Addressing Issues with TJA1040T-CM,118 CAN Bus Termination Resistance

Addressing Issues with TJA1040T/CM, 118 CAN Bus Termination Resistance: Troubleshooting and Solutions

When working with the TJA1040T/CM CAN transceiver and a 118 CAN bus system, several issues can arise due to improper termination resistance or related factors. Here’s a step-by-step guide to diagnosing the problem and resolving it effectively.

Common Faults and Their Causes:

Signal Reflection and Data Errors: Cause: Incomplete or improper CAN bus termination resistance can lead to signal reflections. This results in errors in data transmission and Communication failure. How It Happens: The CAN bus should have a termination resistor at both ends of the bus. If one or both resistors are missing or not properly placed, the signal is reflected back, distorting the data. Bus Communication Dropout: Cause: If the termination resistance is too high or too low, it can cause irregular signal levels, leading to communication dropouts. How It Happens: Termination resistance must match the impedance of the bus. For most CAN networks, this is typically 120 ohms, but if the value deviates too much, it can interrupt communication. High Voltage Spikes or Overcurrent: Cause: Incorrect termination resistance can also lead to high voltage spikes or overcurrent conditions, which might damage the TJA1040T/CM transceiver. How It Happens: Without proper resistance, the voltage levels on the CAN bus could be unstable, possibly resulting in electrical stress on the transceiver.

Steps to Diagnose and Resolve the Issue:

Check Termination Resistor Placement: Step 1: Ensure that there are 120-ohm resistors at both ends of the CAN bus. The placement of these resistors is crucial to maintain proper signal integrity. Step 2: Inspect the resistors for physical damage or incorrect values. If the resistor value is wrong or if there’s any visible damage, replace them. Step 3: If the CAN bus is very short or if it’s a small network, you might not need both resistors, but at least one should still be present at the furthest node. Verify the Resistance Value: Step 1: Using a multimeter, check the resistance across the two termination points. The value should be approximately 120 ohms. Step 2: If it deviates significantly from 120 ohms (e.g., 100 ohms or 150 ohms), replace the resistor with the correct value. Step 3: If the resistance is too high (infinite or very high), ensure the resistor is properly connected to both terminals of the bus. Examine the CAN Bus Wiring: Step 1: Check the wiring for any loose connections or poor contacts, especially where the termination resistors are located. Step 2: If there are any damaged wires, replace them and check if the communication improves. Inspect Transceiver Health (TJA1040T/CM): Step 1: If the above steps do not resolve the issue, verify that the TJA1040T/CM CAN transceiver is functioning correctly. Step 2: Measure the voltage levels at the CANH and CANL pins using an oscilloscope. Ensure that the voltage difference between these pins is within the expected range during data transmission. Step 3: If the transceiver shows abnormal behavior (such as failure to transmit or receive), consider replacing it. Additional Considerations: Step 1: For long CAN bus systems or those with many nodes, consider using additional repeaters or bus extenders. Step 2: Ensure that all other devices connected to the CAN bus comply with the correct voltage and impedance levels.

Conclusion:

The key to addressing CAN bus issues with the TJA1040T/CM transceiver is ensuring correct and stable termination resistance, proper wiring, and the health of your transceiver. Follow these diagnostic steps systematically to identify and resolve any issues in your CAN bus network. Proper termination resistance is vital for maintaining the integrity and reliability of communication in your system.