\endgroup\$
    – HikeOnPast
    Jan 28 '13 at 15:52
  • \$\begingroup\$ @HikeOnPast - You're missing the difference between terminating a transmission line (what you're doing here), and the static resistance of the system. There is an excellent Microchip App-Note here: ww1.microchip.com/downloads/en/appnotes/00228a.pdf \$\endgroup\$ Jan 29 '13 at 6:24

CAN bus uses differential signalling. CAN is wired with twisted pair. Have a look at the standard CAN pinouts. Notice that one of them is for the RJ-45 connector.

CAN cabling guide.

  • \$\begingroup\$ Ok, thank you. I will do the standard method so I can avoid trouble. Thanks for your time \$\endgroup\$ Jan 28 '13 at 8:13

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