The CAN communication standard frame and extended frame is introduced

Introduction

The CAN interface is compliant with specifications 2.0A and 2.0B (active) with bit rates up to 1 Mbit/s.It can receive and transmit standard frames with 11-bit identifiers and extended frames with 29-bit identifiers.

The difference between standard and extended frames is:

1. The arbitration field of Extended frame has 29 bits, and 2^29 messages can appear, and there is a gap on the data link (transparent to the operator), and the frame IDThe range is 0000 0000-1FFF FFFF.(PS: The purpose is to construct a 29-bit CAN ID, which can achieve a larger ID group, and mother no longer has to worry about insufficient IDs!)
Second, the arbitration field of standard frame is continuousThe 11 bits of the frame can appear 2^11 kinds of messages, that is, the frame ID range is 000-7FF;
3. The DLC (data length) in the control frame is exactly the same, but reservedDifferent bits, standard frames IDE, R0, extended frames R1, R0, must be sent at a dominant level (operated by the data link layer), transparent to the programmer;
4. Summary:Everything else is exactly the same, so in fact, the CAN standard frame and the extended frame are only different in the length of the frame ID, so as to expand more CAN nodes and better support the upper layer protocol.

Note: The frame ID here does not indicate the destination address, but the priority of the message accessing the bus (the smaller the frame ID value, the higher the priority, the minimum is 0x00000000).
Standard frame: (0x12)

Extended frame: (0x1314)

CAN standard frame format

CAN standard frame information is 11 bytes (3 + 8), including two parts: information and data parts.The first 3 bytes are the information part.

Byte 1 is frame information.The 7th bit (FF) represents the frame format, in the standard frame, FF=0; the 6th bit (RTR) represents the frame type, RTR=0 represents the data frame, RTR=1 represents the remote frame; DLC represents the data frameThe actual data length of the frame.

Bytes 2 and 3 are the message identification code, 11 bits are valid.

Bytes 4 to 11 are the actual data of the data frame, and are invalid for remote frames.

CAN Extended Frame Format

CAN extended frame information is 13 bytes (5 + 8), including two parts, information and data parts.The first 5 bytes are the information part

Byte 1 is frame information.The 7th bit (FF) represents the frame format, in the extended frame, FF=1; the 6th bit (RTR) represents the frame type, RTR=0 represents the data frame, RTR=1 represents the remote frame; DLC represents the data frameThe actual data length of the frame.

Bytes 2 to 5 are the message identification code, and the upper 29 bits are valid.

The actual data of the data frame in bytes 6~13, invalid in remote frame

Replenishing knowledge

One of the biggest features of the CAN protocol is that the traditional station address encoding is abolished, and the communication data block is encoded instead.The advantage of this method is that the number of nodes in the network is theoretically unlimited.The above different data blocks, this way of coding according to data blocks, can also enable different nodes to receive the same data at the same time, which is very useful in a distributed control system.The length of the data segment is up to 8 bytes, which can meet the general requirements of control commands, working status and test data in common industrial fields.

At the same time, 8 bytes will not occupy the bus for too long, thus ensuring the real-time communication.CAN protocol adopts CRC check and can provide corresponding error handling function, which ensures the reliability of data communication.CAN's excellent characteristics, high reliability and unique design are especially suitable for the interconnection of industrial process monitoring equipment. Therefore, CAN has been paid more and more attention by the industry and has been recognized as one of the most promising field buses.

Appendix:

The latest download address of the above CANTest software: https://www.zlg.cn/Index/Search/search?selc=all&key=cantest
The CAN box needs to be configured to use