Core Role of 120Ω Termination Resistors in RS-485 Bus
Adding 120Ω termination resistors at both ends of an RS-485 bus is primarily for impedance matching to eliminate signal reflection and ensure communication quality. This is like building a "breakwater" that matches the wave characteristics when waves hit the shore, absorbing the energy instead of letting it bounce back and create chaos.
Why Add 120Ω Termination Resistors?
The twisted pair used in RS-485 communication can be regarded as a "transmission line" under high-frequency signals. It has its own characteristic impedance, typically 120Ω. When the electrical signal reaches the end of the bus, if the end impedance does not match the cable's characteristic impedance, part of the signal energy cannot be absorbed and will be reflected.
- Prevent signal reflection: Impedance mismatch at the end (e.g., receiver input impedance as high as 96kΩ) causes signal reflection.
- Ensure signal integrity: Reflected signals superimposed on the original signal cause waveform distortion and ringing, which may lead to communication errors, data packet loss, or even communication failure.
- Reason for choosing 120Ω: The characteristic impedance of standard RS-485 twisted pair is typically 120Ω. When the termination resistor value equals the cable's characteristic impedance, the signal reaching the end is completely absorbed, achieving optimal transmission.
What Happens Without Termination Resistors?
Without termination resistors, the bus will face risks caused by signal reflection:
- Signal waveform distortion: Reflection causes "ringing" or "staircase" distortion in the signal.
- Increased communication error rate: The receiver may not correctly identify the distorted signal, leading to data errors.
- Unstable or interrupted communication: In high-speed (e.g., >19.2Kbps) or long-distance (e.g., >200 meters) communication, reflection may cause complete communication failure.
- Decreased system reliability: Increases debugging difficulty and may cause intermittent failures in harsh industrial environments.
When Must Termination Resistors Be Added?
Termination resistors are not always necessary. Whether to add them mainly depends on communication distance and communication rate. Generally, it is strongly recommended to add them when the following conditions are met:
- Long communication distance: Generally considered over 200 meters or 300 meters.
- High communication rate: Baud rate greater than 19.2Kbps.
- Severe industrial interference: Near strong interference sources such as inverters and large motors.
If the communication distance is very short (e.g., a few meters) and the rate is very low (e.g., 9600bps), it usually works without termination resistors. Blindly adding them may instead reduce signal amplitude due to excessive load, causing new communication problems.
How to Correctly Add Termination Resistors?
Correctly adding termination resistors requires attention to quantity, location, and resistance value:
- Quantity and location: Only and must be added at the two physical ends of the bus, one at each end. Intermediate nodes do not need them. Adding them at all nodes will cause impedance to be too low due to parallel connection, resulting in insufficient driving capability.
- Resistance value: Use 120Ω resistors.
- Design recommendation: Reserve resistor positions during hardware design and decide whether to solder based on actual testing.
"Side Effects" of Adding Termination Resistors
Adding termination resistors increases the power consumption of the transceiver (drive current may rise from 27mA to 83mA) and reduces the amplitude of the drive signal. In low-power or applications with a very large number of nodes, a trade-off is needed.
| Parameter | Without Termination Resistor | With 120Ω Termination Resistor |
|---|---|---|
| Drive current | About 27mA | About 83mA |
| Signal amplitude | Higher | Lower |
| Power consumption | Low | High |
Summary
In simple terms, 120Ω termination resistors are key matching components for ensuring signal quality in high-speed, long-distance RS-485 bus communication, aimed at eliminating signal reflection. However, in low-speed, short-distance scenarios, they are optional and their pros and cons need to be weighed according to actual conditions.
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