CAN, which stands for Controller Area Network, was originally developed by the German company Bosch. It has since evolved into an international standard (ISO 11898) and is one of the most widely used fieldbus systems globally. Initially designed for automotive applications, CAN was recognized for its fast data transmission, high reliability, and flexibility. In the 1990s, it gained widespread adoption in the automotive industry and gradually expanded into other fields such as medical devices, industrial automation, building systems, and transportation.
The CAN protocol is structured into multiple layers, with the data link layer being the core component. This layer includes two sublayers: the Logical Link Control (LLC) and the Media Access Control (MAC). The LLC handles tasks like message filtering, overload notification, and recovery management, while the MAC manages data framing, error detection, and bus access control. These functions are all centered around the process of transmitting information frames.
The Logical Link Control (LLC) sublayer is responsible for managing data transmission and handling remote requests. It confirms that received messages have been successfully processed and provides feedback for recovery and overload conditions. Its design allows for a high degree of flexibility in how messages are handled.
The Media Access Control (MAC) sublayer defines the rules for transmitting data on the bus. It controls frame structure, arbitration, error detection, and fault handling. It also determines whether the bus is free to transmit or if a node should wait before starting a new transmission. Bit timing is another key feature managed by the MAC sublayer.
In terms of frame types, the CAN 2.0B protocol supports two formats: the standard frame with an 11-bit identifier and the extended frame with a 29-bit identifier. While the standard frame format is common in earlier versions, the extended format was introduced in CAN 2.0B. However, not all controllers are required to support the full extended format. Regardless of the frame type, there are four main types of frames used in message transmission: data frames, remote frames, error frames, and overload frames.
A data frame consists of seven bit fields: Start of Frame (SOF), Arbitration Field, Control Field, Data Field, CRC Field, Acknowledgment (ACK) Field, and End of Frame. The data field can be between 0 and 8 bytes long, with each byte containing 8 bits transmitted from the most significant bit first.
The Arbitration Field contains the identifier and the RTR (Remote Transmission Request) bit. In standard frames, the identifier is 11 bits, while in extended frames, it is 29 bits. The IDE (Identifier Extension) bit helps distinguish between the two formats.
The Control Field includes a 4-bit data length code, indicating the number of bytes in the data field. The CRC Field contains a 15-bit checksum and a delimiter bit, ensuring data integrity during transmission.
The ACK Field is used to confirm successful reception of the message, and the End of Frame marks the conclusion of the frame with seven recessive bits.
An error frame is generated when a node detects a transmission error. It consists of an error flag (either active or passive) and an error delimiter. The error flag signals the presence of an error, while the error delimiter marks the end of the error frame.
A remote frame is similar to a data frame but lacks a data field. Instead, it uses the RTR bit to request data from another node. When a data frame and a remote frame with the same identifier are transmitted simultaneously, the data frame takes priority due to the dominant RTR bit.
An overload frame is sent when a node is unable to process incoming data quickly enough. It consists of an overload flag and an overload delimiter, signaling that the receiving node is overloaded and needs time to recover.
Overall, the CAN protocol is a robust and flexible communication system that continues to play a crucial role in various industries, especially in automotive and industrial applications. Its layered structure and well-defined frame formats make it an essential technology for reliable and efficient data exchange.
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