An RS-485 network may look simple because it often uses only two signal wires, but small wiring and termination mistakes can cause serious communication problems. This article explains how RS-485 termination works, why resistor and cable impedance must match, and when termination is necessary. It also covers resistor selection, power rating, termination methods, fail-safe biasing, correct bus wiring, practical test results, and verification procedures.

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Figure 1. RS-485 Daisy-Chain Bus with End Termination

RS-485 Communication and Termination Resistor Overview

RS-485 is a physical-layer standard for serial communication in electrically noisy environments. It transmits data using the voltage difference between two wires, commonly labelled A and B. Because the receiver rejects much of the noise affecting both wires equally, RS-485 is widely used in industrial control, instrumentation, building automation, and security systems.

A termination resistor is connected across A and B at each physical end of the main bus. It provides an electrical load that matches the cable impedance and helps reduce signal reflections. Intermediate nodes normally remain unterminated.

Why RS-485 Commonly Uses 120 Ohms

TIA/EIA-485 recommends a nominal cable characteristic impedance of 120 Ω, which is why 120 Ω termination resistors are widely used in RS-485 networks. However, 120 Ω is a recommended nominal value rather than a requirement that applies to every cable. The differential characteristic impedance of practical twisted-pair cables can vary, commonly from approximately 100 Ω to 150 Ω, depending on conductor geometry, insulation, spacing, and construction.

For proper parallel termination, the termination resistance should be selected to match the actual differential characteristic impedance of the cable as closely as practical. When the termination and cable impedances match, reflections at the cable end are minimized.

The reflection coefficient can be calculated as:

Γ = (R_t − Z₀) / (R_t + Z₀)

where:

• R_t is the termination resistance.

• Z₀ is the cable's differential characteristic impedance.

• Γ is the voltage reflection coefficient.

Idealized Reflection Calculation for a 120 Ω Line

The following table is an idealized calculation for a transmission line with Z₀ = 120 Ω and a purely resistive load at the cable end. It does not include real-world effects such as cable loss, connector impedance, stubs, parasitic capacitance, resistor tolerance, or frequency-dependent impedance.

For example, a cable with a specified differential impedance of approximately 100 Ω should generally use termination closer to 100 Ω rather than automatically using 120 Ω. Likewise, a nominal 120 Ω RS-485 cable is normally terminated with approximately 120 Ω at the required cable ends. The cable datasheet should therefore be checked before selecting the termination resistance.

The termination value must also remain compatible with the transceiver's drive capability because the end resistors contribute to the total electrical load on the bus.

What Happens with Incorrect Termination?

When the termination resistance does not match the cable impedance, part of the signal is reflected from the cable end. The reflected signal travels back along the cable and combines with later signal transitions. Depending on its timing and polarity, it can increase or reduce the voltage seen by the receiver.

Incorrect or missing termination can cause:

• Ringing and overshoot: The signal voltage continues to oscillate after a transition instead of settling quickly.

• Repeated receiver-threshold crossings: Ringing may cause the receiver to detect additional transitions, leading to false data bits.

• Reduced differential voltage: A termination resistance that is too low places a heavier load on the driver and may weaken the voltage difference between A and B.

• CRC errors and missing responses: Distorted signals may be interpreted incorrectly, producing corrupted messages, timeouts, or incomplete communication.

• Failure at longer distances or faster data rates: Reflections become more difficult to tolerate when cable propagation delay is longer or when the receiver has less time to identify each bit.

Termination problems may appear intermittent because their effects depend on cable length, signal rise time, data pattern, electrical noise, node loading, and operating temperature. For this reason, the network should be tested under its worst expected operating conditions.

When Does an RS-485 Network Need Termination?

An RS-485 network needs termination when cable reflections can persist long enough to distort the received waveform. The decision depends mainly on cable length, propagation delay, driver rise and fall time, data rate, and network topology.

Long Cables and High Data Rates

Longer cables have greater propagation delay, so reflected signals take more time to travel between the cable ends. Higher data rates also shorten the available bit period. As these timing margins become smaller, proper termination becomes increasingly important for maintaining signal integrity.

Signal Rise Time and Cable Propagation Delay

Signal rise and fall times are important because transmission-line effects are determined by the speed of the signal edges, not simply by the baud rate. Even a relatively low-data-rate RS-485 transceiver can produce fast edges that create significant reflections on a sufficiently long cable.

A useful engineering guideline is to compare the driver's rise time with cable propagation delay. Analog Devices notes that a cable may generally be treated as electrically short when the signal rise time is more than about four times its one-way propagation delay. Faster edges or longer cables make termination more important.

Practical Screening for an Unterminated Network

For a quick first-pass check, calculate the approximate two-way propagation time of the main cable:

t_loop = 2L / v_p

where:

• t_loop = round-trip propagation time

• L = one-way cable length

• v_p = propagation velocity of the cable

Next, calculate the bit time:

t_BIT = 1 / R_bit

where R_bit is the bit rate in bits per second.

As a practical screening guideline, an unterminated network may be considered when:

t_loop < approximately 0.1 × t_BIT

In other words, the round-trip propagation time should be much shorter than one bit period. This condition gives reflections more time to return and settle before they are likely to interfere with later portions of the data waveform.

For example, consider a 100 m cable with a propagation velocity of 2 × 10⁸ m/s:

t_loop = (2 × 100) / (2 × 10⁸) = 1 µs

At 100 kbps:

t_BIT = 1 / 100,000 = 10 µs

Therefore:

t_loop / t_BIT = 1 µs / 10 µs = 0.1

This network is approximately at the screening limit rather than clearly within the electrically short region.

The 0.1 × t_BIT test is only a preliminary screening method. Passing it does not automatically mean termination can be omitted. Driver edge rate, cable impedance, stub length, connectors, topology, receiver thresholds, and other discontinuities can still produce troublesome reflections. TI guidance likewise emphasizes rise time and physical interconnect dimensions when evaluating reflection effects.

Before using an unterminated configuration, verify the differential waveform with an oscilloscope at important locations on the bus and test communication under worst-case cable length, data rate, node count, temperature, and noise conditions. Omitting termination can reduce DC loading and termination power, but it should only be done when adequate signal margin has been demonstrated.

Industrial Environments with Electrical Noise

Electrical noise does not by itself determine whether termination is required, but it reduces the available signal margin. Reflections can further reduce that margin by causing ringing or unwanted transitions around the receiver threshold.

Proper termination controls reflections, while twisted-pair cabling, suitable grounding, shielding where required, and surge or transient protection address other sources of interference. Termination should therefore be treated as one part of the overall RS-485 signal-integrity design rather than as a substitute for noise-control measures.

How to Select a Termination Resistor

• Match the resistor to the cable impedance: Check the cable datasheet for its differential characteristic impedance. Use approximately 120 Ω for a 120 Ω RS-485 cable or 100 Ω for a 100 Ω cable. A significant mismatch can increase reflections or place unnecessary load on the driver.

• Specify the resistor for the actual operating conditions: Use a ±1% termination resistor and verify that the selected component meets the required continuous-power rating, pulse capability, working-voltage rating, operating-temperature range, temperature derating, and parasitic-inductance requirements of the application. Low parasitic inductance is important because excessive inductance can make the termination impedance depart from its intended resistive value during fast signal edges. Do not select a resistor solely by its nominal resistance, tolerance, or construction type.

• Calculate the required power rating: Determine the power dissipated in each terminator from the maximum differential voltage expected across it:

P_r = Vdiff² / R_t

For the specific example of 3 V across one 120 Ω terminator:

P_r = 3² / 120 = 0.075 W = 75 mW

Two 120 Ω end terminators appear in parallel from the driver's perspective:

R_total = 120 Ω ∥ 120 Ω = 60 Ω

The resulting termination current is:

I_bus = V_diff / R_total

I_bus = 3 / 60 = 0.05 A = 50 mA

The total power dissipated by both terminators is:

P_total = Vdiff² / R_total

P_total = 3² / 60 = 0.15 W = 150 mW

Each terminator therefore dissipates 75 mW under the stated 3 V condition. A 0.25 W resistor may provide suitable margin for this particular example, provided its derated power rating at the actual operating temperature remains above the required dissipation and its other ratings are also satisfied.

The 0.25 W value is not a general recommendation for every RS-485 network. Required resistor power increases with the square of the differential voltage, and higher voltage, elevated temperature, transient conditions, or different termination values can require a higher-rated component. Always calculate the requirement from the worst-case conditions and check the resistor manufacturer's derating and pulse-rating data.

• Confirm the complete bus load: Include the two termination resistors, receiver unit loads, and any external fail-safe bias network when evaluating the load seen by the RS-485 driver. Traditional RS-485 supports a total receiver loading of up to 32 unit loads. Depending on transceiver input loading, this can correspond to 32 one-unit-load, 64 half-unit-load, 128 quarter-unit-load, or theoretically 256 one-eighth-unit-load transceivers. The actual permitted node count depends on the transceivers and total electrical loading. Confirm that the driver can maintain the required differential output voltage under worst-case conditions.

• Check for integrated termination: Some RS-485 transceivers and modules include a switchable 120 Ω termination resistor. Do not add another terminator at the same cable end when the internal termination is enabled.

• Select the appropriate configuration: Use fixed termination when a device will always remain at a physical cable endpoint. Use jumper-selectable or switchable termination when the device may operate either at an endpoint or as an intermediate node.