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New to CANbus and just wondering about something. I want two nodes connected with CANbus. They have different MCU, transceivers and so on. Question is, can I just connect these on the same wire? (see picture 1)enter image description here

If not, how is that different to this (see picture 2)?

They have the same bit rate.

enter image description here

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    \$\begingroup\$ It depends what lengths those wirings are. We can't know if it is within specs or not. \$\endgroup\$
    – Justme
    Commented Sep 16, 2021 at 9:34
  • \$\begingroup\$ can i just connect these... probably not. Details needed. \$\endgroup\$
    – Andy aka
    Commented Sep 16, 2021 at 9:46
  • \$\begingroup\$ Thank you for answering. The bus is within its requirements. The wire length from node 2 is the same length as node 1 +- 1 centimeter. \$\endgroup\$
    – Ben
    Commented Sep 16, 2021 at 9:48
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    \$\begingroup\$ @Ben yes, but this is not about difference in length between two nodes, it is the absolute lengths of wires. The stub from the linear bus to Y connector, and from Y connector ports to nodes 1 and 2. \$\endgroup\$
    – Justme
    Commented Sep 16, 2021 at 10:25
  • \$\begingroup\$ @Justme Thanks again for responding. I will like to point out that i'm connecting Node 2 to a already working system. I just want to "hop on" the same connector as Node 1. \$\endgroup\$
    – Ben
    Commented Sep 16, 2021 at 10:52

1 Answer 1

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It depends on the cable length from the "multi-drop" bus to the physical nodes. This is called a "stub" and the maximum acceptable stub length on classic high speed can is 0.3m, though slightly longer is ok for the lower baudrates.

Notably, your schematic also lacks a signal ground, which must also be routed to every node in addition to CANH and CANL.

See What are the most common causes of CAN bus communication errors? for a summary of common problems like this.

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  • \$\begingroup\$ Thanks for your answer. To make it more understanding: the bus i want to connect on is already a working system. i just want to add one sensor, and its very close-by to another sensor. So the system is functional and working. The system is SAE J1939, so speed is between 250-500 kbps. \$\endgroup\$
    – Ben
    Commented Sep 16, 2021 at 9:52
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    \$\begingroup\$ @Ben Maximum recommended stub length is still around 0.3m then. And you still need a signal ground. \$\endgroup\$
    – Lundin
    Commented Sep 16, 2021 at 13:50
  • \$\begingroup\$ @DavideAndrea No, CAN is a 5 wire bus where shield and V+ are optional. Fundamental electronics knowledge gives that for example your PC and a car will not necessarily have a voltage potential difference that's within the spec of the CAN transceiver. Which is +/-58V on modern CAN transceivers like MCP2562FD and far less on older ones like TJA1050 or MCP2551. Thus we need a signal ground or the communication might fail. \$\endgroup\$
    – Lundin
    Commented Sep 21, 2021 at 13:54
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    \$\begingroup\$ @DavideAndrea There also exists a quackery myth that you don't need to have signal ground across nodes located on the same vehicle, but people saying such cannot possibly have worked with automotive electronics in a professional setting, since the chassis ground is noisy as f*** and can't be used as signal reference, because that ground will be shared with starters, alternators, valves and all other evil things that could be found on some generic vehicle. \$\endgroup\$
    – Lundin
    Commented Sep 21, 2021 at 13:56
  • \$\begingroup\$ OK, I looked up in ISO 11898-2, "Road vehicles — Controller area network (CAN) — Part 2: High-speed medium access unit" iso.org/standard/67244.html. It says: "two-wire cable, relative to a common ground". You are correct, I stand corrected. \$\endgroup\$
    – Davide Andrea
    Commented Sep 21, 2021 at 15:15

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