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How to connect 485 terminal resistance? 485 terminal resistance connection analysis
**First, the Introduction of Terminal Resistance**
When high-frequency signals are transmitted over a line, their wavelength is relatively short compared to the length of the transmission line. This can cause reflections at the end of the line, which interfere with the original signal. To prevent this, a terminating resistor is added at the end of the transmission line. This resistor matches the characteristic impedance of the cable, ensuring that the signal is absorbed rather than reflected. Terminal resistance is typically not used for low-frequency signals, as the effects of reflection are negligible. In long-distance signal transmission, especially in systems like RS-485, it's crucial to add a terminal matching resistor at the receiving end to avoid signal echoes and distortion.
**Second, the Role of Terminal Resistance in Communication**
In RS-485 networks, one key challenge is managing the terminal load resistance. When devices are close together, the network may function without termination resistors. However, as distance increases, signal integrity decreases. Theoretically, if the reflected signal decays enough before sampling, termination might not be necessary. But in practice, this is hard to control. An article from MAXIM suggests an empirical rule: when the signal transition time (rise or fall time) is more than three times the time it takes for the signal to travel along the bus, termination becomes essential.
Generally, terminal resistance is applied by placing a 120Ω resistor at both ends of the RS-485 bus. This value matches the characteristic impedance of most twisted-pair cables. While effective, this method consumes power, making it unsuitable for energy-sensitive systems. An alternative is RC matching, where a capacitor blocks DC current, reducing power use. However, choosing the right capacitor value requires careful balancing between power efficiency and signal quality. Another option is diode-based matching, which clamps the signal to reduce reflections, offering significant power savings, though not true impedance matching.
Under normal conditions, termination resistors aren’t needed unless the communication distance exceeds 300 meters. At that point, adding resistors at both ends helps stabilize the signal and improve reliability.
**Third, the RS-485 Bus Matching Resistor Connection Method**
In field installations, if the RS-485 bus exceeds a certain length, its ability to resist interference weakens. To maintain signal stability, a 120Ω resistor should be placed at both ends of the bus. These resistors are connected across the A and B lines of the bus, matching the cable’s characteristic impedance. Proper installation ensures that signals are absorbed, preventing reflections that could distort data.
The correct way to connect the resistors is to place a 120Ω resistor at each end of the bus, connecting them between the positive and negative terminals. This setup is particularly important in longer networks or when using repeaters or hubs. In some cases, especially when using a 485-to-232 converter or a repeater, the placement of the resistor may vary slightly, but the principle remains the same: to eliminate signal reflections and ensure clean data transmission.
Additional notes:
1. RS-485 requires two termination resistors, one at each end of the bus.
2. For short distances (under 300 meters), termination is usually unnecessary.
3. In some setups, a 120Ω resistor is placed after the last device to suppress noise.
4. RS-485 has a higher common-mode voltage range than RS-422, making it more robust in noisy environments.
By properly implementing terminal resistance, you can significantly improve the performance and reliability of your RS-485 communication system.