Measuring automotive LIN bus signals and waveform analysis – Oscilloscope

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In automotive network communication, in addition to the CAN communication method, there is another low-cost communication method—the LIN system. Its English name is “Local Interconnect Network”. The LIN bus is based on a serial communication protocol known as UART/SCI (Universal Asynchronous Receiver/Transmitter/Serial Interface), and it is primarily used for serial communication between smart sensors and actuators. Data exchange between various LIN bus systems in a vehicle is carried out through the CAN data bus by the control unit. The characteristic of LIN is that it is used in master-slave control systems; one master system can control up to 16 slave systems. The slave systems are only capable of communicating with the master system, they cannot communicate with each other, nor can they communicate with system modules outside the LIN network. LIN is generally used in door control systems; for example, the electric glass control systems for the doors of Ford Mondeo Zetec and Cruise are controlled using LIN. Here, we take the measurement of the wiper motor controlled by Audi’s LIN bus as an example. Connect a BNC to banana plug cable to channel 1 of the oscilloscope. Connect a pin to the red banana plug and insert it into the LIN bus data terminal on the connector of the vehicle. Connect the black connector on the banana plug to the battery negative terminal or to a good chassis ground using a clip. Since the maximum voltage of a LIN bus is generally around 12V, the vertical scaling of the oscilloscope can be set to 2V/div, with the time base set at around 500μs. Then open the oscilloscope’s decoding menu to configure the LIN bus, selecting a baud rate that matches the signal being measured. By adjusting the bus threshold level to the range within which the waveform is displayed, the decoded data can be seen. The triggering method can be changed to bus decoding triggering, and an appropriate frame ID can be set to stabilize the waveform. The figure below shows the LIN bus control signals for the wiper motor in an Audi vehicle. The LIN bus waveform is a square wave that represents the binary states in the serial data stream. The observed waveforms should have no obvious distortion or noise spikes. Decode data packets to display the real-time data content of bus activity in hexadecimal. “The “Frame ID” is displayed in yellow; in the image above, it is 23. The “Data” is shown in white, while the “Checksum” is displayed in green. If the checksum is incorrect, it is indicated by a red “E”. If no information is sent to the LIN data bus (the bus is idle), or if a logic low level is sent to the LIN data bus, then the maximum value on the LIN bus signal is the logic low level. When transmitting a dominant bit, the transceiver in the sending control unit grounds the LIN data bus. It is manifested as the minimum value on the LIN bus signal, that is, the dominant level. The information format of the LIN bus consists of a start message (information header) and a response (reply/information content). The start message (message header) is sent periodically by the LIN master control unit, and it consists of a synchronization pause section, a synchronization delimiter section, a synchronization section, and an identification section ； The response (reply/message content) is sent by the LIN master control unit or the slave control unit.