Introduction to LIN Bus:
The LIN bus (an abbreviation for Local Interconnect Network) doesn’t operate like CAN bus with two wires, but instead uses a single wire between two or more control units. In the LIN bus system, there is a master and a slave; the master sends a message, and the slave receives it. The master is connected to one of the other networks, such as the MOST bus or the CAN bus.
The master can be a control device or a simple switch, and the slave can be a sensor, actuator, or a control device. This can be used, for example, in the control of an A/C compressor or in the operation of a window motor. In this case, the switch is the master, and the window motor is the slave.
Some applications where the LIN bus is used for control include:
- Sunroof
- Mirror adjustment
- Window motors
- Door locks
- Electric seat adjustment
The image on the right shows how the LIN bus can be applied in a door. The master is connected via the CAN bus (orange and green wires) to the gateway. Four slaves are connected to the master; the top one for the mirror adjustment, below that for the door handle electronics, and below left for the lock and right for the window motor.
Compared to the CAN bus, the LIN bus is simple and slow. The speed of the LIN bus is approximately 1 to a maximum of 20Kbit/s (compared to the CAN bus with a maximum speed of 20Mb/s). This makes it significantly cheaper to develop and produce the components. Because it is not crucial for the aforementioned systems to be controlled via a very fast network like the CAN bus, a slow network like the LIN bus is sufficient. Furthermore, the maximum cable length is 40 meters and up to 16 control devices (up to 16 slaves) can be connected.
The LIN bus is connected to the gateway. Communication with other types of networks, like the CAN or MOST bus, can occur via the gateway.
Recessive and Dominant:
The master sends a message to the slave. This information is transmitted using voltages of 0 volts or 12 volts. The LIN bus signal can be measured with the oscilloscope.
At point 1, there is a voltage of 13 volts on the bus. At point 2, the master starts sending a message. The master grounds the bus (point 3). Within 0.1 milliseconds, the line rises back to 13 volts. During the time the bus is grounded, data transmission occurs.
When the voltage on the bus is equal to the battery voltage, it is called recessive. During the recessive voltage, no data is transmitted. The recessive bit is a “0”.
Only when the bus is short-circuited with ground, is a “1” formed. This is called a dominant bit. In the signal, the bus repeatedly becomes dominant and then recessive again. Also, the time the bus is dominant or recessive varies (one horizontal line is wider than the other). By this alternating voltage, a signal with ones and zeros is formed.
The quantity of ones and zeros forms a signal recognized by the slave. The combination 01101100010100 might mean: window motor up. The relevant window motor will use this command to move the window up. If the window has reached the highest position, the window motor (the slave) will send a signal to the master to stop controlling it. In that case, the LIN bus does not become entirely recessive, but the data bytes in the signal change.
The LIN bus never becomes fully recessive during the car’s operation; there is always communication between the master and the slaves. When the slave doesn’t communicate because the LIN bus wire is interrupted, or when the slave has a power or ground problem and can’t be switched on, the master will ensure an error code is stored in the control device.
Data Frames:
A LIN bus signal consists of a frame made up of different fields. The signal below shows how a data frame is structured.
- Break field (Break): the Break field is used to activate all connected slaves to listen to the subsequent parts of the frame. The breakfield consists of a start bit and at least 13 dominant bits (in the dominant part the voltage is 0 volts), followed by a recessive bit. The Break field thus serves as a start-of-frame message for all slaves on the bus.
- Synchronization field (Synch): due to the absence of crystals in the slaves, the transmission time must be determined anew with each message. By measuring the time between the established up and down edges, the master clock is synchronized, and thus the transmission speed is determined. The internal baud rate is recalculated with each message.
- Identifier (ID): the identifier indicates whether the message is a transmit frame or a response frame. The transmit frame is sent by the master (called TX-ID) and the response frame is sent by the slave (RX-ID). Both messages contain the breakfield, the synch, and the message ID fields generated by the master. Depending on whether it is a Tx or an Rx frame, the message is supplemented by either the master or the slave. The Tx and Rx frames are sent alternately. This allows the master to request information from the slave. The master sends the first part of the message up to the id field, the slave fills it in with all data blocks up to and including the checksum.
- Data fields (Data 1 & 2): these contain the data bytes and contain the information to be sent (for example, the actual command from the master to the slave, or sensor information from the slave to the master).
- Checksum (Check): The checksum is a control field used to verify that all data has been received. A calculation is performed with the data in the checksum field, which must match the data received in the data fields. If the result is positive, the message is accepted. In case of a negative result, an error handling is performed. The attempt is initially made again.
- Interframe Space (IFS): the LIN bus is made recessive for a number of bits before a new message is sent. After the IFS, the master can send a new message.
Between the different fields, the bus is recessive for a certain time. This time is recorded in the protocol. Then follows the Break field of the next message. The scope image below shows a LIN bus message from a wiper motor.
LIN Bus Communication of the Seat Heating Button:
This section provides an example of controlling seat heating via the LIN bus. The air conditioning control panel contains a button for seat heating. Under the button are three LEDs that indicate the heating level. Pressing the button multiple times changes the heating setting (level 1 is the lowest, and level 3 is the highest). In the image below, three LEDs light up as an indication of the highest seat heating level. In this section, a diagram explains the communication via the LIN bus for controlling the LEDs when the switch is operated.
The wiring diagram below is of the seat heating. The air conditioning control panel is also the control device G600. The switches and LEDs for seat heating on the left and right are visible in the control panel. The arrows next to the control devices indicate that the control device is larger than depicted in the diagram; the control device extends into other diagrams.
When a seat heating button on the control panel is pressed, it sends a signal via the LIN bus to the comfort electronics control device (G100).
Control device G100 will activate the seat heating by supplying voltage to pin 21 or 55 on connector T45. The voltage is adjusted according to the switch setting (low voltage in setting 1, maximum voltage in setting 3). Next to the heating element is a symbol of a thermistor, which is an NTC sensor that sends the temperature to the control device and thus protects the seat heating elements from overheating.
When operating the switch, the slave will convert this physical position of the switch into a bit value. After the master sends a response frame, the slave will place this bit value in the data bytes (see the change in the Data 1 frame in image 2). This bit value is transmitted until the switch is released. When the button returns to the rest position, the signal will change back to the original signal (image 1).
Image 1: Signal with the button in the rest position in the response frame:
Image 2: Signal with the button pressed in the response frame:
After the master receives the bit values of the pressed switch, it activates the LED in the switch by placing a bit value in the data bytes of the transmit frame. The voltage pattern in Data 1 or Data 2 changes in that case, as in the example above. The LED remains on until the master sends a command to switch off the LED.
LIN Bus Communication of the Wiper Motor:
More and more often, the wiper motor is controlled via the LIN bus. The operation and advantages over the conventional system are described on the page Wiper Motor. On this page, the signals are examined, and oscilloscope images of possible malfunctions are shown.
As previously described, in the LIN bus system, there is a master and one or more slaves. In the above diagram, the ECU (central electronics control unit) is the master, and the RLS (rain/light sensor) and the RWM (wiper motor) are the slaves. In the scope image below, three signals are visible, placed one after another on the LIN bus.
In each signal, the Break and Synch fields are clearly visible. The subsequent signals do not reveal origin or exact content. However, we know that in the Identification field, the master specifies for which slave the message is intended. The ID field also indicates whether the slave must receive the message (Transmit frame) or whether the slave must send a reply, thus responding (Response frame). A Transmit frame could mean the slave must drive the actuator, such as turning the wiper motor on or off. With a Response frame, the master can request the current moisture value on the windshield from the rain sensor. With this value, the master (the ECU) can determine the wiper motor speed. The actual data to be sent is placed in the Data fields. This could be the speed at which the wiper motor must be driven. Multiple data fields are possible.
The scope image is with the wiper motor off and in a situation where no moisture is detected on the windshield. Yet, communication between the master and slaves continues.a0The ECU in the wiper motor recognizes a change of one or more bits in this signal indicating it should be activated.
Communication Malfunction with the Wiper Motor:
When the wiper motor is disconnected, the master attempts to reach the slave. This can happen if the motor has a power problem or if the LIN bus wire is interrupted. The master sends the Break, Sync, and ID fields with a Response bit, but the wiper motor does not respond.a0
In the oscilloscope image nearby, it is evident from the third data frame that the master sends the data frame, but the slave does not respond. The master will then store a DTC fault code related to the communication problem. Such a fault code is indicated with U (User Network). The master will also continuously attempt to reach the slave to resume communication.
In this case, measurements should be taken on the connector attached to the wiper motor. If the LIN bus wire is interrupted, or the power (12 volts) or ground wire is broken, the wiper motor will also no longer be able to communicate. A blown fuse is also a possibility.
On the page: LIN Bus Diagnosis, multiple fault scenarios with solutions are described.
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