What this means that while the bus is physically a broadcast (all the nodes are connected to the same cable), the protocol makes it work logically as multicast.
As the bus can have a lot of traffic on it, but most nodes only care about some specific frame IDs and does not have the computational resources to handle the full traffic, each CAN bus controller implements some sort of “acceptance filter”, which is usually a bitmask to decide which range of frame addresses should be received by the node, while all the others are discarded silently. what’s the meaning of the payload), and all nodes on the bus can send and receive any type of frame.
The idea behind this is that address are used to identify the type of frame (i.e. (Image from Analog Devices AN-1123 Application Note datasheet) The unusual thing is that we think of frame address, not source or destination address, in fact CAN bus nodes does not have an address at all! What is really interesting about CAN bus is the actual usage model: each packet has single address of either 11 or 29 bits (the latter is called extended frame), and a maximum payload of 8 bytes. The bus runs up to 1Mbps, and the speed directly influences the max bus length.
The bus is physically deployed as a terminated differential twisted pair with a ground reference, and it’s usually in a trunk-drop topology, where many nodes are connected to a main trunk cable with short drop lines, sometimes using T junctions. If you are new to field busses, CAN bus may appear weird at first sight. This is based on my experiences hacking into my Toyota… Toyothack! In this post I’ll show how to tap into a modern car local bus, dump a bunch of data and analyze the trace offline to write a decoder from scratch using the SocketCAN APIs and utilities. The Linux kernel has native CAN bus support at network layer since some years, with a lot of drivers for both embedded and USB CAN bus controllers, so it’s now fairly easy to add a CAN bus interface to any Linux laptop and have a playaround with it. CAN bus is an automation fieldbus commonly used in the automotive industry as the main network bus to allow communications between the many on-board ECUs on modern vehicles.
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