Contact
The goal of creating a cost-effective communication architecture for the sensor/actuator area has significantly influenced the design of the LIN physical layer. For cost reasons, in comparison to CAN the design omits a communication controller and communication line. In place of a communication controller, a microcontroller handles the communication protocol. Physical communication is executed via the serial communications interface (SCI) that is implemented on nearly all microcontrollers. As a result, LIN communication is based on transmission of so-called SCI frames: Eight useful bits are framed by a start and stop bit.
In the meantime, microcontrollers with ESCI (Enhanced SCI) or LIN-SCI have become available. These modified SCIs relieve the microcontroller considerably in its LIN communication tasks. The most important task of the LIN transceiver is to adapt the logical level to the physical bus level. The LIN transceiver is equipped with a Tx driver, a Rx section and sleep and wake-up logic.
In terms of electronic circuitry, a LIN network is an open collector circuit. Therefore, a distinction is made between a dominant level (logic 0) and a recessive level (logic 1). The recessive level is detected by the receiver when the bus voltage exhibits at least 60 percent of its maximum possible voltage. At below 40 percent the receiver detects a dominant level.
The lack of differential signal transmission goes hand in hand with a reduction in noise immunity. This effect can be mitigated by connecting the single-wire line to the car's electrical system voltage. However, this results in elevated emissions. Excessive emissions are avoided by means of a relatively low data rate (20 KBit/sec) and a maximum edge rise time (1-3 V/µs).
The maximum network extension length and number of nodes are not specified, but they can be calculated from LIN specification parameters for node resistances and capacitances, as well as cluster time constants T. A maximum node count of 16 is recommended for line lengths up to 40 meters; this is based on node and line capacitances and the maximum allowable time constant for a LIN cluster.
 |
 |