Introduction to the types of oscilloscope CAN frames
As the number of automotive electronic devices continues to increase, it is both reliable and economical to use serial buses to achieve multi-channel transmission and form an automotive electronic network.
In the original traditional automotive circuits, the connections between the powertrain module and the body module were point-to-point connections, which made the circuits increasingly complex. The increase in circuits would also lead to an increase in vehicle failure rates.
Later, CAN bus was used more and more widely in automobiles. The so-called multiplex transmission refers to the method of mixing or crossing multiple types of information through a communication channel in a computer local area network. A network with multiplexing capabilities allows multiple computers to access it simultaneously.
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. The modules are connected to each other by ordinary twisted pairs and use the data link socket as the diagnostic interface. Information is exchanged in a manner similar to a telephone party line, with modules communicating using messages and proprietary enterprise standard protocols. The information content involves control, status or diagnostic information and operating parameters. Twisted pair cable has the advantage of providing redundancy backup, that is, when one line is interrupted, the other line can ensure the system operation. Moreover, twisted pairs reduce external electronic interference to the multi-channel communication network, and also reduce electronic interference generated by the multi-channel communication network itself.
Let's take a look at how to use an oscilloscope to measure the car's CAN bus signal. First, find the car's OBD interface.
Let's take a look at the interface pin definitions:
4. Body ground 5. Signal ground 6. CAN high (ISO 15765-4)
14.CAN low (ISO15765-4) 16.Battery voltage
3.CAN high (standby) 11.CAN low (standby)
Connect channels 1 and 2 of the oscilloscope to the BNC to banana cable, connect the black banana cable to an alligator clip, and connect pin 4 to ground. Connect channel one to OBD's PIN6 (CAN_H), channel two to OBD's 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 stabilize the data frame ID waveform. Adjust the vertical gear 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 the same, but with opposite polarity.
When the CAN-BUS system is in sleep state, the electronic control unit ECU introduces 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 ground, CAN-L is a normal transmission signal waveform, and the CAN-H signal voltage is 0V.
When the CAN-L line is short-circuited to ground, CAN-H is a normal transmission signal waveform, and the CAN-L signal voltage is 0V.
When the CAN-H and CAN-L lines are both short-circuited to ground, both signals are at 0V voltage.
When the CAN-H and CAN-L lines are short-circuited to each other, their signal voltages have the same polarity and the 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, the voltage of both is the battery voltage.
When the CAN-H line is disconnected, the CAN-H line waveform is still normal, while the CAN-L line is always at 0 potential.
When the CAN-L line is disconnected, the CAN-L line voltage is at a high potential and remains 5V, while the CAN-H line waveform is still normal.
Types of CAN frames:
Data Frame: Data frame, used to transfer 0-8byte data.
Remote Frame: Remote frame, used to require 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.
