Access to the CAN Bus

Due to multi-master architecture or event orientation, the message traffic in a CAN network is not predictable – it is random. Message interferences, i.e. collisions, cannot be ruled out. Collisions put the fulfillment of time conditions – and consequently timely data transmission – at risk. This risk increases with growing bus load. The goal of guaranteeing nondestructive data transmission and providing for orderly bus access is attained by utilizing the CSMA/CA bus access method (Carrier Sense Multiple Access with Collision Avoidance).

The CSMA/CA bus access method restricts bus access: The CAN bus may only be accessed when the CAN bus is available. Bus access is not allowed while a CAN frame is being transmitted (nondestructive data transmission). CAN nodes wishing to send must wait until the CAN bus is available. In the case of simultaneous bus access, bus access is decided within an arbitration phase based on the priorities assigned to individual CAN frames. In the example, CAN node 1 may transmit first, then CAN node 2 may send.