RS-485 Bias Resistor

Formula

Description

RS-485 bias resistors hold the bus in a known state when no driver is active, preventing the receiver from interpreting noise as valid data. A pull-up resistor on the non-inverting line (A) and a pull-down on the inverting line (B) create a differential voltage greater than the receiver threshold (typically 200 mV). The bias resistance must be large enough to avoid overloading the drivers but small enough to maintain the minimum differential voltage against termination loading.

Variables

• R — Bias resistance in ohms (Ω)
• Vcc — Supply voltage (V)
• V_diff_min — Minimum required differential voltage (V)
• I_bias — Required bias current (A)

Practical Notes

Typical bias resistor values range from 390Ω to 750Ω for a 5V or 3.3V supply. The bias network must overcome the current draw from two termination resistors (typically 120Ω each) in parallel. Many modern RS-485 transceivers include internal fail-safe biasing that eliminates the need for external bias resistors.