CAN Bus

CAN bus is a Communication Protocol and networking standard designed for communicating between devices in vehicles. Because of its versatility and relatively low complexity, it is used in the vast majority of vehicles manufactured today. By letting multiple devices communicate with each-other through a common interface, wiring harness complexity can be reduced.

Types of CAN Bus Networks

There are several types of CAN networks: low–speed CAN and high–speed CAN. There is also #CAN FD, described below.

High–Speed CAN

Example of a high–speed CAN network.

High–speed CAN (ISO 11898-2) is probably the most common version of CAN. It can support speeds up to 1 Mb/s, and uses a linear bus. High–speed CAN does not work when hooked up as a star network, as star networks can lead to strange behavior at higher frequencies. The stubs coming off of the linear bus should be kept as short as possible. If you are designing a custom PCB that uses CAN, you can do this branching on the PCB instead of in the wiring harness. A 120 ohm termination resistor is required at either end of the bus, i.e. the DC resistance measurement between the 2 CAN wires should be 60 ohms when hooked up properly. The cable should theoretically have an impedance between 108 and 132 Ohms at the bus frequency, but in practice this isn't necessary and most cable will work just fine.

Low–Speed CAN

Example of a low–speed CAN network.

Low speed is limited to a speed of 125 Kb/s, but it is more flexible in its routing. It can be connected as a star bus with no issues, and each device on the network has two termination resistors. Their value can be calculated based on the method described on page 4-10 of this document from National Instruments: [1].

CAN FD

CAN FD (Flexible Data-rate) is an extension of the original CAN protocol. It can dynamically switch between different data rates and longer or shorter messages, resulting in faster data speeds and higher data capacity.