Kvaser CAN Protocol Course: CAN Message Types (Part 2)

Summary

This video script provides a detailed overview of the CAN (Controller Area Network) messaging system. It covers the four types of CAN frames: data, remote, error, and overload frames. The script explains the structure of data frames, including their arbitration and acknowledgment processes. It also highlights how remote frames request data and how error and overload frames manage bus faults and congestion. The standard and extended CAN formats are discussed, with emphasis on compatibility between different CAN controller types. The video offers a comprehensive understanding of CAN protocols and their applications in modern communication systems.

Takeaways

• 😀 CAN messages are short and have a maximum utility load of 94 bits.
• 😀 CAN messages are not explicitly addressed; their content implicitly determines their address.
• 😀 There are four types of CAN frames: Data frame, Remote frame, Error frame, and Overload frame.
• 😀 The Data frame is the most common message type and includes fields like Arbitration, Data, CRC, and Acknowledgement.
• 😀 The Remote frame requests the transmission of a Data frame, but it doesn't contain data itself.
• 😀 The Error frame is transmitted when a fault is detected, and it causes other nodes to detect the fault as well.
• 😀 The Overload frame is used when a node becomes too busy to process messages, though it’s rarely used with modern CAN controllers.
• 😀 The Arbitration field in a Data frame determines the priority of the message, with different formats for CAN 2.0A and 2.0B.
• 😀 The Acknowledgement bit ensures that the message has been received correctly, but it doesn’t guarantee it was received by the intended node.
• 😀 The Extended CAN (2.0B) supports a 29-bit identifier, providing more flexibility in addressing than the original 11-bit identifier (2.0A).

Q & A

What is the maximum utility load of a CAN message?

The maximum utility load of a CAN message is 94 bits.

How are messages addressed in a CAN network?

Messages in a CAN network are not explicitly addressed. Instead, they are content-addressed, meaning their contents implicitly determine the message's address.

What are the four different types of CAN messages?

The four types of CAN messages are: 1) Data Frame, 2) Remote Frame, 3) Error Frame, and 4) Overload Frame.

What is the purpose of the Arbitration Field in a CAN Data Frame?

The Arbitration Field determines the priority of a message when two or more nodes contend for the bus. It contains an identifier and the RTR bit, which is dominant for data frames.

What is the role of the Acknowledgement (ACK) bit in a CAN Data Frame?

The Acknowledgement bit is used for error detection. Any CAN controller that has correctly received the message sends an ACK bit at the end of the message. If no ACK is detected, the message will be retransmitted.

What distinguishes a Remote Frame from a Data Frame?

A Remote Frame is similar to a Data Frame, but with two key differences: the RTR bit in the Arbitration Field is recessive, and it does not contain a data field. Its purpose is to request the transmission of a corresponding Data Frame.

How does the CAN standard handle error detection?

The Error Frame is used to signal a fault, and it causes all other nodes to detect the error as well. Error frames include an Error Flag, which violates the bit stuffing rule, and an Error Delimiter, which allows other nodes to send their Error Flags.

What is the purpose of the Overload Frame in a CAN network?

The Overload Frame is transmitted by a node that becomes too busy to process messages. It is similar in format to the Error Frame but is less commonly used in modern CAN controllers.

How did the CAN protocol evolve to accommodate longer identifiers?

Originally, the CAN protocol defined an 11-bit identifier, but customer demand led to the introduction of the Extended CAN format, which uses a 29-bit identifier. This was made possible by a reserved bit in the control field to differentiate between standard and extended frames.

Can nodes with different versions of CAN (2.0A and 2.0B) coexist on the same bus?

Yes, nodes with different versions of CAN (2.0A and 2.0B) can coexist on the same bus. However, a 2.0B passive node will discard extended frames, while a 2.0B active node can both transmit and receive them.