RS-485 Bias Resistor

R_bias = (Vcc − V_diff_min) / I_bias

Quick answer

This calculator solves R_bias = (Vcc − V_diff_min) / I_bias for Bias Resistance. With Supply Voltage 120 V, Min Differential Voltage 56.4 V, Bias Current 0.2 A, the result is 0.318 kΩ. It solves in 4 directions, so you can enter the values you have and get the one you need.

Calculator

Bias Resistance (Ω)
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Formula

R_bias = (Vcc − V_diff_min) / I_bias

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.

Symbols and units

Symbol Meaning Default unit Accepted units
R Bias Resistance Ω mΩ, Ω, kΩ, MΩ
Vcc Supply Voltage V pV, nV, µV, mV, V, kV
Vdiff Min Differential Voltage V pV, nV, µV, mV, V, kV
I Bias Current mA pA, nA, µA, mA, A, kA

Step by step

  1. 1. What you know: Supply Voltage 120 V, Min Differential Voltage 56.4 V, Bias Current 0.2 A
  2. 2. The relation: R_bias = (Vcc − V_diff_min) / I_bias
  3. 3. Substitute: Vcc = 120 V, Vdiff = 56.4 V, I = 0.2 A
  4. 4. Result: Bias Resistance = 0.318 kΩ

Related quantities from the same inputs

  • P = V × I = 24 W

Nearest standard component values

Series Nearest value Error
E12 0.33 kΩ 3.8%
E24 0.33 kΩ 3.8%

Worked examples

Minimum range

Given: Supply Voltage 3.3 V, Min Differential Voltage 1.55 V, Bias Current 1 mA

Result: Bias Resistance = 1.75 kΩ

Low range

Given: Supply Voltage 33 V, Min Differential Voltage 15.5 V, Bias Current 10 mA

Result: Bias Resistance = 1.75 kΩ

Typical

Given: Supply Voltage 120 V, Min Differential Voltage 56.4 V, Bias Current 0.2 A

Result: Bias Resistance = 0.318 kΩ

High range

Given: Supply Voltage 2.3 kV, Min Differential Voltage 1.08 kV, Bias Current 5 A

Result: Bias Resistance = 0.244 kΩ

Maximum range

Given: Supply Voltage 23 kV, Min Differential Voltage 10.8 kV, Bias Current 50 A

Result: Bias Resistance = 0.244 kΩ

Solving for each variable

Solve for Bias Resistance

Supply Voltage 3.3 V, Min Differential Voltage 1.55 V, Bias Current 1 mA → 1.75 kΩ

Solve for Supply Voltage

Bias Resistance 1 Ω, Min Differential Voltage 0.726 V, Bias Current 100 µA → 0.7261 V

Solve for Min Differential Voltage

Bias Resistance 1 Ω, Supply Voltage 0.33 V, Bias Current 100 µA → 0.3299 V

Solve for Bias Current

Bias Resistance 1 Ω, Supply Voltage 0.33 V, Min Differential Voltage 0.726 V → -0.396 A

How Bias Resistance changes with Supply Voltage

Supply Voltage (input) Bias Resistance (result)
60 V 18 Ω
90 V 0.168 kΩ
120 V 0.318 kΩ
0.18 kV 0.618 kΩ
0.24 kV 0.918 kΩ
0.48 kV 2.118 kΩ
0.84 kV 3.918 kΩ
1.2 kV 5.718 kΩ

The same result in other units

Unit Value
mΩ 318000
Ω 318
kΩ 0.318
MΩ 0.000318

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Frequently asked questions

How do you calculate RS-485 Bias Resistor?

Use R_bias = (Vcc − V_diff_min) / I_bias. With Supply Voltage 120 V, Min Differential Voltage 56.4 V, Bias Current 0.2 A, Bias Resistance works out to 0.318 kΩ.

What is the formula for RS-485 Bias Resistor?

The formula is R_bias = (Vcc − V_diff_min) / I_bias. 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.

How do you find Supply Voltage when you know the other values?

Rearrange for Supply Voltage: with Bias Resistance 1 Ω, Min Differential Voltage 0.726 V, Bias Current 100 µA, Supply Voltage is 0.7261 V.

How do you find Min Differential Voltage when you know the other values?

Rearrange for Min Differential Voltage: with Bias Resistance 1 Ω, Supply Voltage 0.33 V, Bias Current 100 µA, Min Differential Voltage is 0.3299 V.

What units does RS-485 Bias Resistor use?

Bias Resistance is shown in mΩ, Ω, kΩ, MΩ. Values are held in SI base units internally, so you can mix prefixes without converting by hand.

Related Concepts

All Communications formulas →
UART Baud Rate Error RS-485 Termination Resistor RS-485 Bias Resistor Shannon Channel Capacity Bit Error Rate (BPSK) Wireless Link Range Manchester Encoding Bandwidth I²C Pull-up Resistor

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