Designing a CAN network

When designing a CAN network, you have to make some general decisions. First of all, you need to estimate the required data throughput. In theory, you can't overload the CAN network. You can only delay the transmission of lower-prior data frames, but then your application does not work properly. For a short time, busloads above 50 percent are acceptable. But an average busload of 50 percent should not be exceeded as a rule of thumbs.

Now you can calculate the necessary transmission speed, which is the number of data frames bits to be sent in a dedicated time window (e.g., one second). You can and should ask the application programmer to reduce the communication as much as possible. It is recommended to communicate as little as possible, but as much as necessary. Avoid communication without new information (CANopen offers you many options to do so, due to its flexible parametrization of PDO messages). If you have optimized your busload and know the minimum bit rate you need, you can calculate the maximal achievable network length.

You can also do it the other way around, of course: You take the needed network length and determine the possible maximum bit rate. Here, you should optimize the communication to a minimum too. If possible, always use the minimum bit rate, because the lower the speed, the more robust the communication.

Today, there are two CAN data link layer protocol options: CAN CC (classic) and CAN FD. CAN CC is limited to a maximum bit rate of 1 Mbit/s and up to an 8-byte data field. CAN FD provides up to 64 byte in the data field and enables a higher bit rate in the data-phase depending on your network topology and the chosen transceivers. Using CAN SIC (signal improvement capability) transceivers, you reach bit rates of up to 8 Mbit/s even when using challenging hybrid topologies.

The third option, CAN XL, is not yet real, because MCUs (microcontroller units) and CAN SIC XL transceivers are not yet available in high volumes. CAN XL components are still in prototype phase. But this can change quickly.

Independent of the selected CAN protocol version, you need to choose carefully your network topology: It is highly recommended to use a line topology terminated at both ends with resistors matching the impedance of all other physical layer elements (cable, connector, etc.). When using bit rates much higher than 1 Mbit/s (CAN FD), even the impedance of micro-controller ports can be an issue. Star and mixed topologies limit the achievable speed/length ratio. It is recommended to keep the not terminated stubs and branches as short as possible. Even carmakers often using star topologies consider going for line topologies when introducing CAN FD communication.

If the network is quite long, it is recommended to use galvanic isolation. But this shortens the maximum length or speed due to the additional delay caused by the transformer or optocoupler. You should also use twisted-pair cables and if necessary, you should shield them, in particular in noisy environments.

Most new designs are based on the CAN HS (high-speed) transceivers (specified in ISO 11898-2). The transceivers are optionally available with low-power functionality. This means, the ISO 11898-3 approach (low-power, fault-tolerant limited to 125 kbit/s) is not state-of-the-art any longer. The single wire CAN (SAE J1411) is no option anyway, due to being slow and to be not robust.