RS-485 Termination Resistor

Formula

Description

RS-485 bus termination prevents signal reflections that degrade data integrity at high baud rates or long cable runs. A termination resistor equal to the characteristic impedance of the cable is placed at each end of the bus. Standard twisted-pair cables used in RS-485 networks typically have a characteristic impedance of 100-120 ohms. Without proper termination, reflected signals can cause bit errors, especially at baud rates above 100 kbaud or cable lengths above 10 meters.

Variables

• R — Termination resistance in ohms (Ω)
• Z — Characteristic impedance of the cable in ohms (Ω)

Practical Notes

Only the two physical ends of the bus should be terminated. Intermediate nodes (stubs) should not have termination resistors. For short, slow buses, termination may be omitted. Some RS-485 transceivers have built-in fail-safe biasing, but external termination is still recommended for robust communication.