Several important test items for CAN node conformance testing will be explained one by one

The CAN conformance test is to reduce the difference of nodes in the CAN network, ensure the stability of the CAN network environment, and effectively improve the anti-interference ability of the CAN network; therefore, the conformance test of the CAN node is particularly important. This article will focus on several important test items. Explain one by one.
With the development of concepts such as new energy and intelligent networking, the body CAN bus environment has become complicated and disordered, and the unstable quality of CAN nodes poses a great threat to the safety of OEMs. Therefore, CAN conformance testing has become an important means to ensure the safe operation of CAN networks. The content of CAN conformance testing covers the physical layer, link layer, application layer and other test requirements, as shown in Table 1 CAN conformance test content (excerpt); It includes key test items for input threshold, output voltage, sampling point, and bit width tolerance.

  1
Input threshold test
Threshold test is divided into invisible input voltage threshold and explicit input voltage threshold.
When the node Vdiff is greater than 0.9V, it must be recognized as dominant, when it is less than 0.5V, it must be recognized as recessive. Between 0.5V and 0.9V, it belongs to an uncertain area; when Vdiff≤0.5V, the node can send messages normally, otherwise it means the node The work is in an abnormal state; when Vdiff>0.9V, the node must stop sending. If it does not stop, it means that the node is still recognized as a recessive level, and there is a misjudgment of level judgment; therefore, it is particularly important to test the input threshold of the device.
The purpose of the test is to check whether the explicit/recessive input voltage thresholds of CAN_H and CAN_L of the DUT comply with the definition of ISO 11898-2. The specific input voltage threshold standard is shown in Table 2 ISO 11898-2 input voltage threshold standard.

Test principle:
Under the bus load and common mode voltage conditions shown in Table 2 ISO 11898-2 input voltage threshold standard, the implicit/dominant input voltage is carried out according to the test principle of Fig. 1 implicit input voltage test principle and Fig. 2 explicit input voltage test principle Threshold test.

  Judgments based:
DUT can send messages normally when Vdiff≤0.5V (users can customize this range). At least in the case of Vdiff>0.9V (user-input), the DUT should stop sending frames.
  2
output voltage test
The signal amplitude on the CAN bus is a guarantee that the receiving node can correctly identify the logic signal; in the recessive state, if the CAN_diff voltage is greater than 0.9V, the bus will be in a continuous dominant state, causing the bus to be paralyzed; in the dominant state, if the CAN_diff voltage is greater than 0.9V The CAN_diff voltage is lower than 1.5V, indicating that the node's driving ability is weak, which will lead to an error in the dominant level judgment, and it is prone to level inversion in a strong interference environment, leading to bus failures; as shown in Figure 3, the output voltage amplitude causes errors. .

Figure 3 The output voltage amplitude causes an error
Therefore, each manufacturer must test the output voltage amplitude of the CAN node DUT (device under test) before the product is put into use. The purpose of the test is to check whether the recessive/dominant output voltage of CAN_H and CAN_L of the DUT complies with the definition of ISO11898-2. As shown in Table 3, the ISO11898-2 output voltage standard is the test standard ISO 11898-2 output voltage standard.

Table 3 ISO11898-2 output voltage standard
Test principle:
Under the bus load condition shown in Table 3 ISO11898-2 output voltage standard, the output voltage test is carried out according to the test principle of the output voltage test principle in Figure 4.

Figure 4 Principle of output voltage test
  Judgments based:
If the test result meets the ISO 11898-2 output voltage standard in Table 3 ISO11898-2 output voltage standard, the test will be passed.
3
Sampling point test
The sampling point is the location where the receiving node judges the signal logic. CAN communication is asynchronous communication, and continuous resynchronization is required to ensure accurate sampling of the receiving node; if the sampling point is too far forward, sampling errors are likely to occur due to the cable; If the sampling point is too far behind, because of SJW, the synchronization jump width of the receiving node will be adjusted (adjust its own clock to match). If the adjustment is not good, errors will easily occur; therefore, it is particularly important to test the sampling point of the CAN node.
The purpose of sampling point test is to check whether the sampling point of the CAN message signal sent by the DUT complies with the specification requirements; test principle:
Start CANstress receiving message interference function, as shown in Figure 5 CANstress receiving message interference function. From the 45% position of the bit time, the interference increases by 1% each time until 100%; the error message of CANScope-Pro is recorded through statistics, and the interference point of the error message is equal to the sampling point.

Figure 5 CANstress receive message interference function
  Judgments based:
The sampling point of the CAN message signal sent by the DUT should be within the range defined by the specification: 75%≤SP≤83.4%.
4
bit width tolerance test
Tolerance represents the baud rate range (bit time range). Increasing the tolerance can help to receive messages from other nodes; sampling point and tolerance are important factors for evaluating the adaptability of CAN nodes. SJW (Synchronous Jump Width) determines whether the receiving node can have better compatibility. If the receiving node SJW does not adjust the baud rate well, it will lead to insufficient bit width tolerance and errors; therefore, bit width tolerance test It is also one of the important test items to ensure the stability of the signal.
The bit width tolerance test is used to check whether the bit width tolerance of the CAN message sent by the DUT complies with the definition of ISO 11898-2.
Test principle:
Use the bit width tolerance test function, as shown in Figure 6 bit width tolerance test. Based on the DUT baud rate, the configuration baud rate deviation is gradually increased by 1%, and the message is sent to the CAN network; it is judged whether the network has message error information feedback, if there is error feedback, the test is ended, and the result is judged whether it is in the standard Within the interval.

Figure 6-bit width tolerance test
  Judgments based:
The tolerance range of bit width should meet at least -4≤SJW≤4% (the range can be set by the user).
5
CANDT conformance test system
The CANDT conformance test system released by ZLG Zhiyuan Electronics can automatically complete CAN node physical layer, link layer and application layer conformance tests. It is an instrument that can perform complete physical layer automated tests and export reports in the current CAN bus test field. It avoids the error of manual measurement and statistics, and at the same time cooperates with the automatic test method to reduce the waste of test time, improve the accuracy of the test, and greatly save labor costs.
Users can check the required test items on the software page for fully automated testing. If you only want to test the output voltage, check the voltage test items separately. After the test is completed, the user can export the automated test, as shown in Table 4 Test. According to the test, the standard source of test items, test procedures, and judgment basis can also be tracked. The OEM can evaluate the quality of CAN nodes based on this, as the basis for the entry of the OEM, and ensure the stability of the CAN network environment of the vehicle.

Table 4 Test
The CANDT conformance test system is based on the underlying analysis capabilities of CANScope, integrates oscilloscopes, power supplies and other necessary equipment, which can cover the CAN conformance test standards of host manufacturers, and establish a CAN bus test and guarantee system for host manufacturers and component companies.