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Physical CAN bus layout

CANBus Communications Decoded

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I'm trying to connect a motorsport dash on a PQ35 Audi and the closest point where the powertrain CAN is present it probably the gear selector, so I was thinking of just tapping there, but that could easily exceed the recommended stub length. Anyone knows how the physical layout of the CAN network looks in these cars as I'm trying to locate the main bus trunk and tap directly into that or should I just ignore that and tap anywhere?


You should consider cutting the existing CAN lines and extending them to nearer your dash for the stub splice. It just means you need to run two pairs of CAN wires from the existing location to your new connector.

You could also locate the bus node, and extend from there, making your dash a new node. Just pay attention to terminating resistors.

@David, wouldn't that effectively lengthen the stub length to the current CAN node (gear selector), since that part is already at the end of a stub. I'm trying to understand where the main CAN trunk in the car is.

@Rikko, yes, but if that node is already a non-terminated node with a stub length close to the limit, adding something from there will exceed that limit for sure. That's why I'm trying to find the main trunk, since based on the wiring diagram the physical topology should be a real bus, and not a node-to-node topology.

Usually OEM CAN buses aren't spliced with stubs of any significant length. So, I figured you are at one end of the main CAN trunk. Often you will find two CAN wires going to a connector, and you can just extend the bus from one of the wires.

But worry about it all you want. Or you can just try it and see what the CAN signals look like with an oscilloscope.

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