Basic Features of Controller Area Network (CAN)

Controller Area Network (CAN) is a serial communication protocol developed primarily for applications in the automotive industry. It is also capable of offering good performance in other time-critical industrial applications.

The CAN protocol is optimized for short messages and uses a carrier sense multiple access (CSMA)/Arbitration message priority (AMP) medium access control technique. Therefore CAN protocol is message oriented, and each message has a specific priority that is used to arbitrate access to the bus in instances of simultaneous transmission.

Related: Basic Features of Foundation Fieldbus (FF)

The bit stream of a transmission is synchronized on the start bit, and the arbitration is performed on the following message identifier, in which a logic zero is dominant over a logic one. A node that wants to transmit a message waits until the bus is free and then starts to send the identifier of its message bit by bit. Conflicts of access to the bus are solved during the transmission by an arbitration process at the bit level of the arbitration field, which is the initial part of each frame. Therefore, if two devices want to send messages at the same time, they first continue to send the message frames and then listen to the network. If one of them receives a bit different from the one it sends out, it loses the right to continue to send its message, and the other wins the arbitration. With this message method, an ongoing transmission is never corrupted, and collisions are non-destructive.

A typical example of technology that is based on CAN specification is the DeviceNet which has received considerable acceptance in the manufacturing applications at the device level. The DeviceNet specification is based on the standard CAN protocol with an additional application and physical layer specification.

The DeviceNet frame has a total overhead of 47 bits which include: start of frame, arbitration (11-bit identifier), control, cyclic redundancy check (CRC), acknowledgement packet (ACK), end of frame, and intermission fields. The size of a data field is between 0 and 8 bytes. The DeviceNet protocol employs the arbitration field to provide source and destination addressing as well as message prioritization.

Related: Features of HART Communication Protocol

Shortcomings of Controller Area Network (CAN)

The major shortcoming of CAN network compared with other networks is the slow data rate, limited by the network length. Because of the bit synchronization, the same data must appear at both ends of the network simultaneously.

DeviceNet which is based on CAN network specification has a maximum data rate of 500 kbps for a network of 100 m. Therefore the throughput is limited compared with other control networks.

CAN network is also not suitable for the transmission of message of large data sizes, though it does support fragmentation of data that is more than 8 bytes into multiple messages.

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