CAN Bus System Operation and Data Transmission

How the CAN Bus System Operates

The control units connected to the CAN bus system are those that need to share information, regardless of whether they belong to the same system. In automotive applications, one line typically connects the engine control units, ABS, and automatic transmission, while another lower-speed line manages control units associated with comfort systems.

Message-Oriented Communication

The CAN bus system is message-oriented rather than recipient-oriented. Information is transmitted as structured messages where an identifier indicates the type of data contained. All control units receive the message, but they filter and use only the specific data they require. Naturally, all control units integrated into the system are capable of both introducing and collecting line messages. When the bus is free, any connected unit may start transmitting a new message.

Priority and Arbitration

In the event that two or more units intend to introduce a message at the same time, the one with the higher priority will prevail. This priority is determined by the message identifier.

The Data Transmission Cycle

The data transmission process follows a cycle consisting of several distinct phases:

• Provision of Information

  A control unit receives information from its associated sensors (such as engine RPM, speed, engine temperature, or door status). Its microprocessor passes this information to the controller, where it is maintained and conditioned. It is then passed to the transmitter-receiver (transceiver), where it is transformed into electrical signals.

• Data Transmission

  The unit's driver transfers the data and its identifier along with a transmission request. It ensures the message is successfully transmitted to all associated control units. To convey the message, the bus must be free; in the event of a collision with another unit, the message must have a higher priority. Once transmission begins, the remaining control units become recipients.

• Receipt of the Message

  When a control unit receives a message, it verifies the ID to determine if the data is relevant. If the control unit requires the data, it processes the message; if not, the message is ignored.