LIN - 1700

Overview
The LIN bus (Local Interconnect Network) was designed to cover the communication requirements for Class A systems
(see Table 1) with the most cost-effective hardware possible in the node. Typical applications are the door module with
door lock, the power-window units, door- mirror adjustment, and air-conditioning system (transmission of signals from the
control element, activation of the fresh-air blower).
The current LIN specification can be accessed on the LIN Consortium website [4].
Important features of the LIN bus are:
– Single-master and multi-slave concepts,
– Low hardware costs on account of data transfer via non-shielded single-wire cable,
– Self-synchronization of the slaves even without a quartz oscillator,
– Communication in the form of very short messages,
– Transfer rate max. 20 kbit/s,
– Bus length up to 40 m and up to 16 nodes.

Transfer system
The LIN bus is designed as a non-shielded single-wire cable. The bus level can assume two logic states. The dominant level corresponds to a voltage of approximately 0 V (ground) and represents logic 0. The recessive level corresponds to the battery voltage Ubatt and represents the logic 1 state. Because of the different design variations of the circuitry, the levels may be subject to differences. The definition of tolerances for transmitting and receiving in the field of the recessive and dominant levels ensures a stable data transfer. The tolerance bands are wider at the receiving end (Figure 9) so that valid signals can also be received in spite of interference radiation. The transfer rate of the LIN bus is limited to 20 kbit/s. This is a compromise between the demand for high edge steepness to synchronize the slaves easily on the one hand and the demand for low edge steepness to improve EMC performance on the other. Recommended transfer rates are
2,400 bit/s, 9,600 bit/s and 19,200 Bit/s. The minimum permissible value for the transfer rate is 1 kbit/s. The maximum number of nodes is not stipulated in the LIN specification. Theoretically, it is limited by the number of available content related message identifiers. Line and node capacities and edge steepnesses limit the combination of length and
node number of a LIN network; a maximum of 16 nodes is recommended. The bus users are usually arranged in
a linear bus topology; however, this topology is not explicitly prescribed.

 

Bus access
In the LIN bus, access is provided on the basis of the master-slave access method. The network features a master, which
initiates each message. The slave has the opportunity to respond. The messages are exchanged between the master and one, several or all the slaves. The following relationships are possible during communication between master
and slave:
– Message with slave response: The master transmits a message to one or more slaves and asks for data (e. g.
switch states of measured values).
– Message with master instruction: The master issues control instructions to a slave (e. g. switching on a servomotor).
– Message for initialization: The master initiates a communication between two slaves.

 

LIN protocol
Data frameData frame
The information on the LIN bus is embedded in a defined data frame (LIN frame) (Figure 10). A message initiated by the
master begins with a header. The message field (response) contains different information depending on the type of
message. If the master transmits control instructions for a slave, it describes the message field with the data to be utilized
by the slave. In the event of a data request, the addressed slave describes the message field with the data requested by
the master.

HeaderHeader
The header is made up of the synchronization break (Synch Break), the synchronization field (Synch Field), and the
identifier field (Ident Field).

 

Synchronization
A synchronization takes place at the start of each data frame to ensure a consistent data transfer between master and slaves. First the start of a data frame is clearly identified by the Synch Break. It consists of at least 13 consecutive dominant levels and one recessive level. After the Synch Break, the master transmits the Synch Field, consisting of
the bit sequence 01010101. The slaves thus have the opportunity to adapt themselves to the master’s time base. The
clock pulse of the master should not deviate from the nominal value by more than ± 0.5 %. The clock pulse of the slaves may deviate prior to synchronization by up to ± 15 % if the synchronization achieves a deviation of max. ± 2 % up to the end of the message. The slaves can thus be designed without an expensive quartz oscillator, for example with a cost-effective RC circuit.

 

Identifier
The third byte in the header is used as the LIN identifier. Similarly to the CAN bus, content-based addressing is used –
the identifier therefore gives information about the content of a message. All the nodes connected to the bus decide on
the basis of this information whether they intend to receive and process the message or ignore it (acceptance filtering).
Six of the eight bits of the identifier field determine the identifier itself, from which 64 possible identifiers (ID) are obtained.
They have the following meanings:
– ID = 0 – 59: Transmission of signals,
– ID = 60: Master request for commands and diagnostics,
– ID = 61: Slave response to ID 60,
– ID = 62: Reserved for manufacturerspecific communication,
– ID = 63: Reserved for future expansions of the protocol.
Of the 64 possible messages, 32 may contain only two data bytes, 16 four data bytes, and the remaining 16 eight data
bytes. The last two bits in the Ident Field contain two checksums, with which the identifier is protected against transmission errors and resulting incorrect message allocations.

 

Data field
Transmission of the actual data begins after the master node has transmitted the header. The slaves identify from the
transmitted identifier whether they are addressed and, if necessary, transmit back the response in the data field.
Several signals can be packed into a data frame. Here, each signal has exactly one generator, i. e. it is always described
by the same network node. During operation it is not permitted to change the signal allocation to another generator, as
would be possible in other time-controlled networks. The data in the slave response are safeguarded by a checksum (CS).

 

LIN description file
The configuration of the LIN bus, i. e. the specification of network users, signals and data frames, is performed in the LIN
description file. The LIN specification provides for a suitable configuration language for this purpose. From the LIN description file, tools automatically generate program sections which are used to implement the master and slave functions in the ECUs located on the bus. The LIN description file thus serves to configure the entire LIN network. It is a common interface between the vehicle manufacturer and the suppliers of the master and slave modules.

 

Message scheduling
The scheduling table in the LIN description file determines the order and time frame in which the messages are trans-
mitted. Frequently needed information is transmitted from time to time. When the table has been worked through, the
master begins again with the first message. The sequence of processing can be altered depending on the operating state
(e. g. diagnostics active or inactive, ignition on or off) Thus, the transmission frame of each message is known. The deterministic performance is guaranteed by the fact that all the transmissions are initiated by the master in the case of master-slave access control.

 

Network management
The nodes of a LIN network can be placed in sleep mode in order to minimize closed-circuit current. Sleep mode can be
achieved in two ways. The master transmits the “Go to Sleep” command with the reserved identifier 60, or the slaves automatically go into sleep mode if there has been no data transfer on the bus for an extended period of time (four seconds). Both the master and the slaves can wake up the network again. The wake-up signal must be transmitted for this purpose. This consists of a data byte with the number 128 as content. After a break of 4 to 64 bit times (wake-up delimiter), all the nodes must be initialized and be able to respond to the master.