Sep 22, 2022 伝言を残す

Comparison of the Advantages and Disadvantages of CAN Bus and RS485

Comparison of the Advantages and Disadvantages of CAN Bus and RS485

1. Speed and distance: CAN and RS485 high-speed 1Mbit/S transmission distance does not exceed 100M, which is similar to high-speed distance. However, at low speed, it can reach 10KM at 5Kbit/S, while the 485 can only reach a low speed of about 1219 meters. It can be seen that CAN has an absolute advantage in long-distance transmission.

2. The utilization rate of the bus: RS485 is a single master-slave structure, that is, the bus can only have one host, the communication is initiated by the host, the host has no commands, and the next node cannot send it, and it will return after sending. After receiving the reply, the host will go to the next one. Node polling to prevent multiple nodes from sending data to the bus and causing data clutter. The CAN-bus is a multi-host structure, each node has a CAN controller, multiple nodes send, and the ID number is automatically arbitrated, which can prevent the bus data from being cluttered, while one node sends, the other node can detect the bus idle speed, And send it immediately, thereby saving the query of the host, improving the bus utilization, and increasing the speed. Therefore, in systems such as automobiles, which have higher practical requirements, CAN bus or other similar buses are used.

3. Error detection mechanism: rs485 only provides the physical layer, not the data link layer, so it cannot identify errors unless some physical errors such as short circuits occur. This can easily cause the destruction of a node, sending data to the bus frantically (continuously sending 1), which will cause the entire bus to be paralyzed. Therefore, as long as rs485 damages one node, the bus network will be completely interrupted. However, the CAN bus has a CAN controller, which can check any errors on the bus. If the error exceeds 128, the bus will be protected by automatic blocking. If another node error or its own error is detected, an error frame is sent to the bus to alert other nodes that the data is wrong. In this way, even if a node cpu program runs away, the CAN bus will be automatically locked by the controller to protect the bus. Therefore, the powerful features of CAN will be used in the case of high network security requirements.

4. Price and training costs: The price of CAN equipment is about twice that of RS485. In this way, RS485 communication is very convenient in software, as long as you understand serial communication, you can program, while CAN requires the underlying engineer to understand the complex level of CAN, and the host computer software also needs to understand the CAN protocol. It can be said that the training cost is high.

5. The CAN bus can connect the two output terminals CANH and CANL of the interface circuit chip 82C250 to the physical bus through the CAN controller, and the status information of the CANH terminal can only be high level or floating state, and the CANL terminal can only be low level. or suspended state. This needs to ensure that enterprises will not appear in the RS-485 network. When there is an error in the system and multiple nodes can send relevant data to the bus at the same time, the service bus will be short-circuited, resulting in damage to some important nodes. Moreover, the CAN node develops the function of automatically shutting down the output management in the case of serious errors, so that the operation of other nodes on the bus will not be affected, so as to ensure that there will be no abnormality in the network. problem, leaving the bus in a "deadlocked" state.

6. CAN has a complete communication protocol, which can be realized by the CAN controller chip and its interface chip, which greatly reduces the difficulty of system development and shortens the development cycle, which is incomparable with RS485, which only has an electrical protocol.