Auto repair oscilloscope measurement of automobile CAN-BUS bus signal and waveform analysis

With the increasing number of automotive electronic devices, it is a reliable and economical approach to use serial bus to achieve multi-channel transmission and form an automotive electronic network.

In the original traditional automobile circuits, the connection between the powertrain module and the body module is point-to-point, so the circuit will become increasingly complex. The increase in circuits will also lead to an increase in the failure rate of the vehicle.

Later, CAN bus was used more and more widely in automobiles. The so-called multiplexing refers to the way of mixing or crossing multiple information through a communication channel in a computer local area network. A network with multiplexing function allows multiple computers to access it at the same time.

The application of CAN (multi-channel transmission technology) in automobiles can simplify wiring, reduce costs, make communication between electronic control units simpler and faster, reduce the number of sensors, and realize information resource sharing.

Multiplexed communication networks are used in multi-module operating systems. Modules are interconnected by ordinary twisted pair cables and use data link jacks as diagnostic interfaces. Information is exchanged in a manner similar to telephone party lines, and modules communicate using information and dedicated enterprise standard protocols. Information content involves control, status or diagnostic information and operating parameters. Twisted pair cables have the advantage of providing redundant backup, that is, when one line is interrupted, the other line can ensure system operation. In addition, twisted pair cables reduce external electronic interference to the multi-channel communication network, and also reduce the electronic interference generated by the multi-channel communication network itself.

Let's see how to use an oscilloscope to measure the CAN bus signal of a car. First, find the OBD interface of the car.

Let's take a look at the interface pin definition:

4. Body ground 5. Signal ground 6. CAN high (ISO 15765-4)

14.CAN low (ISO15765-4) 16.Battery voltage

3.CAN high (spare) 11.CAN low (spare)

Connect oscilloscope channels 1 and 2 to BNC to banana cables, connect the black banana plug to an alligator clip, and connect pin 4 to ground. Connect channel 1 to OBD PIN6 (CAN_H), and channel 2 to OBD PIN14 (CAN_L). Open the oscilloscope decoding menu and configure the CAN bus. Adjust the bus threshold level to obtain decoded data, set the trigger mode to decode trigger, and the data frame ID stable waveform. Adjust the vertical scale and time base to observe the signal.

The above is the normal waveform of CAN-BUS. The waveforms of CAN-H and CAN-L are consistent, but the polarities are opposite.

When the CAN-BUS system is in sleep mode, the ECU imports the battery voltage into the CAN-H and CAN-L lines through the EN and STB connectors. At this time, the CAN-H voltage is close to 12V and the CAN-L voltage is close to 0V

If the CAN-H line is short-circuited to the ground, the CAN-L signal will be transmitted normally and the CAN-H signal voltage will be 0V.

When the CAN-L line is short-circuited to the ground, the CAN-H signal waveform is normal and the CAN-L signal voltage is 0V.

When both CAN-H and CAN-L lines are short-circuited to ground, both signals are 0V.

When the CAN-H and CAN-L lines are short-circuited to each other, their signal voltage polarities are the same and their waveforms tend to be consistent.

When the CAN-H line is short-circuited to the power supply, its voltage is always 12V and the CAN-L line waveform is normal.

When the CAN-L line is short-circuited to the power supply, its voltage is always 12V and the CAN-H line waveform is normal.

When both CAN-L and CAN-H are short-circuited to the power supply, their voltages are both the battery voltage.

When the CAN-H line is broken, the CAN-H line waveform remains normal, while the CAN-L line is always at 0 potential.

When the CAN-L line is broken, the CAN-L line voltage is at a high potential and remains at 5V, while the CAN-H line waveform remains normal.

Types of CAN frames:

Data Frame: Data frame, used to transfer 0-8 bytes of data.

Remote Frame: Remote frame is used to request other nodes to send data frames with the same ID.

Error Frame: Error frame. Any node on the bus can send an error frame if it finds an error.

Overload Frame: Overload frame, generated between data frames or remote frames when the bus load is too high.

Some oscilloscopes have built-in setting software, which can automatically set the relevant parameters with just a click.