Exploring the Effects of Incorrect Termination on SN65HVD82DR Performance

Exploring the Effects of Incorrect Termination on SN65HVD82DR Performance

Analyzing the Cause of Faults in SN65HVD82DR Due to Incorrect Termination

The SN65HVD82DR is a high-speed, differential line transceiver used in RS-485/RS-422 Communication systems. When termination is not correctly implemented, it can cause a range of issues that degrade its performance. This analysis will explore the causes of faulty performance due to incorrect termination, the underlying reasons, and provide a step-by-step guide to resolve these issues.

Cause of Fault: Incorrect Termination

In communication systems like the RS-485, proper termination at the ends of the communication lines is essential to prevent reflections, signal degradation, and data errors. The incorrect termination can lead to several problems:

Signal Reflection: If termination Resistors are not correctly placed or of the wrong value, the signals traveling along the transmission line may bounce back, causing reflections. These reflections interfere with the main signal, potentially leading to data corruption.

Increased Noise: Incorrect termination can introduce noise into the communication lines. This noise can interfere with the data signal and cause the transceiver (like the SN65HVD82DR) to misinterpret the received signals.

Reduced Signal Integrity: Termination resistance that is either too high or too low can distort the signal, making it difficult for the transceiver to detect the correct logic levels. This causes errors in communication.

Bus Contention: If termination is not applied properly, multiple devices on the bus may drive conflicting signals, leading to bus contention, which can damage the transceiver or cause data loss.

Reasons for Faulty Termination:

Incorrect Resistor Value: The most common mistake is using a resistor value that doesn't match the characteristic impedance of the cable (usually 120 ohms for RS-485 systems). A mismatch can cause improper termination and signal issues.

Termination Resistor Missing: In many cases, a termination resistor might not be placed at the end of the transmission line. Without proper termination at both ends of the bus, signals will reflect, leading to data corruption.

Improper Placement of Termination Resistors: The termination resistor should be placed at the last device on the bus, not somewhere in the middle of the transmission line. Placing it incorrectly can lead to signal degradation.

Incorrect Placement of Biasing Resistors: Biasing resistors are needed to ensure that the bus remains in a defined state when no device is actively transmitting. Missing or incorrectly placed biasing resistors can cause floating signals, leading to communication failures.

How to Solve This Issue: Check the Termination Resistor Value: Ensure that the termination resistor is 120 ohms, which matches the characteristic impedance of most RS-485 cables. This resistor should be placed at both ends of the transmission line. Procedure: If you are using the SN65HVD82DR with an RS-485 network, confirm that a 120-ohm resistor is placed across the A and B lines at both ends of the bus. If the transceiver is the last device in the network, ensure the resistor is placed correctly to terminate the bus properly. Verify Proper Placement of Termination Resistors: At the Last Device: Make sure that the termination resistor is located at the farthest point of the bus, which is typically the last device in the RS-485 chain. Procedure: If termination resistors are incorrectly placed elsewhere or missing, remove them and place them correctly at the ends of the bus. Check Biasing Resistors: Biasing Resistors Placement: Ensure proper biasing at both ends of the bus. Biasing resistors should be installed to maintain a defined logic level (high or low) when no active device is driving the bus. Procedure: Add biasing resistors (typically 680 ohms to 1k ohms) at both ends of the bus to ensure the system is not left floating. Examine Cable Length: If the cable length is too long, signal integrity may be compromised, even with proper termination. Consider shortening the cable length or using repeaters to ensure the signals are properly transmitted. Test the Communication: After ensuring correct termination and biasing, run tests to check for communication errors. You can use an oscilloscope to check the signal integrity on the bus. Procedure: Inspect the waveforms for any reflections or distortion. If the signals look clean with no significant reflection, the termination is correct. Use Differential Transceivers with Built-in Termination (If Available): Some transceivers, like the SN65HVD82DR, may offer integrated termination options. If so, use the internal termination feature by setting the appropriate configuration pins to enable or disable termination, depending on the design of the network. Conclusion:

Incorrect termination is a common issue that can cause severe performance degradation in RS-485 networks using the SN65HVD82DR transceiver. To address these issues, ensure that:

A 120-ohm termination resistor is placed at both ends of the transmission line. Proper biasing resistors are installed to avoid floating states. Resistor values and placement are correct, and the communication system is tested for integrity.

By following these steps carefully, you can restore reliable communication and prevent further transmission errors, ensuring smooth operation of your RS-485 network.