Relationship between CAN bus packet ID and device

bigbat

Relationship between CAN bus packet ID and device [Copy link]

 

When I talked about CAN bus with my friends, I always thought that the device and the package ID were bound, that is, the function of a device and the package ID were fixed. A device can have multiple functions, and each function is bound to an ID. For example, a thermometer and hygrometer has two parameters, temperature, humidity, and dew point. Then the parameters can be assigned three IDs.

But my friend said: CAN's total ID is a feature of the package, so there is no restriction that the ID and device must be bound, so any device can send the ID of other devices.

I don't know how you understand it, please give me some scientific knowledge.

韵湖葱白

Simply understand, two points:

(1) CAN ID is part of the protocol. - means you can define it at will.

(2) CAN filters data frames by ID. - This allows you to customize and filter different devices/protocols.

数码小叶

Regardless of the usage scenario, any device can send a CAN signal with any ID. But in reality, this is not the case. For example, in a car, there are strict regulations on what each device can send, especially gateway frames and diagnostic frames. The original poster did not understand the difference between a CAN ID and a CAN message. Temperature and humidity can be under the same CAN ID. If you look at the CANFD protocol, you can also see who sent it and to whom from the CAN ID It's a bit complicated

bigbat

My understanding is that every CAN MESSAGE packet must have an ID. This ID is used to identify who the message is sent to, regardless of who sends it. That is why the above question arises. As long as my understanding is correct, it doesn't matter whether the correspondence between devices and messages should be managed.

bigbat

Digital Leaf published on 2024-3-18 15:46 Regardless of the usage scenario, any device can send a CAN signal of any ID. But in reality, for example, in a car, what each device can send is strictly...

I'm talking about the protocol layer, not management and application. CAN packets only have one ID, unlike TCP/IP packets which have two address fields.

freebsder

can id or package id?

数码小叶

bigbat posted on 2024-3-18 16:04 I am talking about the protocol layer, not management and application. The CAN packet has only one ID, unlike the TCP/IP packet which has two address fields.

The standard CAN does not have it, but it does have it after the protocol is added. The source address and the destination address are an ordinary ID, which contains a lot of information.

小陈想吃粉鹅肝

When receiving this ID, the data it carries can be used as temperature data; CAN ID is designed at the software level, so different devices can send the same ID, which is what your friend said, there is no restriction that the ID and the device must be bound.

LitchiCheng

The relationship between can id and device is name. You can call the other party any name, and the other party will know it. If no filter is set, all devices can receive all the data on the bus. They just use the name they know, or the data they want, and ignore the ones they don't care about.

LitchiCheng

LitchiCheng posted on 2024-3-19 09:42 The relationship between can id and device is the name. You can call the other party any name, and the other party will know it. If you don't set a filter, all devices can receive the total...

You can take a look at Zhou Ligong's can easy begin

lugl4313820

You may think that the ID you understand is the address of the hardware firmware. In fact, the CAN ID is also used to identify the communication method in order to achieve efficient communication and filter out some information that he does not care about. However, it is not fixed and is specified in the configuration. A device can be bound to several IDs.

bigbat

lugl4313820 posted on 2024-3-19 16:30 You may think that the ID you understand is the address of the hardware firmware. In fact, the CAN ID is also used to identify the communication method in order to achieve efficient communication and filter out some...

The CAN protocol only defines the physical layer and data link layer of ISO 7. The function of message ID is used to manage the destination of the message and also to manage the priority of the message.

When applied, it is the filter setting, just like the RS485 address

lugl4313820

bigbat posted on 2024-3-19 18:40 The CAN protocol only defines the physical layer and data link layer of ISO 7. The function of the message ID is used to manage the destination of the message and also to manage the priority of the message...

It is better than 485. 485 is a question-and-answer mode, and users can talk freely when the bus is idle.

bigbat

lugl4313820 posted on 2024-3-19 21:40 It is better than 485. 485 is a question-and-answer system, but here you can talk freely when the bus is idle.

RS485 has no priority management, but CAN does. Therefore, CAN can achieve better real-time performance when communicating with multiple machines. Moreover, the physical characteristic of CAN is that the electrical design of each device occupying the bus can be "wired and connected". 485 has no such requirement, but only requires differential signals. The address signal of RS485 is realized by increasing the signal length, while CAN is realized by wired and connected. Therefore, I only said that the address is similar to that of RS485.