2
\$\begingroup\$

I just looked at the datasheet of the TJA1050 CAN transceiver and noticed that both CANL and CANH tolerate -27 to +40 V.

Standards aside, does that mean I could pull the entire CAN bus to e.g. 12V to provide power to devices along the bus? With GND running along a chassis, would that make it a 2 wire data- and powerbus solution?

edit: Even if the transceiver would not tolerate that, it should still be possible to do it the old-fashioned way by adding input capacitors to decouple the transceiver from the phantom power, right?

\$\endgroup\$
15
  • 2
    \$\begingroup\$ That rating means that the transceiver should survive an accidental short to 12VDC/24VDC, not that it guarantees that meaningful data transmission will be possible at the same time as there is an accidental short circuit present on the bus. The dominant state for either CAN line will 2.5V, in which case the transceiver will actively drive that signal level. Also, using chassis for signal ground is a really bad idea in general. Just as inventing some secret 2-wire non-standard is a really bad idea. \$\endgroup\$
    – Lundin
    Commented Feb 27, 2023 at 10:19
  • 1
    \$\begingroup\$ The CAN transceiver will survive -27V..+40V on the differential lines. But when transmitting a dominant bit, it'll drive CAN_H to 3.0..4.25V and CAN_L to 0.5..1.75V. And during recessive bits both lines will be between 2.0..3.0V. \$\endgroup\$
    – Velvet
    Commented Feb 27, 2023 at 10:56
  • 1
    \$\begingroup\$ Anyway, what is the actual problem here? Why is 6 wires a problem? Is the actual problem a cabling one? Is the problem cable thickness, EMC, what? \$\endgroup\$
    – Lundin
    Commented Feb 27, 2023 at 11:01
  • 1
    \$\begingroup\$ "Even if the transceiver would not tolerate that" It does tolerate those voltages. That's the point. "..by adding input capacitors to decouple the transceiver from the phantom power" That sounds like a bad idea. CAN is robust when implemented properly. But I've come to the conclusion that it very easily backfires when being abused. Just do it the proper way and add two separate V+/GND lines. \$\endgroup\$
    – Velvet
    Commented Feb 27, 2023 at 11:07
  • 1
    \$\begingroup\$ There are ways to multiplex power lines for various purposes, but CAN is not one of them: it needs DC coupling so you can't also send DC power through it. For example, Ethernet is AC coupled so can be multiplexed with DC (power over Ethernet); see also communication over powerline schemes (ranging from home intercoms to Ethernet and more). \$\endgroup\$
    – Tim Williams
    Commented Feb 27, 2023 at 11:10

0

Reset to default

Browse other questions tagged or ask your own question.