- LIN bus general
- Recessive and dominant
- data frames
- Transmit frame and Response frame
- LIN bus communication of the seat heating button
- LIN bus communication of the wiper motor
- Fault in communication with the wiper motor
- Fault due to contact resistance in the LIN bus wire
LIN bus general:
The LIN bus (short for Local Interconnect Network) does not work like a CAN bus with two wires, but with one wire between two or more control units. The LIN bus has a master and a slave; the master sends a message and the slave receives it. The master is in contact with one of the other networks, such as the MOST bus or the CAN-bus.
The master can control unit or be a simple switch and the slave a sensor, actuator or a control device. This could be, for example, when controlling an air conditioning compressor, or when operating a window motor. The switch is the master and the window motor the slave.
Some applications where LIN bus is used for control include:
- Sliding tilt roof
- Mirror adjustment
- Window motors
- door locks
- Electric seat adjustment
In the image on the right you can see how LIN bus can be used in a door. The master is connected to the gateway via the CAN bus (orange and green wires). Four slaves are coupled to the master; the top one for the mirror adjustment, the bottom one for the door handle electronics and the bottom one on the left for the lock and on the right for the window motor.
Compared to CAN bus, LIN bus is simple and slow. The speed of the LIN bus is approximately 1 to a maximum of 20Kbit/s (versus CAN bus with a maximum speed of 20Mb/s). This makes it a lot cheaper in the development and production of the parts. Since it is not important for the above systems that they are controlled via a very fast network as a CAN bus, a slow network such as LIN bus is sufficient. Furthermore, the maximum length of the cabling is 40 meters and a maximum of 16 control units (ie up to 16 slaves) can be connected.
The LIN bus is connected to the Gateway. Via the gateway it is possible to communicate with other types of networks, such as the CAN or MOST bus.
Recessive and dominant:
The master sends a message to the slave. This information is passed on using voltages that are 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 switches the bus to ground (point 3). Within 0,1 millisecond the line rises again to 13 volts. In the time that the bus is connected to ground, an information transfer takes place.
When the voltage on the bus is equal to the battery voltage, it is called recessive. During the recessive voltage, no information is transmitted. The recessive bit is a “0”.
Only when the bus is shorted to ground, a “1” is formed. This is called a dominant bit. In the signal, the bus becomes dominant several times and then recessive again. The time that the bus is dominant or recessive also differs (one horizontal line is wider than the other). This alternating voltage creates a signal with ones and zeros.
The amount of ones and zeros forms a signal that is recognized by the slave. The combination 01101100010100 can mean: window motor up. The relevant window motor will move the window up with this command. When the window has reached the highest position, the window motor (the slave) will give a signal to the master that it stops driving. In that case, the LIN bus does not become fully recessive, but the data bytes in the signal change.
The LIN bus never becomes fully recessive during car use; there is always communication between the master and the slaves. If the slave is not communicating because the LIN bus wire is interrupted, or if the slave has a power or ground problem and cannot be switched on, the master will cause an error code to be stored in the control unit.
A LIN bus signal consists of a frame made up of several fields. The signal below shows how a data frame is constructed.
- Break field (Break): The Break field is used to activate all connected slaves to listen to the next 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 therefore serves as a start-of-frame message for all slaves on the bus.
- Synchronization field (Synch): because of the missing crystals in the slaves, the transmission time has to be determined again for each message. By measuring the time between the determined rising and falling edges, the master clock is synchronized and thus the transmission speed is determined. The internal baud rate is recalculated every message.
- Identifier (ID): The identifier indicates whether the message is a transmit frame or a response frame. The transmit and response frames are described in the next section.
- Datafields (Data 1 & 2): 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 check field that checks whether all data has been received. The data in the checksum field performs a calculation that must match the data received in the data fields. If the result is positive, the message is accepted. If the result is negative, an error handler is performed. It will be retried initially.
- Interframe Space (IFS): The LIN bus is recessed for a few bits before sending a new message. After the IFS, the master can send a new message.
The bus is recessive for a certain time between the different fields. This time is laid down in the protocol. This is followed by the Break field of the next sent message.
Transmit frame and Response frame:
The identifier in the message indicates whether it is a transmit frame or a response frame. The transmit frame is sent by the master (this is called a TX-ID) and the response frame is sent by the slave (RX-ID). Both messages contain the breakfield, synch and message ID fields generated by the master. Depending on whether it is a Tx or an Rx frame, the message is completed by the master or the slave. The Tx and Rx frames are transmitted alternately.
LIN bus communication of the seat heating button:
This section gives an example of controlling the seat heating via the LIN bus. There is a button for the seat heating in the air conditioning control panel. Below the button there are three LEDs that indicate the position of the seat heating. Pressing the button several times will change the seat heating position (position 1 is the lowest and position 3 is the highest). In the illustration below, three LEDs light up to indicate the seat heating's highest setting. This section explains with the aid of a diagram how to communicate via the LIN bus to control the LEDs when the switch is operated.
Below electrical diagram is the seat heating. The control panel of the air conditioner is also the control unit G600. In the control panel, the switches and the LEDs for the left and right seat heating are visible. The arrows next to the control units indicate that the control unit is larger than shown on the schematic; the control unit continues in other schemes.
When a seat heating button on the control panel is pressed, it sends a signal via the LIN bus to the control unit of the comfort electronics (G100).
Control unit G100 will switch on the seat heating by applying voltage to pin 21 or 55 on connector T45. The voltage is adjusted to the position of the switch (low voltage in position 1, maximum voltage in position 3). A thermal sensor symbol is shown next to the heating element. This is an NTC sensor that sends the temperature to the control unit and thus protects the seat heating elements from overheating.
When operating the switch, the slave will convert this physical position of the switch to 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 picture 2). This bit value is passed on until the switch is released. When the button is back in the idle position, the signal will change back to the original signal (image 1).
Picture 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 has received the bit values from the pressed switch, it drives the LED in the switch by placing a bit value in the data bytes of the transmit frame. Also in that case the voltage image changes to Data 1 or Data 2 as in the example above. The LED remains on until the master sends a command that the LED must be switched off.
Wiper motor LIN bus communication:
The wiper motor is increasingly controlled via the LIN bus. The operation and advantages over the conventional system are described on the page wiper motor. This page examines the signals and shows scope images of malfunctions that may occur.
As described earlier, the LIN bus consists of 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 RWM (wiper motor) are the slaves. The scope image below shows three signals that are placed one after the other on the LIN bus.
The Break and Synch fields are clearly visible in each signal. It is not possible to deduce from the subsequent signals where they come from or what exactly is being sent. What we do know is that the master indicates in the Identification field for which slave the message is intended. It is also indicated in the ID field whether the slave should receive the message (Transmit frame) or whether the slave should send a message back, i.e. reply (Response frame). A Transmit frame could be that the slave has to control the actuator, such as switching the wiper motor on or off. With a Response frame, the master can request the current value of the moisture on the windscreen from the rain sensor. With this value, the master (the ECU) can determine the speed at which the wiper motor should be controlled. The actual data to be sent is placed in the Data fields. This can be, for example, the speed at which the wiper motor should be controlled. Multiple data fields can be possible.
The scope image is with the wiper motor switched off and in a situation where no moisture is registered on the windscreen. Nevertheless, there is continuous communication between the master and the slaves.
The ECU in the wiper motor recognizes a change of one or more bits in this signal that it should be switched on.
Fault in communication with the wiper motor:
When the wiper motor is disconnected, the master tries to reach the slave. This can happen when the motor has a power problem, or when 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. In that case, the master will store a DTC trouble code related to the communication problem. Such an error code is indicated by U (User Network). It will also continuously try to reach the slave to resume communication.
To resolve this fault, the LIN bus wire of the wiper motor must be checked. Moisture may have entered the connector causing corrosion and broken the connection between the wire and the wiper motor. Another possibility is that the LIN bus wire is interrupted somewhere in the wiring harness.
Fault due to contact resistance in the LIN bus wire
Damage to a wire because it has been pinched, has rubbed against something or when someone has punctured the wire with a test probe, can eventually lead to a contact resistance with a voltage drop as a result. A voltage drop in a power supply wire of a consumer ensures that the consumer has less voltage to function properly. In that case, the location of the contact resistance can be traced with a V4 measurement.
A contact resistance in a LIN bus wire does not cause the recessive voltage to drop. However, it does have a major influence on the signal. Too large a contact resistance can ensure that the signal on the oscilloscope is still visible, but that the quality is too poor for good communication. In that case, the slaves on the relevant LIN bus will no longer perform anything.
The scope image serves as an example for the following two signals where there is a contact resistance.
The second scope image is of a signal where a contact resistance has caused the signal to change. The rising and falling flanks in the image are more slanted and have a point shape at the top and bottom instead of being flattened.
Almost nothing is left of the signal from the third scope image. This results in an even higher contact resistance. The break field, the synchronization field and a number of broad recessive parts in the signal can be recognized, but are unusable.
Thus, when the scope signal has a sawtooth shape, there may be a transition resistance, despite the recessive voltage level being equal to the battery voltage. Keep in mind that the flanks are never exactly vertical, but always slightly slanted. However, the difference in the signals shows a clear deviation. To find the location of the damaged wire, in many cases the wiring harness between the master and the multiple slaves will have to be checked. Where the wiring harness is next to seams of the bodywork or sharp dashboard parts, or places where traces of disassembly/assembly work of other parts can be found, the first attention should be paid. Repairing a part of the wire where the damage is located is often sufficient. You can also choose to disconnect the old LIN bus wire at all ends at the master and slaves and install a completely new LIN bus wire.