RS485 Termination Resistor Calculator

Enter Cable and Bias Network Parameters

Z₀ Cable Impedance (default: 120Ω)

V_sup Bias Supply Voltage

R_pu Pull-Up on A+ Line

R_pd Pull-Down on B- Line

N Devices on Bus (optional)

L Cable Length (optional)

RS-485 Bus Analysis

Termination R
—

V_A+ (idle)

—

V_B- (idle)

—

Vdiff (A – B)
—

Bias Current

—

Bias Power

—

Unit Loads

—

Cable / Baud Rate

—

RS-485 Bus Termination and Bias

An RS-485 bus uses differential signalling over twisted pair cable. Termination resistors (matching the cable impedance, typically 120 Ω) are placed at each end of the bus to prevent reflections. Bias resistors pull the A+ line high and B- line low to ensure a valid idle state when no driver is active.

Termination = cable impedance at each end. Bias resistors ensure a valid idle state (A+ > B-) when no device is driving the bus.

RS-485 Termination Resistor Calculator

RS-485 is a differential serial bus used by Modbus RTU, DMX512, BACnet MS/TP, PROFIBUS, and many industrial protocols. Two things must be correct for reliable communication: termination resistors at each end of the cable to prevent signal reflections, and bias resistors to hold the bus in a known idle state when no device is transmitting. The calculator above takes your cable impedance, bias supply voltage, and bias resistor values and tells you whether the idle differential voltage meets the RS-485 specification’s 200 mV minimum threshold.

Why Termination and Bias Matter

Termination Resistors

A differential signal travelling down a twisted pair cable reflects back at any point where the impedance changes. An unterminated cable end is the worst case — the reflection can be nearly as large as the original signal, corrupting data. Placing a termination resistor equal to the cable’s characteristic impedance (Z₀) at each end of the bus absorbs the signal energy and eliminates reflections. Standard RS-485 cable has Z₀ = 120 Ω, so each end gets a 120 Ω resistor between A+ and B−. For understanding impedance in more detail, see our Impedance Calculator.

Bias Resistors

RS-485 is a multi-drop half-duplex bus. Between transmissions, every driver goes high-impedance and the bus floats. If the differential voltage between A+ and B− drops below 200 mV during this idle period, receivers cannot determine the bus state and may output garbage data. Bias resistors force a defined idle condition: a pull-up on A+ (to V_sup) and a pull-down on B− (to ground) create a positive differential voltage that all receivers interpret as a logic “1” (idle/mark).

Idle Voltage Calculation

The bias and termination resistors form a voltage divider network. The two 120 Ω termination resistors in parallel give 60 Ω between A+ and B−. The pull-up and pull-down resistors complete the circuit:

R_{term(parallel)} = Z₀ / 2 — two terminations in parallel (60 Ω for 120 Ω cable)

V_B− = V_sup × R_pd / (R_pu + R_{term(parallel)} + R_pd)
V_A+ = V_B− × (R_{term(parallel)} + R_pd) / R_pd
V_diff = V_A+ − V_B−

Pass if V_diff ≥ 200 mV

Worked Example — 5 V Supply, 560 Ω Bias

Z₀ = 120 Ω, V_sup = 5 V, R_pu = 560 Ω, R_pd = 560 Ω, 10 devices at 1 unit load each.

R_{term(parallel)} = 120 / 2 = 60 Ω
Total loop resistance = 560 + 60 + 560 = 1,180 Ω
I_bias = 5 / 1,180 ≈ 4.2 mA
V_diff = 4.2 mA × 60 Ω ≈ 254 mV → PASS
P_bias = 5 × 4.2 mA ≈ 21 mW

The 560 Ω / 5 V combination passes with 54 mV of margin above the 200 mV minimum. Bias current draw is about 4.2 mA — negligible for most power budgets.

3.3 V Supply — Tighter Margins

With V_sup = 3.3 V and R_pu = R_pd = 390 Ω: total loop resistance = 390 + 60 + 390 = 840 Ω. I_bias = 3.3 / 840 ≈ 3.9 mA. V_diff = 3.9 × 60 ≈ 236 mV — still passing but with less margin. On a long bus with many devices adding input loading, this margin shrinks further.

Bias Resistor Sizing Guidelines

Bias resistors that are too small waste current and reduce the voltage swing available for data transmission — drivers have to fight the bias to change the bus state. Bias resistors that are too large do not produce enough differential voltage at idle, risking false data. Common values:

5 V supply: 390 Ω to 680 Ω pull-up and pull-down. 560 Ω is the most common default.

3.3 V supply: 330 Ω to 560 Ω. Use the lower end if you have many devices loading the bus.

Modbus RTU standard: 680 Ω pull-up and pull-down with 5 V supply is a widely used convention.

Only one device on the bus should provide bias. If multiple devices each have their own bias resistors, the parallel combination lowers the effective bias resistance and draws excessive current. Designate the master or a dedicated junction box as the bias source.

Cable Length vs. Baud Rate

RS-485 supports up to 1200 m (4000 ft) at low baud rates. As cable length increases, signal attenuation and propagation delay limit the maximum reliable speed:

≤ 100 m → up to 10 Mbps
≤ 300 m → up to 1 Mbps
≤ 500 m → up to 115.2 kbps
≤ 1200 m → up to 9600 bps

These are guidelines, not hard limits — actual performance depends on cable quality, noise environment, and transceiver speed. Enter your cable length in the calculator to see the recommended maximum baud rate.

Device Count and Unit Loads

The RS-485 standard (TIA-485-A) defines a “unit load” as the input impedance of a standard transceiver (≥12 kΩ). A bus supports 32 unit loads maximum — meaning 32 standard transceivers. Modern 1/8 unit load transceivers (≥96 kΩ input impedance) allow up to 256 devices on the same bus. The calculator checks whether your device count exceeds the unit load limit for the transceiver type you select.

Common Protocol Configurations

Modbus RTU

120 Ω termination at each end, 680 Ω bias resistors with 5 V supply. Up to 32 devices (or 247 addresses with 1/8 unit load transceivers). Typical baud rates: 9600, 19200, 38400, 115200.

DMX512

120 Ω termination at the last fixture in the daisy chain. Bias is usually handled internally by the DMX controller. Up to 512 channels at 250 kbps. Cable runs up to 300 m between controller and last fixture.

BACnet MS/TP

120 Ω termination, EIA-485 bias. Up to 128 devices with 1/8 unit load transceivers. Baud rates: 9600 to 76800. Maximum cable length 1200 m at 9600 bps.

PROFIBUS DP

Termination is built into the PROFIBUS connector (active termination with 390 Ω to V+ and 390 Ω to GND, with a 220 Ω line-to-line resistor). Do not add external bias resistors — the PROFIBUS connector handles everything. Up to 32 devices per segment at speeds up to 12 Mbps.

Frequently Asked Questions

Do I always need termination resistors?

At low baud rates (9600 bps) over short cables (under 10 m), you can often get away without termination. Above 100 kbps or over 50 m, reflections become a real problem and termination is required. The safe practice is to always terminate both ends.

What if I only terminate one end?

Reflections from the unterminated end bounce back and forth, adding noise to the signal. It works at low speeds and short distances but fails as baud rate or cable length increases. Always terminate both ends of the bus.

Can I use a 100 Ω or 150 Ω termination resistor?

Only if your cable’s characteristic impedance is 100 Ω or 150 Ω. The termination resistor must match Z₀ of the cable, not a generic standard. Most RS-485 cable is 120 Ω, but check your cable’s data sheet.

Where should the bias resistors be installed?

At one location on the bus — typically the master device or main junction point. Only one set of bias resistors should be active. Multiple bias sources in parallel lower the effective resistance and draw excessive current.

My differential voltage is below 200 mV — what do I fix?

Lower the bias resistor values to push more current through the termination resistors. If you are already at 390 Ω and still failing, check for multiple termination resistors (there should be exactly two — one at each cable end) or excessive device loading. Each additional device lowers the effective bus impedance and reduces V_diff.

What is the idle power consumption of the bias network?

P = V_sup × I_bias. For 5 V / 560 Ω bias: about 21 mW. For 3.3 V / 390 Ω: about 13 mW. Negligible in most industrial installations but worth checking in battery-powered or low-power designs.

Last updated: March 2026