Abstract: In the bus communication system, the signal quality of each node directly affects the communication quality of the entire bus. It is essential to ensure that each node has a highly consistent signal quality. This article will be detailed for everyone. If you do a good job of signal characteristics.
The CAN bus design specification strictly defines the characteristics of the differential level bits of the CAN node. If the differential level signal characteristics of the node do not conform to the specifications, the abnormal working state is likely to occur after the field networking, and the nodes appear between the nodes. Communication failure. The specific requirements are shown in Table 1, which is the test standard "GMW3122 Signal Characteristics Standard".
Table 1 GMW3122 signal characteristics standard
In the past tests, we can only measure the end amplitude of the 1bit differential level dominant bit through the oscilloscope, and then divide the difference in the first half (50%) of the first half of the measurement, if the difference is between 81%~ 150% passed;
Then measure the maximum value of the differential level in the second half (50%), divided by the end amplitude of the sub-dominant dominant bit, if it passes at 95%~105%;
It can be seen that although the method can roughly measure the characteristics of the differential level signal of the CAN node, the measurement method is difficult to operate because of the small number of samples used for analysis, so in order to improve the accuracy of the measurement result, the reduction is Labor costs, Zhou Ligong Zhiyuan Electronics improved the test method, using CANScope-Pro bus analyzer, CANScope-StressZ expansion board, using the supporting software's eye diagram and symmetry test to measure.
The test plan is as follows:
1. As shown in Figure 1, make a test connection. The test uses CANScope-Pro's eye diagram function to perform statistics to achieve high test accuracy. After the DUT is powered on, it must always send CAN messages. CANScope does not check the bus response. Its black test leads (ground) must be shared with the DUT's CAN transceiver.
Figure 1 signal characteristic test connection diagram
2. Open CANScope. After the DUT sends the message normally, click Start as shown in Figure 2. Turn on the eye diagram function of the device and obtain the eye waveform as shown in Figure 3.
Figure 2 Start eye diagram function 
Figure 3 Eye function
3. Using the cursor, first measure the amplitude of the end of the eye diagram signal, as shown in Figure 3, which is 2.15V. Then measure the maximum amplitude of the first half of the bit, as shown in Figure 4, 2.73V. Then 2.73/2.15=127%, in line with GMW3122.
Figure 4 Maximum amplitude of the first half
4. Then measure the maximum amplitude of the second half of the bit, as shown in Figure 5, which is 2.25V. Then 2.25 ÷ 2.15 = 104%, in line with the GMW3122 signal characteristic standard.
Figure 5 The last half of the maximum amplitude
5. In order to accurately test the voltage value of CANH+CANL, you need to use the symmetry test function. As shown in Figure 6, click “Start symmetry test†in “Testâ€.
Figure 6 symmetry test
6. As shown in Figure 7, in the “symmetry test†operation window, first click “Automatic setting†to adjust the voltage range, and then adjust the error voltage to 0.4, then click the “Start Test†button and wait for the test result to be judged. .
Note: Since GMW3122 specifies that the voltage of CANH+CANL should be 4.2V~5.8V, it is normally 5V. Therefore, when the symmetry test is used, the (CANH+CANL)/2 is used for discrimination. So the standard range is 2.1V~2.9V, and the normal is 2.5V. Therefore, the error voltage in Figure 7 needs to be filled in 0.4.
Figure 7 Symmetry test CANH+CANL
If the test result is passed, the voltage of CANH+CANL is 4.2V~5.8V.
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