In industrial control, security surveillance, and smart device applications, RS-485 communication is widely adopted in various instrumentation systems due to its high interference immunity, long transmission distances, and support for multipoint communication. However, when debugging RS-485 communication, many engineers frequently encounter a question: Why must a 120-ohm terminating resistor be added at both ends of the bus? This article will thoroughly analyze the function and principles of terminating resistors by integrating instrumentation applications with real-world operating conditions.

I. Introduction to RS-485 Communication

RS-485 communication is a differential transmission method, whose main characteristics include:

Strong anti-interference capability: Differential signals effectively suppress electromagnetic interference (EMI).

Long transmission distance: RS-485 supports a maximum transmission distance of up to 1200 meters.

Multi-point communication: RS-485 supports communication among up to 32 devices (nodes) on a single bus.

In practical applications, the RS-485 communication bus typically adopts a bus-type structure. Impedance matching at both ends of the communication line requires special attention, which is the key consideration in terminal resistor design.

II. The Necessity of Termination Resistors

In RS-485 communication, termination resistors are not optional but an essential component for ensuring communication quality. Specifically, the role of termination resistors is reflected in the following aspects:

Preventing Signal Reflection: During data transmission, signals encountering impedance mismatch at the end of the transmission line may reflect back onto the bus. This reflection introduces interference on the bus, compromising data integrity and causing communication instability.

Achieving Impedance Matching: The characteristic impedance of RS-485 communication cables is typically around 120 ohms. To ensure effective signal transmission and prevent reflections, a 120-ohm terminating resistor must be used to absorb excess signal energy, thereby matching the cable's characteristic impedance.

Ensuring Data Integrity: Without a terminating resistor, reflected signals superimpose onto the original signal, causing waveform distortion that leads to data errors or communication interruptions. This issue becomes particularly pronounced during long-distance or high-speed transmission.

III. Why Choose 120 Euros?

The RS-485 communication standard specifies that the characteristic impedance of the communication cable is approximately 120 ohms. Therefore, adding 120-ohm terminating resistors at both ends of the bus effectively achieves impedance matching, thereby ensuring stable communication.

What happens if there is an impedance mismatch?

Excessively high resistance: Signal reflections remain unabsorbed and continue to interfere with communication.

Excessively low resistance: Signal energy loss becomes excessive, adversely affecting transmission distance.

Therefore, the 120-ohm terminating resistor has been proven through extensive practical testing to be the optimal choice. It delivers the best matching effect—simple and effective.

IV. Correct Procedures in Practice

Terminating resistor placement: Terminating resistors should only be added at both ends of the bus, not at intermediate nodes. Each node does not require additional resistors, as this may affect impedance matching and cause unnecessary signal loss.

Common resistor selection: For most applications, 120-ohm, 1/4-watt metal-film resistors suffice. While termination may not be mandatory for short-range communication (e.g., within a few meters), it remains critical for long-distance transmission or high-speed communication.

V. Practical Case Study: The Importance of Termination Resistors

A chemical plant initially failed to install termination resistors when using an RS-485 bus for long-distance sensor data transmission. This resulted in data packet loss and frequent alarm issues, with communication stability significantly deteriorating as the number of nodes increased. After adding termination resistors, signal reflection phenomena disappeared, and data transmission became more stable and reliable. This case vividly demonstrates the critical role termination resistors play in ensuring the stability of 485 communication.

VI. Common Misconceptions

Each node requires a terminating resistor: Incorrect! Terminating resistors should only be placed at both ends of the bus. Resistors should not be added between nodes, as this would compromise overall impedance matching.

Short-distance communication does not require terminating resistors: While communication may sometimes function normally over short distances, terminating resistors still enhance reliability when signals face interference or operate at higher transmission rates.

Terminating resistors can have arbitrary values: Incorrect! The resistor value must match the bus's characteristic impedance; selecting 120 ohms is best practice.

VII. Summary and Recommendations

Although seemingly insignificant, the terminating resistor is a critical component in RS-485 communication, ensuring stable and reliable data transmission. When designing and debugging RS-485 communication systems, it is imperative to connect 120-ohm terminating resistors at both ends of the bus in accordance with standards. Particular attention should be paid to this design detail during long-distance transmission or multi-node communication to prevent potential communication instability. Properly configuring the terminating resistor significantly enhances the overall communication quality of the system, ensuring the stable operation of instruments and control equipment.