The role of terminal resistors in CAN networks
1. Prevent signal reflection
Basic principle: CAN networks use differential signal transmission, where the voltage difference between CAN_H and CAN_L represents the signal state.
To ensure the signal's integrity on the bus, both ends of the CAN bus must be equipped with appropriate terminal resistors.
Impedance matching: The characteristic impedance of the CAN bus is usually 120 ohms. To achieve impedance matching and prevent signal reflection, a 120-ohm terminal resistor must be connected to each end of the CAN bus.
In this way, the total impedance of the bus is kept at around 60 ohms, effectively absorbing the energy at the end of the signal and preventing the signal from being reflected to the bus, causing signal distortion or communication errors.
2. Signal integrity
Signal absorption: The terminal resistor can not only prevent signal reflection but also effectively absorb the remaining energy of the signal to ensure stable transmission of the signal on the bus. When there is no terminal resistor or the terminal resistor is misconfigured, the signal may produce an echo effect, resulting in timing and level distortion of the data frame.
Bus stability: Correctly configured terminal resistors ensure that the differential signal on the bus can be accurately transmitted between nodes, which is crucial to the system's stability.
Configuration of terminal resistors
1. Resistor value selection
Standard value: In a typical CAN network, the standard value of the terminal resistor is 120 ohms (±1% accuracy). This value corresponds to the characteristic impedance of the CAN bus, so a 120-ohm resistor must be connected to each end of the bus.
Layout location: The terminal resistors should be placed at both physical ends of the bus to achieve optimal signal absorption and minimal reflection. This is especially important for long buses (over 10 meters) or high-speed communications (such as 1 Mbps).
2. Installation method
Built-in terminal resistors: Some CAN devices are designed with built-in terminal resistors, which is very convenient in some simple network topologies. However, in complex networks, it may be necessary to manually configure external terminal resistors to ensure the correct matching of the bus impedance.
Manual configuration: In systems without built-in terminal resistors, the terminal resistors usually need to be manually soldered or connected to the physical ends of the CAN bus through sockets. Make sure that the resistors are firmly connected and correctly connected to CAN_H and CAN_L.
Common Problems and Troubleshooting
1. Missing or Excessive Terminal Resistors
Missing: If a terminal resistor is missing, the total impedance of the CAN bus will be greater than the expected value, which will cause signal reflection, unstable communication, or packet loss.
Too many: If the bus has too many terminal resistors (for example, three or more terminal resistors are connected by mistake), the total impedance will be reduced, resulting in increased signal attenuation and affecting communication quality.
2. Unmatched resistance values
Too high resistance: If the terminal resistance value is too high, the signal reflection problem will become serious, resulting in an increase in communication error rate.
Too low resistance: If the terminal resistance value is too low, the signal may be excessively attenuated, making it impossible for the remote node to receive the signal correctly.
3. Troubleshooting methods
Physical inspection: First check whether the terminal resistors are connected at both ends of the bus and make sure the resistance value is 120 ohms. If the resistors are loose or have poor contact, it may also cause failure.
Measure the total impedance: Use a multimeter or impedance analyzer to measure the total impedance value at both ends of the bus. Under normal circumstances, it should be about 60 ohms (two 120 ohm resistors in parallel).
Waveform analysis: Use an oscilloscope to analyze the signal waveforms of CAN_H and CAN_L. If reflection or distortion is found, it may be related to improper configuration of the terminal resistor.
Signal integrity test: Run the signal integrity test in the system to evaluate whether the terminal resistor is configured correctly by the error rate and packet loss rate of the data packet.
Summary
The role of the terminal resistor in the CAN network cannot be ignored. It not only ensures the integrity of the signal, but also directly affects the stability and communication quality of the system.
Correctly selecting and configuring the terminal resistor, and timely detecting and eliminating faults caused by the terminal resistor are key factors to ensure the reliable operation of the electric vehicle CAN network.
