The CAN protocol is a widely used communication standard in automotive and industrial applications. It has been standardized by the International Organization for Standardization (ISO) with two main versions: ISO 11898 and ISO 11519-2. Both standards define the data link layer in the same way, but they differ significantly in their physical layer specifications.
ISO 11898 is primarily designed for high-speed CAN communication, typically operating at speeds up to 1 Mbps. Originally, this standard included both the data link layer and the physical layer. However, it was later divided into two separate parts: ISO 11898-1, which covers the data link layer, and ISO 11898-2, which focuses on the physical layer. This separation allows for more flexibility in system design and implementation.
On the other hand, ISO 11519-2 is intended for low-speed CAN communication, supporting data rates up to 125 kbps. It is commonly used in applications where cost and simplicity are more important than speed, such as in certain vehicle subsystems or industrial control systems.
One of the key differences between ISO 11898 and ISO 11519-2 lies in their physical layer characteristics. For example, the termination impedance, dominant and recessive voltage levels, and signal timing are defined differently in each standard. These variations affect how the bus is connected, how signals are transmitted, and the maximum length of the network based on the communication speed.
Figure 6 illustrates the physical layer sublayers, including the Physical Signaling Sublayer (PLS), the Physical Medium Attachment Sublayer (PMA), and the Medium Dependent Interface Sublayer (MDI). While PMA and MDI have similar functions, their specific definitions vary between the two standards.
Table 3 outlines the differences in physical layer parameters between ISO 11898 and ISO 11519-2. Additionally, Figure 7 shows the relationship between communication speed and bus length, emphasizing that users must configure these parameters according to their system requirements.
The CAN bus typically consists of two wires: CAN_High and CAN_Low. The bus operates using two voltage levels—dominant and recessive—which correspond to logic 0 and logic 1, respectively. A transmitting node changes the bus level to send a message, while the receiving node interprets the signal accordingly.
Transceivers play a crucial role in ensuring compatibility with the respective standards. ISO 11898 and ISO 11519-2 require transceivers that meet their specific electrical and timing requirements. Table 4 lists some common transceiver ICs that support these standards.
Beyond ISO, the CAN protocol is also standardized by other organizations such as SAE (Society of Automotive Engineers), NMEA (National Marine Electronics Association), and SDS (Smart Distributed System). These standards often apply to specific industries or applications, providing additional guidelines for implementation.
Figure 9 classifies car communication protocols based on their data rate, highlighting the different layers and types of CAN used in various vehicle systems. Understanding these standards is essential for designing and maintaining reliable CAN-based systems.
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