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I am currently probing what seems to me the CAN bus twisted pair of my car. But the signals are not what I expect. One of the pair is like a typical CAN HI signal, however the idle state is a 0V and a logical high is 5V. The CAN LO signal on the other hand is idle at 5V and a logic high at 0V. Therefore, this is not the standard differential signal with 2.5V common mode. My oscilloscope can however correctly decode this as a CAN bus signal and read the packets etc.

My question is this some variant of the CAN bus protocol?

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  • \$\begingroup\$ What's the bit rate? Does it and the electrical specs match the specs for low speed fault tolerant CAN bus? If yes, that's it then. \$\endgroup\$
    – Justme
    Commented Apr 30, 2022 at 15:14
  • \$\begingroup\$ bit rate is slower than what I would have expected. It is 83.3 kbps \$\endgroup\$
    – MAM
    Commented Apr 30, 2022 at 15:16

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The bus is likely ISO 11898-3 low-speed or fault-tolerant CAN bus, as the voltages and bit rates match the specifications for it.

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  • \$\begingroup\$ There's also the oddball J1850 bus that was popular in cars for a while. I believe it uses 5V differential signals as well. \$\endgroup\$
    – Lundin
    Commented May 2, 2022 at 6:34
  • \$\begingroup\$ @Lundin Maybe, but that would not be decodable with CAN bus protocol analyzer. \$\endgroup\$
    – Justme
    Commented May 2, 2022 at 7:51

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