For those engaged in the automotive industry, CAN bus (Controller Area Network) is a well-known communication standard among various electronic control units (ECUs) in today's vehicles. Almost all automotive manufacturers now choose CAN bus for communication. Why is CAN bus favored by automotive designers? What outstanding advantages does it possess? In this article, we will discuss the origin and evolution of the CAN bus.
The term "CAN" in CAN bus should be capitalized as it is not a word but an abbreviation for "Controller Area Network."
CAN bus originated from the renowned German company Bosch and emerged with the development of the automotive industry. During the 1970s and 1980s, the automotive industry experienced significant growth, leading to an increase in electronic control units (ECUs) in vehicles. The rising need for signal exchange between ECUs resulted in a growing number of wiring harnesses, creating a contradiction between the complex and bulky wiring harnesses and the limited wiring space in vehicles. The multitude of wiring harnesses led to decreased reliability in the electrical system, increased weight, and posed challenges for manufacturing and assembly, hindering the realization of lightweight vehicles.
Back then, as no existing bus standard was suitable to address the issue of increasing wiring harnesses in the automotive industry, engineers and researchers from Bosch, Mercedes-Benz, Intel, and two German universities began researching a new type of bus standard suitable for communication between internal controllers in vehicles.
In 1986, Bosch first proposed the CAN bus standard at the Society of Automotive Engineers (SAE) conference in the United States. The following year, Intel introduced the first CAN bus controller chip, the 82526. Soon after, Philips Semiconductors launched the CAN bus controller chip 82C200.
The CAN bus interconnected various electronic control units (ECUs) within vehicles, forming a local area network and facilitating information sharing. This innovation significantly reduced the complexity of automotive wiring harnesses, as illustrated in the schematic diagram below:
The CAN bus utilizes differential signaling for transmission. Its physical layer transmission medium consists of two twisted-pair wires: one is called CAN_H (CAN High), and the other is called CAN_L (CAN Low). The CAN bus uses the voltage difference between these two wires to transmit logical signals. During bus idle periods, the voltages of CAN_H and CAN_L with respect to ground are both approximately 2.5V, resulting in a voltage difference of 0V between the two lines. This voltage level is termed as the recessive level, representing logical "1". When a node needs to send a signal, it raises the voltage of CAN_H to around 3.75V and lowers the voltage of CAN_L to around 1.25V. This creates a voltage difference of 2.5V between the two lines, known as the dominant level, representing logical "0". The illustration below depicts this process:
When electromagnetic interference occurs, it may simultaneously affect the voltages of CAN_H and CAN_L. However, it doesn't significantly impact the voltage difference between the two lines. This robustness makes the CAN bus highly resistant to interference, ensuring the correct transmission of signals.
A typical CAN bus network node consists of a microcontroller (MCU), CAN controller, and CAN transceiver, for example, a 51 microcontroller + SJA1000 + PCA82C250 (5V). With the widespread use of the CAN bus, many microcontrollers now integrate CAN controllers internally (e.g., STM32 series microcontrollers), eliminating the need for a separate CAN controller. In such cases, only a microcontroller and a CAN transceiver are required.
To match the impedance of the transmission cable and eliminate terminal signal reflections during long-distance transmission, terminal resistors (typically 120 ohms) are added at the ends of the network, as shown in the diagram below:
The use of the CAN bus has simplified automotive wiring design, saving time and resource costs. It has also reduced the weight of wiring harnesses, facilitating lightweight automotive designs. The strong resistance to interference offered by CAN's differential signal transmission enhances the reliability of automotive electronic control systems. Additionally, the flexibility to easily add or remove network nodes has significantly increased design flexibility.
The various advantages of the CAN bus have led to its widespread adoption in automotive design and its gradual expansion into other fields such as aerospace, maritime, machinery, healthcare, home automation, and more. It is a highly regarded bus standard, and in future articles, we will delve deeper into the knowledge of the CAN bus (friendly reminder: you can leave comments on this article).
Related Reference Articles:
A Comprehensive Guide to CAN Bus Communication - Understanding Data Frames