As the name suggests, a Local Interconnect Network is a communication network protocol. It is a Single wire-serial network protocol that supports communications up to 19.2 Kbit/s at a bus length of 40 meters with a speed of 20 bit/s. Though knowing these specifications, one may realise the mightiness of CAN Bus, but that is the very reason why LIN Bus found its existence in the world of communication network protocols. LIN Bus satisfies the need for a low-cost alternative for the components with considerably low speed and low bandwidth requirements. Practically, CAN bus can be costly to be used for every single component of the car. Hence, LIN serves as a low-cost alternative to enable communication between mostly mechatronic components of the vehicles.
Local Interconnect Network
Typically, the components that do not affect the vehicle's performance make use of LIN Buses for communication. It was developed as an initiative by five automakers; namely, BMW, Volkswagen Group, Audi, Volvo Cars, Mercedes-Benz), with the technologies such as networking and hardware supplied from Volcano Automotive Group and Motorola. LIN version 1.3 was the first fully implemented version of the new LIN specification (November 2002). Version 2.0 was introduced in September 2003, with expanded configuration capabilities and provisions for significant additional diagnostics features and tool interfaces.
Features of LIN Bus:
There are a bunch of features that have added to the popularity of the LIN bus. Some being:
Low cost than the other protocols.
Reliable to use.
Uses less time-trigger scheduling.
Accuracy in latency time.
Error detect and checksum.
Error detect and checksum.
No protocol license is needed.
Supports sleep and wake-up mode.
Variable Enabler for hierarchical networks.
Operating voltage of 12 V.
Length of the data frame (2, 4, and 8 bytes).
Work Principles of LIN Bus:
LIN makes use of only one wire (single wire bus). On one such network, 16 nodes can be accommodated. One of the 16 nodes functions as a "Master", the others automatically serve as the "Slaves". Unlike CAN, which is a pure Master-Master setup, LIN is a typical Master-slave configuration. Where one node passes the instructions/commands and the other follow. Here, there can be one to 15 slaves, and the master device contains both a master task and a slave task. The master and slaves are typically microcontrollers. The Communication network inside a vehicle is a collaboration of CAN and LIN buses. LIN Bus is used for body electronics, and CAN is implemented for mainstream powertrain and body communications. In LIN, the sub-systems can be connected by a backbone network using a CAN Bus.
Many components can be interfaced using a LIN Bus. Here, speed and cost requirements become significant decision-makers in the use of either CAN or LIN.
LIN can be implemented relatively inexpensively using the standard serial universal asynchronous receiver/transmitter (embedded into most modern low-cost 8-bit microcontrollers where CAN serves high-speed, error-sensitive needs and operates on a 5-V differential bus. LIN, however, serves low-speed, low-bandwidth requirements on a 12-V single-wire bus.
Frame structure: Data is transferred across the bus in the fixed form of messages (selectable lengths). The master task transmits a header that consists of a break signal followed by synchronisation and identifier fields. The slaves respond with the information (in the form of a data frame that consists of between 2, 4, and 8 data bytes, plus 3 bytes of control information) whenever requested. The Salves do not communicate on their own; only the master can enable them to communicate. All messages are initiated by the master, with at most one slave replying to a given message identifier. If the slave task needs to publish a response, it transmits one to eight data bytes to the bus followed by a checksum byte. The master node can also function as a slave by replying to its messages. As the master initiates all communications, it is not necessary to implement collision detection.
The LIN message frame is divided into two major parts, namely
The "message header" is always transmitted only by the master node and consists of break, sync (synchronisation), and identifier fields.
The "message response" can be transmitted by both the master and slave nodes and consists of data and checksum fields.
Break: Marks the Start of Frame. It is used to enable all the slaves to listen to the upcoming parts of the header. It comprises 13 bits (dominant bits) and 1 bit (recessive) break delimiter (Hexadecimal value 0X00).
Sync: It allows the slave devices to adjust their internal baud rates to synchronise with the bus by allowing slave devices that perform automatic baud rate detection to measure the period of the baud rate. (Hexadecimal value 0X55).
Identifier: This field provides identification for each message and specifies which nodes in the network needs to receive or respond to each transmission. All slave nodes listen for ID fields and verify their parities to determine if they have to transmit or receive a particular identifier (The lower six bits containing the raw ID and the upper two bits containing the parity.
The LIN bus provides a total of 64 IDs.
Response Space: the time gap between the ID field and the first data byte message response part of the LIN frame. When a particular LIN frame is transmitted completely, comprising of a header and a response by the LIN master node, it uses the entire response space to calculate when to send the response after sending the header. If the response part of the LIN frame is coming from a physically different slave node, then each node (master & slave) utilise 50% of the response space-time in their timeout calculations.
Data: Marks the beginning of the message response. It is transmitted by one of the LIN slaves onto the network as a response in the form of one to eight bytes of payload data bytes.
Checksum: The LIN bus defines the use of one of two checksum algorithms to calculate the value in the eight-bit checksum field. The classic checksum is calculated by summing the data bytes alone, and the enhanced checksum is calculated by summing the data bytes and the protected ID.
Advanced Frame Types of LIN Bus: Depending upon the timing of how a frame is transmitted or the content of the data byte that they hold, LIN frames are classified into six advanced types.
Despite LIN being a cost-effective method of communication, it is not a complete replacement for the CAN bus. It simply is a good alternative wherever low speed/bandwidth does not pose any serious issues, and the cost can be effectively manipulated. Typically, it is used as sub-system support to the central system CAN Bus connecting the modules, which are not critical to vehicle performance or safety. At Influx Technologies, data loggers support LIN bus along with CAN bus (digital and analogue). A "complete data logger" making it an ideal choice for your vehicle networking system.
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