LOCAL INTERCONNECT NETWORK
When talking about LIN, this means the Local Interconnect Network, a system for serial communication. Basically, this network can also be called a fieldbus, as it is a one-wire bus, i.e. a bus running over a single wire. The development of LIN served a communication in motor vehicles for which the costs were to be lowered. Transfer of the data here is performed between the sensors and the actuators. By now, the Local Interconnect Network has established itself as a de-facto standard and is thus considered to be generally accepted. This fieldbus is used mainly in the field of motor vehicles, where the bandwidth or the variety of functions of the CAN bus are not required and where its use would only use up capacity unnecessarily.
The Specification of the Local Interconnect Network
As this network is a standard, there is also a standard format used for description of the entire system, also covering the interface between the network and the application. This format is called the LIN protocol. Protocols serve to structure the entire process of the transfer of data, and they are an essential building block within the network architecture. The Local Interconnect Network is composed of two essential components, the master and several slaves.
Accordingly, it is a one-master/multiple-slaves system, which is usual for single-wire buses.
Here, the master contains all knowledge concerning the time sequence of the data to be transferred, i.e. which data block should be sent when. However, the files are transferred by the slaves, as soon as the master has performed the corresponding initialization for this. The request for data transfer within the Local Interconnect Network is made by sending a header, as this has a specific message address. In this way, the slave knows which data are required, and he puts them on the bus in the form of bytes and sends them back to the master.
The Structure of a Local Interconnect Network Message
When a message is sent from the master to an addressed slave, it consists of a header and the data. A maximum of eight payload bytes can be transmitted per message, which any slave can receive. The receiver-selective system decides which slaves is assigned the message. After all, only the control units use an acceptance filter or a message filter to decide whether the message is processed only by one control unit, by several, or by all control units. The Local Interconnect Network has various regulations specified and diverse possibilities to prevent collisions and other problems:
- It is never possible to transmit two messages at the same time.
- All messages are transmitted within a cycle.
- The time sequence for transmitting the messages is specified in a schedule.
- Multiple slaves can answer simultaneously to an identifier only when new data needs be transmitted, as this can reduce the bus load.
- When a possible collision is found, corresponding polling frames are sent, which can be answered only by a single slave.
The Communication Flow Step by Step
To obtain a data transmission considered to completely sent, five steps must be completed for communication by means of the Local Interconnect Network, where the master task in the header and the slave task are used.
- Break – this is the start point of each frame. The break consists of 13 nominal and dominant bits and a delimiter. A nominal recessive bit is at the end of the frame. By this step, each node coupled to the bus is informed of the beginning of the frame.
- Sync – this is the synchronization field in the Local Interconnect Network, which enables those slave devices that run through automatic baud rate detection to measure the baud rate period, and which in addition offers the option of synchronization between the internal baud rate and the bus.
- ID – this is the last field transmitted by the master task. It serves for identification of the message and specification of which nodes receive and answer which transmissions.
- Data bytes – this step of the communication is taken over by the slave task from the Local Internal Network. Here, the entire field is transmitted within one answer. With this, one field can contain only one bit or max. eight bits.
- Sum of the digits – this field is also transmitted by the slave task in one answer. The Local Internal Network uses two versions of an algorithm to calculate the value of such a sum-of-the-digits field, where either the classical sum of the digits or the expanded sum of the digits is the result.