CAN Physical Layer

There are two different physical layers: The high-speed CAN and low-speed CAN physical layers. The high-speed CAN physical layer is primarily used in the powertrain area of the automobile. The low-speed CAN physical layer is used in the convenience/body area. The high-speed CAN physical layer is discussed in the following.

The high-speed CAN physical layer is described by ISO document 11898-2. ISO 11898-2 primarily defines differential signal transmission at a maximum data transfer rate of 1 Mbit/sec with a maximum network length of 40 meters and maximum of 30 bus nodes. Due to the high data transfer rate, ISO 11898-2 calls for a bus termination resistor at each line end. It is not difficult to extend the network by reducing the transmission rate.



The differential voltages defined in ISO 11898 are implemented with the help of the CAN transceivers. The figure shows the physical (recessive and dominant) levels when a high-speed CAN transceiver is used. The recessive bus level (logic "1") is characterized by a differential voltage of 0 Volt. Both communication lines lie at a voltage level of 2.5 Volt. In the case of a dominant bus level (logic "0"), the CAN high line assumes a voltage of 3.5 Volt, and the CAN low line a voltage of 1.5 Volt. This yields a differential voltage of 2 Volt.



Comment: The low-speed CAN physical layer, described in ISO document 11898-3 supports data rates of up to 125 KBit/sec.