485 Differential Bus Utility

RS-485 Bias & Termination Calculator

Calculate equal-value RS-485 failsafe bias resistors, idle differential voltage, parallel termination load, receiver loading, bias current, resistor power, driver current and transmission-line termination mismatch.

✓ Bias Resistors ✓ 120 Ω Termination ✓ Idle Differential ✓ Receiver Loading ✓ Driver Current ✓ Reflection Coefficient

485

Bias & Termination Network

● Ready

Bias Supply Voltage (V) Supply used by the external pull-up and pull-down bias network.

Target Idle Differential Voltage (V) Desired A-to-B differential produced by the external failsafe bias network.

Cable Characteristic Impedance (Ω) Typical twisted-pair RS-485 cable is often near 120 Ω.

Termination Resistor Each (Ω) Each physical termination resistor placed across the differential pair.

Number of Terminations A conventional linear RS-485 trunk is normally terminated at the two ends.

Receiver Count Number of receiver inputs loading the differential bus.

Receiver Differential Input Resistance (Ω Each) Use the relevant transceiver input resistance from its datasheet.

Driver Differential Voltage for Load Estimate (V) Used only to estimate differential bus current and load power while driven.

Bias model: the calculator assumes one pull-up resistor from the bias supply to one bus conductor and one equal pull-down resistor from the other conductor to ground. The differential load includes the selected termination resistors and receiver input resistances in parallel. Real transceivers can include internal failsafe features, so always compare the result with the device datasheet.

RS-485 Bias & Termination Result Calculated

Recommended Equal Bias Resistors —

Bias Resistor Each —

Actual Idle Differential —

Bias Current —

Power Per Bias Resistor —

Termination Parallel Load —

Receiver Parallel Load —

Total Differential Load —

Driver Current Estimate —

Bus A Idle Voltage —

Bus B Idle Voltage —

Idle Common Mode —

Reflection Coefficient —

Termination Mismatch —

Power Each Termination —

Total Driven Load Power —

Target Margin —

Calculation Breakdown

—

What Is RS-485 Biasing?

An RS-485 bus can temporarily have no active driver. External bias resistors can establish a defined differential voltage during that idle condition. A common arrangement uses one pull-up resistor on one conductor and one pull-down resistor on the other.

The resistors must be strong enough to create the intended idle differential voltage while the termination resistors and receiver inputs load the bus, but not unnecessarily low because stronger biasing increases current and power.

RS-485 Bias Resistor Formula

For two equal bias resistors with resistance Rb and an equivalent differential bus load Rload, the simplified DC network is a series path from the bias supply through Rb, Rload and the second Rb.

Idle differential voltage:

Vdiff =
Vcc × Rload
-----------------
2 × Rb + Rload


Solving for equal bias resistors:

Rb =
Rload × (Vcc / Vdiff - 1)
--------------------------
2

The calculator applies this equation after combining termination and receiver loads.

Two 120 Ω Termination Resistors

When two 120 Ω termination resistors are connected across the same differential bus, their DC parallel resistance is 60 Ω.

Rt = 120 Ω
Terminations = 2

Equivalent:

1 / R =
1 / 120 + 1 / 120

R = 60 Ω

That 60 Ω load is important when sizing external failsafe bias resistors. It does not mean that the transmission-line impedance became 60 Ω; each end is still individually terminated against the cable’s characteristic impedance.

Receiver Loading

RS-485 receivers also present finite differential input resistance. Multiple receivers therefore create another parallel load across the bus.

32 receivers
12,000 Ω each

Parallel receiver resistance:

12,000 / 32
= 375 Ω

The calculator combines this receiver load with the termination load before calculating bias resistance.

Total Differential Bus Load

Termination load:
60 Ω

Receiver load:
375 Ω

Combined load:

1 / Rtotal =
1 / 60 + 1 / 375

Rtotal ≈ 51.7 Ω

This combined DC resistance is the load seen by the simplified external bias network.

Bias Current

Once the equal bias resistor value is known, the simplified idle current is:

Ibias =
Vcc
-----------------
2 × Rb + Rload

The same current flows through each external bias resistor in this simplified DC model.

Bias Resistor Power

The power dissipated in each equal bias resistor is calculated from:

Pbias =
I² × Rb

A practical design should select a resistor power rating with suitable margin rather than using a component exactly at the calculated dissipation.

RS-485 Termination Resistance

Termination is primarily a transmission-line issue. Each termination resistor should be selected to approximately match the characteristic impedance of the cable at the end of the line.

Typical example:

Cable Z0:
120 Ω

Termination:
120 Ω

Long buses and high data rates are generally more sensitive to termination quality than short, slow links.

Reflection Coefficient

The calculator reports the voltage reflection coefficient for one termination using:

Γ =
Rt - Z0
--------
Rt + Z0

A perfectly matched resistive termination gives Γ = 0. Positive or negative values indicate an impedance mismatch.

Termination Examples

Cable Z0	Termination	Reflection Coefficient
120 Ω	120 Ω	0
120 Ω	100 Ω	About -0.091
120 Ω	150 Ω	About +0.111
100 Ω	120 Ω	About +0.091

Where Should RS-485 Termination Be Installed?

For a conventional daisy-chain or linear RS-485 trunk, termination is normally placed at the two physical ends of the main cable rather than at every device. Placing 120 Ω across many nodes would create an excessively low differential load.

Networks with unusual topology, very low data rate or specialized transceivers can require different treatment, so the physical installation should be evaluated rather than applying a fixed resistor count blindly.

Failsafe Bias and Modern RS-485 Transceivers

Many modern RS-485 receivers include built-in failsafe behavior for open, shorted or idle bus states. In such systems external bias resistors may be unnecessary or can use different values from older networks.

The calculator therefore treats external biasing as an electrical network calculation rather than declaring that every RS-485 bus requires external bias resistors.

RS-485 A and B Polarity

Different equipment documentation can use A/B labels differently. For that reason the calculator describes its bias arrangement mathematically as one conductor being pulled toward the positive supply and the other toward ground.

When implementing the circuit, verify the polarity convention used by the actual transceiver and equipment documentation.

Driver Current Estimate

The calculator can estimate differential bus current from a user-supplied driver differential voltage and the calculated parallel bus load:

Idriver =
Vdriver
-------
Rload

This is a simplified resistive-load estimate. The actual transceiver output voltage depends on its output stage, supply, bus loading and device specifications.

Termination Power

When the driver produces a differential voltage V across a termination Rt, the approximate power in each active termination is:

Pterm =
V² / Rt

For two equal end terminations, both dissipate power while the driver is holding a differential level.

RS-485 Bias & Termination Calculator FAQs

What termination value is commonly used for RS-485?

120 Ω is common because many RS-485 twisted-pair cables have a characteristic impedance near 120 Ω. The correct value should match the actual cable.

Why do two 120 Ω terminations equal a 60 Ω DC load?

Both resistors are connected across the same two conductors, so from a DC load perspective they are in parallel.

Should every RS-485 device have a termination resistor?

Normally no. A conventional linear bus is typically terminated at its two physical ends.

What are RS-485 bias resistors?

They are pull-up and pull-down resistors used to establish a known differential bus state when no active driver controls the line.

Do all RS-485 networks require external biasing?

No. Many modern transceivers have built-in failsafe receiver behavior. Check the transceiver datasheet before adding external bias.

Why does receiver count affect bias resistor sizing?

Receiver inputs have finite differential resistance. Many receivers connected in parallel lower the effective bus resistance and therefore load the bias network more strongly.

What is the reflection coefficient?

It is a measure of transmission-line impedance mismatch. A matched resistive load has a reflection coefficient of zero.

Does this calculator include cable capacitance?

No. The bias calculation is a DC resistive model. Cable capacitance and edge rate matter for dynamic signal integrity but are separate from this resistor calculation.

Does the calculator account for transceiver output resistance?

No. Driver current is shown as a simplified load estimate. Actual output performance must be checked against the transceiver datasheet.

Can I use 3.3 V biasing?

Yes, if the selected transceiver and system electrical limits permit it. Enter 3.3 V and the calculator will size the resistors for that supply.

Calculate RS-485 Bias and Termination Loading

Estimate external failsafe resistor values, idle differential voltage, termination and receiver loading, driver current, resistor dissipation and transmission-line matching for an RS-485 differential bus.