Local Interconnect Network (LIN) allows the many different components of motor vehicles to communicate. In technical terms, the current LIN protocol is “an automotive focused low speed universal asynchronous receiver transmitter (UART) based network”. This communication infrastructure is essential to the different processes of automobiles, as its applications help operate varying sections, such as door module and air conditioning systems.
LIN is the standard for serial network protocol, but it is not the only network option for automobile component communications. Prior to the establishment of the Local Interconnect Network bus, the Controller Area Network (CAN bus), a network consisting of multiple controllers in communication with each other, was the primary option.
The LIN Consortium was founded in the late 1990s, and the first established LIN specification (LIN 1.3) was released in 2002. LIN was developed to create a low-cost alternative for intelligent sensors and actuators in motor vehicles that could be used wherever the bandwidth and versatility of CAN was not needed.
However, the establishment of the Local Interconnect Network has not made the CAN bus obsolete. Instead, many modern automotive networks use a combination of LIN for body electronics and CAN for mainstream powertrain and body communications. CAN is implemented in hardware in microcontrollers of about 25 chip manufacturers.
In general, the LIN protocol provides the exchange of information through signal-based communication between different nodes. Each node acts as part of a master/slave system, in which the LIN master is “connected to a backbone network and schedules the frames”, and the LIN slave is the “node that serves the communication requests of a LIN master”. A frame is an “entity that consists of the header and the PDU including payload and a frame verification byte at the end”.
Other key characteristics used within the LIN protocol are schedule table based frame transfer, error detection (through the master/slave system), and diagnostic service transportation. Overall, this system is cheaper and easier to use, and it has made vehicles more reliable.
The LIN protocol had lasted for almost two decades as a de facto standard within the automotive industry. However, the latest LIN specification (2.2.A) has been transcribed to ISO, the new version of the protocol has been further standardized, and its concepts refined. Now, as an ISO standard, the specification (now covered in ISO 17987) is based on the open systems interconnection (OSI) Basic Reference Model as specified in ISO/IEC 7498-1.
The OSI model structures data communication into seven layers: application layer, presentation layer, session layer, transport layer, network layer, data link layer, and physical layer. These layers are addressed in the seven parts of the document, which have been written to make the services reusable for other types of services than the LIN. These are:
ISO 17987-1:2016 – Road vehicles – Local Interconnect Network (LIN) – Part 1: General information and use case definition
ISO 17987-2:2016 – Road vehicles – Local Interconnect Network (LIN) – Part 2: Transport protocol and network layer services
ISO 17987-3:2016 – Road vehicles – Local Interconnect Network (LIN) – Part 3: Protocol specification
ISO 17987-4:2016 – Road vehicles – Local Interconnect Network (LIN) – Part 4: Electrical physical layer (EPL) specification 12 V/24 V
ISO/TR 17987-5:2016 – Road vehicles – Local Interconnect Network (LIN) – Part 5: Application programmers interface (API)
ISO 17987-6:2016 – Road vehicles – Local Interconnect Network (LIN) – Part 6: Protocol conformance test specification
ISO 14229-7:2022 – Road vehicles – Unified diagnostic services (UDS) – Part 7: UDS on local interconnect network (UDSonLIN)
While the different parts of ISO 14229 serve to establish the standard itself, SAE International has published a series of documents intended to address implementations for the LIN protocol in harmonization with relevant SAE standards. This includes recommendations that are not covered by the protocol itself. These documents include:
SAE J 2602-1-2021- LIN Network for Vehicle Applications
SAE J 2602-2-2021- LIN Network for Vehicle Applications Conformance Test
SAE J 2602-3-2021- File Structures for a Node Capability File (NCF) and LIN Description File (LDF)
All of these standards are available on the ANSI Webstore.