RS-485 Bias and Termination Calculator

Estimate first-order RS-485 failsafe bias behavior from pull-up, pull-down, and termination values, including idle differential voltage and resistor power.

RS-485 failsafe bias calculator

Inputs

Supply voltage (V) Termination resistor (ohms)

Bias pull-up resistor (ohms) Bias pull-down resistor (ohms)

Number of termination resistors Failsafe threshold (mV)

Node count (informational) Transceiver has internal failsafe

Outputs

Effective termination-

Bias current-

Idle differential voltage-

Estimated line A voltage-

Estimated line B voltage-

Threshold check-

Pull-up resistor power-

Pull-down resistor power-

Termination power-

Node-load note-

Enter RS-485 bias values to estimate idle differential voltage and current.

What is RS-485 failsafe biasing?

Failsafe biasing keeps the RS-485 bus in a known idle state when no node is actively driving. A pull-up and pull-down pair creates a small differential voltage across A and B.

Pull-up, pull-down and termination resistors

Termination loads the bias network. With two 120 ohm terminators, the effective line resistance is 60 ohm. Stronger bias resistors increase idle differential voltage but also current.

Idle differential voltage

This calculator uses a first-order model: Vdiff = I x RtermEff. Compare the result against your failsafe threshold target (for example 200 mV).

Bias current and resistor power

Lower bias resistor values increase current and dissipation. Check resistor power and transceiver loading before finalizing your network design.

Internal failsafe transceivers

Many modern transceivers include internal failsafe behavior. External bias may still be needed depending on noise, bus length, and idle robustness requirements.

RS-485 bias reference examples

Vcc	Bias resistors	Termination	Idle differential	Notes
5 V	680 + 680 ohm	2 x 120 ohm	about 0.21 V	Common external bias example
5 V	560 + 560 ohm	2 x 120 ohm	about 0.25 V	Stronger bias current
3.3 V	680 + 680 ohm	2 x 120 ohm	about 0.14 V	May be low for 200 mV threshold
3.3 V	390 + 390 ohm	2 x 120 ohm	about 0.23 V	Higher current bias example

FAQ

What are RS-485 bias resistors?

They are pull-up and pull-down resistors that create a known idle differential voltage so receivers do not float between states.

Do I always need RS-485 failsafe biasing?

Not always. Some transceivers provide internal failsafe. External bias is still common in noisy or long-bus applications.

Why are there two 120 ohm termination resistors?

A properly terminated RS-485 bus typically has one terminator at each end of the main cable run, so two 120 ohm resistors in total.

How much idle differential voltage is enough?

A practical target is often above receiver threshold with margin (for example around or above 200 mV), but requirements vary by transceiver.

What happens if the RS-485 bias resistors are too small?

Idle differential increases, but current and power also increase. Excessive current can stress power budgets and reduce efficiency.