In the CAN bus test, it will encounter a mistake for the cause, which may ignore a tool-termination resistance that solves an abnormal. What is the important role and impact on the Terminal resistance to the CAN bus?
The propagation and light of the electrical signal are similar to the light, and the emission and refraction of light from air from air, and the electrical signal will occur during the impedance discontinuity or a mutation, the electrical signal is transmitted. The reflection process is more complicated, and even multiple reflections may occur, and the transmitted signal may be confused on the normal signal, causing the level change, resulting in an error in data transmission.
1 What is a terminal resistance
Keep the impedance of the transfer cable continuously can effectively eliminate the emission, but the cable always has endpoints, the impedance of the endpoint will definitely occur, in order to keep the end-point impedance continue continuous, the CAN-BUS specifies the matching resistance at both ends or one end of the cable. - Terminal resistance, as shown in Figure 1, a connection method of high speed CAN-Bus termination resistance.
Figure 1 Link of high speed CAN-Bus termination resistance
summary
1. The role of the terminal resistance is to absorb reflection and echoes, and the maximum source of signal reflection is to impedance discontinuities and mismatches.
2. If it is added to the separate two lines, it is equivalent to a state of open-loop, depending on the source of the signal reflection, that is, this connection method can cause a single-line impedance even more discontinuous, and suddenly become 0 at the end, it can cause Reflection is doubled.
3. The two 120Ω resistors added by the high-speed CAN actually simulate the characteristic impedance (not actual impedance) generated on the transmission line when the harness connection is infinite, which is a typical experience value, depending on the type of harness used.
2 Terminal resistance and critical disaster scenes do not match
Understand the role of the terminal resistance, what does the CAN bus do actually affect?
1. Terminal resistance is too small to cause small signal amplitude, resulting in signal identification problems, so that the entire network data is confusing, sometimes communication, but is extremely unstable, as shown in Figure 2.
figure 2
2, the terminal resistance is too large, the signal amplitude is large, the driving force is too large, the signal is overshoot, the signal falling edge is slow, that is, the discharge time is long, the final bit width recognition error, as shown in FIG.
image 3
In the scene, I encountered the above situation, and the Stressz expansion board was connected in parallel to the queued bus by Canscope, first simulated resistance test, found the resistance of the bus impedance, and then added to the bus end, the signal is normal, as shown in Figure 4.
Figure 4
The terminal resistance of the bus terminal is not negligible. The blocking value of each node is not negligible. Many car mills' CAN node test specification has an internal resistance test item, including: Power and ground CAN_H internal resistance, power supply, and The CAN_L internal resistance of normal connection is lost, and the input internal resistance of the measurement node is lost, and the input internal resistance of the measured node is lost, and the differential resistance between CAN_H and CAN_L. The size of the internal resistance is one of the factors that the bus can have sufficient driving force to drive each node. The internal resistance test is also an effective way to directly determine if the node internal resistance is faulty.
3 internal resistance test principle and method
Figure 5 Internal resistance test test wiring diagram
The RTEST in Figure 5 is a test resistor, 5kΩ. Test according to internal resistance test standards. At Canl, CANL is detected in the ground, u = -2V and 7V test once, then open the way to CANH and CANL, test the ground voltage Vcan_h and Vcan_L. Then adjusted U adjusted to -2V and 7V each access test, respectively, respectively, with the following formula, take the average value (Vcan_h and Vcan_L as open circuit CANH and CANL pairs):
RIN_H = RTEST × (VCAN_H - V) / (V- U)
RIN_L = RTEST × (VCAN_L - V) / (V- U)
When the Differences of Canl on Canl test, u = 5V, then open the CANH and CANL, and test the differential voltage VDIFF. Then adjusted U adjusted to 5V access test. Calculated with the following formula (VDIFF is an open circuit differential voltage):
RDIFF = RTEST × (VDIFF - V) / (V- U)
The current situation of the current industry is through external power supply, current source and other peripherals, manual completion of testing, mainly wiring complex, more links, low test efficiency, and does not guarantee the accuracy of test data. How can I do it to be tested, accurate and fast?
See here
4CANDT consistency test system
The CANDT consistency test system automates the completion of the CAN node physical layer, the link layer, and application layer consistency test. It is the only instrument and equipment that can be perfectly perfectly perfect in the field of CAN bus test. Details architecture is shown in Figure 6 .
Image 6
ISO11898-2 internal blocking standard test standard
According to the test standard, use CANDT to consistency with the internal resistance of each node on the bus, the test results can generate a report, as shown in Figure 7.
Figure 7
After the test is complete, the user can export the automated test report. For the unauthorized test item, the CAN bus analyzer can be used to interfere with the fault to exclude and carry reliability test, to enhance the robustness of the test equipment, and the remote electronic CAN bus Experts will assist users and analyze the rectification recommendations for unlike the project;
According to the test report, you can track the standards, test procedures, and judgments of the test entries;
The host factory can evaluate the quality of CAN nodes based on this report as the main factory access. Read the full article, original title: Terminal resistance and internal resistance test of the CAN consistency test system
Article Source: [Micro Signal: Zlg_zhiyuan, WeChat Public Number: Nordic Semiconductor] Welcome to add attention! Please indicate the source of the article.
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