RS-485 is widely used in industrial control and building automation for its noise immunity, long reach (up to 1200 m), and multi-drop capability. However, without proper signal integrity practices—especially bus termination—even high-quality transceivers can suffer data errors or communication dropouts.
When Is Termination Needed?
Termination prevents signal reflections caused by impedance mismatches at cable ends. Reflections distort waveforms (ringing, overshoot), corrupting data. Use this rule of thumb:
If cable length (in meters) > signal rise time (in ns) × 0.1, termination is recommended.
Practically:
Data rate ≥ 115.2 kbps or distance ≥ 30 meters → add termination.
Even at lower speeds, if you see intermittent CRC errors or glitches, try adding terminators as a diagnostic step.
How to Terminate Correctly
Standard RS-485 twisted-pair cable has a characteristic impedance of 120Ω. Place a 120Ω ±1% resistor across the A (D+) and B (D–) lines only at the two farthest ends of the bus.
⚠️ Critical Notes:
Never install terminators at intermediate nodes—this overloads the driver and may cause failure;
For “T-tap” branches, keep stubs shorter than 0.3 meters; terminate only the main trunk ends;
At very low speeds (<9600 bps) and short distances (<10 m), termination can often be omitted to save power.
Advanced: Biasing for Fail-Safe Operation
Termination alone may leave the bus in an indeterminate state when idle (A ≈ B), causing false receives. To fix this, add a bias network:
Pull-up resistor (~680Ω) from A to VCC;
Pull-down resistor (~680Ω) from B to GND;
This ensures A–B > +200 mV when idle, forcing a known logic state.
Many modern transceivers include built-in fail-safe receivers, eliminating the need for external bias resistors.
Common Mistakes
Adding 120Ω at every node → total bus impedance drops too low;
Using incorrect resistor values (e.g., 100Ω) → incomplete reflection suppression;
Ignoring PCB trace impedance on the board side.
In Summary
RS-485 termination isn’t optional for robust communication—it’s essential for high-speed or long-distance links. Remember: “120Ω at both ends only; skip for slow/short runs; use biasing or fail-safe ICs to prevent glitches.” Combine good layout, proper components, and real-world testing to build a reliable industrial network.
