Introduction to automotive electronic control unit and CAN bus technology Chengdu Panfeng Technology

What is a bus?
Just like our telephone lines and network cables are communication lines used for communication, but here they are called buses. All devices connected to this bus can exchange information through this bus. The simple understanding of the bus is that the two communication lines used for communication are one at a time. Only when the two lines are connected correctly can communication be possible.
Why use the bus?
The use of a bus can reduce the number of wires used to exchange information between multiple input and output devices. For example, there are two buildings meters apart, which we call building and building. There is a light bulb installed in the building and a switch in the building correspondingly controls the turning on and off of the light. Using the traditional control method, we need to connect a live wire (AC power supply) from the building to the building (using a public neutral ground wire). If bus control is used, one computer in the building manages all the light bulbs, and one computer in the building manages all the switches. We only need to connect a network cable between the two computers, which reduces the difficulty of wiring and reduces the cost of wiring. reduced cable costs. The same principle applies to bus applications in cars. Buses can allow light bulbs and switches distributed on the car body to be managed by computers distributed in various parts and then connected together through a bus, reducing the number of connecting lines between switches and light bulbs in the car. . The computer that manages the light bulbs and switches in the car body is what we often call it (electronic control unit, or car computer). The greater the number of switches and light bulbs, the farther the control distance from each other is, the greater the advantage of using bus + control.
What are the technical advantages of the bus?
Why should a bus be used as a communication line in a car instead of telephone lines and network cables? Because when the bus is used as a communication line in a car, it has advantages that other communication lines do not have.
Multi-master communication: This feature is just like when we speak freely in a meeting, anyone can speak and anyone can hear. But in order to prevent one person's words from interfering with another person's words, we only allow one person to speak at the same time and do not allow anyone to interrupt the person who is speaking. This is our next feature:
Lossless bus Moderation: This function is to let the person who is speaking finish speaking. While this person is speaking, there are two or more other people who are ready to speak, but who should speak first? This is another feature that follows:
Bus priority arbitration: Say the important things first and the unimportant things first, and then consider whether to say it or not after the important things have been said.
When we have meetings, there are often people walking around outside the conference room door, talking, and the sound of car horns coming from outside the window. These sounds will inevitably cause interference to the meeting. Resistance to interference is the next feature:
Strong anti-interference ability: In most cases, people in meetings can still conduct normal activities no matter how busy it is outside the door or outside the window. comminicate. But occasionally, due to too much interference outside the window and door, the speaker's words were not heard by others. In this case, the speaker will take the initiative to say it again. This is the next characteristic:
Super strong bus error correction ability: When you learn that what you said was interfered by the outside world and cannot express complete information, you can say it again.
During a meeting, not all words need to be executed by everyone, but some people do not execute them after hearing these words, and some people need to execute them. This is the high-level communication protocol of the bus that speaks to everyone, but not everyone needs to execute it.
In the meeting, some people may catch a cold and sneeze during the meeting. In order to prevent these people from sneezing and affecting the efficiency of the meeting, these people will automatically exit the meeting room. This is another advantage:
Problem nodes automatically exit the node: Automatically exit the meeting when sneezing affects the meeting many times.
In the above content: the conference room is like the bus and the people are like the words spoken are like the bus communication messages. The external interference to the meeting is like the electromagnetic environment interference inside and outside the car.
The above advantages of the bus allow it to exchange information between multiple devices in the car and allow important message information (the importance of message information in bus communication is determined based on its D). The smaller the D The higher the priority, the higher the message information appears on the bus than other messages. When sending, the message information must be sent to completion. The sending process is not interrupted by other more important message information. There are several different messages. When information is to be sent, important messages are sent first. In addition, the bus also has the characteristics of fast communication speed and long communication distance.
These characteristics have led to the wide application of bus in the automotive field.
In addition, since the bus adopts a conference-style information exchange method, it becomes easier to join and exit the bus. This flexibility makes it easier to add and remove features from the car.
concept?
That is the electronic control unit or on-board computer. Just like our commonly used computers, there are input devices (computer input devices such as mouse and keyboard are used to transmit external operating information to the computer) and output devices (computer monitors, speakers, etc. are used to output information processed by the computer to used by the outside world) is to implement signal input processing and output processing according to the preinstalled program.
What are the advantages of networking via a bus?
After multiple computers are connected to the Internet, resources can be shared between computers. For example, if three computers are connected to the Internet, the documents in the computer can be transferred to the computer through the network and the processing speed will be very fast or very slow. When a piece of data needs to be processed, it can be notified through the network to help process it. The same principle applies to cars being networked via buses.
There are more and more in cars because the functions are becoming more and more complex. There are more than one functions on Audi, each with different functions, but they are often related to each other. For example, headlight follow-up steering is controlled by a headlight, but controlling the headlight steering direction and steering angle combines vehicle speed and steering wheel angle information. This information does not require the headlight to be obtained personally, but relies on other operators. This information is sent to the bus and the headlights obtain it from the bus information.
The advantage of networking through a bus is that you can share each other's input and output without having to let those who need the information detect the information or perform the actions in person.
The use of the Internet makes cars smarter. There are fewer connecting wires in the car and it becomes easier to add functions.
What is the impact of car maintenance and diagnosis after being connected to the bus?
After networking through the bus, the sharing of input and output has also caused an input problem on one node, resulting in no execution output or output error on another node.
The problem of a non-output no longer being this may be that another one is not told through the bus to let this perform the output action. Therefore, when performing maintenance and diagnosis, we need to master the fault determination method of the bus system, know the correlation between systems, and know how to find problems systematically. Only with a systematic fault diagnosis method can we better engage in automobile maintenance related work.