The automotive industry has seen tremendous advancements in recent years, particularly in the realm of vehicle communication systems. IT employs various types of buses to suit different needs. One such technology that is introduced to accommodate low-cost interfaces is the automotive LIN bus aka Local Interconnect Network. In this comprehensive introduction, we will delve into the world of LIN bus, exploring its physical layers, frame formats, applications, variants and later higher-level protocols, market share, advantages, disadvantages, and its future in the automotive industry. So, let's embark on this journey to understand how the LIN bus is simplifying communication in modern vehicles.

What is an automotive LIN bus?

The automotive LIN bus (Local Interconnect Network) is a communication system that allows various electronic components within a vehicle to exchange data. It serves as a low-cost and low-speed supplement to the more complex and expensive CAN (Controller Area Network) bus. In fact, it is designed specifically to leverage the UART peripheral available in smaller microcontroller with minimal changes. The LIN bus is primarily used for communication between different sensors and actuators, such as temperature sensors, window switches, and door lock actuators.

The Local Interconnect Network was originally developed by the OIN consortium and after a few revisions standardized as ISO 17897. The LIN protocol specification covers the physical layer and data link layers of the OSI layer. It employs a single wire communication up to 20 kbps over 40 m length.

Automotive LIN bus network

The automotive LIN bus operates on a master-slave architecture – called the Commander-Responder, wherein a single master device (Commander) controls multiple slave device (Responder). The Commander always initiates communication and sends commands to the Responder devices, which in turn respond accordingly. The communication is organized as message structure with IDs assigned for each frame. The protocol also supports multicast frames. This simple and hierarchical structure makes the LIN bus ideal for applications where high bandwidth and extensive network capabilities are not required. The typical LIN network is captured below:

The automotive LIN bus uses a single-wire serial communication protocol, which significantly reduces the complexity and cost of the wiring harness in vehicles. Two more wires are used to run the power and ground connections typically from the vehicle battery.

LIN Protocol Physical Layer

The LIN Protocol physical layer is connected in an open collector configuration where when no transmitter is driving the bus, the bus is in the high voltage level – called the recessive state. When the transmitted is driving, the transceiver pulls the bus low actively drawing current – called the dominant state. Since the bus is powered by battery, whose level is prone to significant variations in automotive environment, there is no standard voltage to consider for state detection. Rather the transmitting node bust drives the bus to less than 20% battery level in dominant state and above 80% in recessive state. The receiver considers voltage below 40% as dominant and above 60% as recessive.

Since the Commander always initiates the transmission and one of the responders is going to respond, there is no need for a collision detect mechanism. The signaling is like the standard UART interface with a large tolerance – 14% given for bit sampling. This enables the systems to run from a cheaper RC oscillator rather than a crystal clock source.

LIN Protocol Message Structure

The LIN Protocol employs message structure with following fields:

  • Synchronization Break: At least 13 bits of dominant bits followed by one recessive delimiter bit.
  • Sync Field: A fixed value of 0x55 – alternate 1’s and 0’s
  • Identifier: 6-bit Identifier followed by 2 parity bits. Based on the value, the responders can choose to respond or listen to the answering node or simply want for the start of next message.

    Followed by the header, the response block is transmitted by the responder. It has the sequence of data followed by the checksum.
  • Data: Sequence of bytes carrying the transmitted data – either 2, 4 or 8 bytes. The length is determined by the ID.
  • Check sum: CRC check to validate the received packet. In classic mode calculated the CRC only for data portion while the enhanced mode also includes the ID field.

LIN Frame types

LIN bus supports the following frame types.

Frame Type Identifier Description
Unconditional frame 00 to 3B Default mode where the intended responders respond
Event-triggered frame 00 to 3B Used to poll data-updated responders. In case of collision, Commander uses Unconditional frames to poll
Sporadic frame 00 to 3B Sent by Commander to transmit data
Diagnostic frame 3C to 3D Used for Diagnostic purposes
User-defined frame 3E Custom frame type to carry arbitrary information
Reserved frame 3F Not to be used

Based on the received frame types, the responders respond to the commander. This simple protocol is quiet enough to meet many automotive functionalities.

Variants of Local Interconnect Network protocols

The Local Interconnect Network bus protocol has evolved over the years, leading to the development of various variants with different capabilities and features. The most commonly used variant is LIN 2.0, which supports data transfer rates of up to 19.2 kbps and provides sufficient bandwidth for most of the applications in a vehicle's electrical system.

However, as vehicle systems become more complex and require higher data transfer rates, newer variants such as LIN 2.1, LIN 2.2, and LIN 2.3 have emerged. These variants offer increased data rates, expanded addressing capabilities, and improved error handling mechanisms. They are particularly useful in applications that demand real-time data communication or involve more extensive sensor networks.

The continuous evolution of Local Interconnect Network protocols ensures that the communication needs of modern vehicles are met while maintaining compatibility with existing LIN networks. This adaptability and backward compatibility make the LIN bus an attractive choice for automotive applications.

Conclusion

The automotive LIN bus has emerged as a vital communication system in modern vehicles, simplifying communication between various electronic components. Its low-cost, low-speed, and hierarchical architecture make it an attractive choice for applications that do not require high bandwidth and extensive network capabilities. With the foundations for basics of LIN communication strongly set, we will explore the applications of LIN in vehicle communication in the upcoming article.

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