RS-485 Termination Resistors: Why 120 Ohms, Wiring, and Testing

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.

Catalog

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.

Termination	Reflection Coefficient	Idealized Result
120 Ω	0%	Matched load; no reflected voltage
100 Ω	−9.1%	Small reflection with opposite polarity
54 Ω	−37.9%	Larger reflection with opposite polarity
Open end	+100%	Full reflection with the same polarity
Short circuit	−100%	Full reflection with opposite polarity

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.

RS-485 Termination Methods Compared

The values below assume a 3 V differential signal, a 120 Ω cable, and termination at both cable ends. Receiver and fail-safe bias loading are excluded.

Termination method	Connection and effective load	DC current and power	Suitable operating conditions	Main trade-off and best use
Parallel termination	One 120 Ω resistor is connected directly across A and B at each cable end, creating a 60 Ω effective driver load from the two end terminators.	50 mA and 150 mW total, or 75 mW per resistor, for the stated 3 V example. See Section 5 for the calculation.	Long cables, fast signal edges, and high data rates within the transceiver's limits.	Provides strong reflection control but creates continuous differential loading and power loss. It is the standard approach for many RS-485 networks.
Split termination	Two matched 60 Ω resistors are connected in series between A and B at each terminated cable end. Their combined resistance is 120 Ω, so the differential termination remains matched to a 120 Ω cable. The resistor midpoint may be connected to a suitable reference through a capacitor.	The DC differential load is essentially the same as a single 120 Ω parallel terminator. The midpoint capacitor ideally carries no steady-state DC current.	Networks that require normal differential termination while also benefiting from additional high-frequency common-mode noise filtering.	Maintains the required differential termination while the midpoint capacitor can shunt high-frequency common-mode noise. Resistor matching, capacitor value, PCB layout, and the choice of reference connection must be carefully designed to avoid degrading signal integrity.
AC termination	A resistor and capacitor are connected in series across A and B at each cable end. The capacitor passes fast transitions but blocks steady-state DC.	Steady-state DC termination current is nearly zero. Power is mainly consumed during signal transitions and depends on RC values, switching rate, and data pattern.	Known low-to-moderate data rates where the RC network has been validated for the expected cable and data pattern.	Reduces DC loading and can control some reflections, but incorrect RC values can slow transitions or distort data.
No termination	No resistor is connected across A and B, leaving the cable ends electrically unmatched.	Termination current and power are approximately zero, although receivers and bias networks may still draw current.	Short, relatively low-speed links where propagation-time, edge-rate, and waveform testing confirm that reflections settle adequately.	Uses the least termination power and fewest components but provides no controlled absorption of cable-end reflections.

RS-485 Termination vs Fail-Safe Biasing

Termination and fail-safe biasing solve different electrical problems in an RS-485 network:

• Termination controls signal reflections during communication: A matching resistor absorbs signal energy at each cable end, helping reduce ringing and waveform distortion.

• Fail-safe biasing establishes a known idle state: Pull-up and pull-down resistors create a small differential voltage when all drivers are disabled, preventing an undefined receiver output.

• Neither method can replace the other: Bias resistors do not match the cable impedance or control reflections. Termination resistors do not create a reliable idle voltage for receivers without built-in fail-safe protection.

How Termination Affects the Bias Network

Two 120 Ω end terminators appear as an effective resistance of approximately 60 Ω across the bus. This load reduces the idle differential voltage created by the bias resistors. Therefore, the pull-up, pull-down, and termination values must be calculated together. The network must produce a reliable idle voltage while avoiding excessive current or overloading the active driver.

For a two-wire half-duplex bus, only one external bias network is normally installed. Adding bias resistors at several devices can create excessive loading and change the intended idle voltage. In a four-wire network, each independently driven pair must be evaluated separately.

When External Fail-Safe Biasing May Be Required

External bias resistors may be needed when:

• The transceivers do not include built-in fail-safe receivers.

• Older and newer transceivers are connected to the same bus.

• The application requires a specific idle-state polarity.

• Electrical noise could exceed the receiver’s internal fail-safe margin.

Many modern RS-485 transceivers include built-in fail-safe receivers, so external bias resistors may not be necessary. Check the transceiver datasheet to confirm whether it guarantees a defined output during idle, open-circuit, short-circuit, and terminated-bus conditions. Fail-safe behavior and input thresholds vary between devices.

RS-485 Termination Wiring Examples

Correct Two-Wire RS-485 Wiring

A two-wire RS-485 network uses one twisted pair for both transmitting and receiving. All transceivers connect in parallel to the same A and B lines, but only one driver should transmit at a time. This arrangement is also called half-duplex RS-485.

For a terminated linear bus, place one termination resistor across A and B at each physical end of the main cable. Intermediate devices should remain unterminated. A master does not have to be located at a physical end of the bus, but if it is connected at an intermediate point, its connection to the main trunk should be an electrically short stub. The acceptable stub length depends on cable propagation delay and signal rise and fall times, so it should be kept as short as practical and verified for the actual network. Long stubs introduce impedance discontinuities that can cause additional reflections.

Correct Four-Wire RS-485 Wiring

Four-wire RS-485 uses two twisted pairs, allowing transmission and reception to use separate differential paths. In the Analog Devices full-duplex master/slave topology illustrated in Figure 3, one pair carries data from the master transmitter to the slave receivers, while the other carries responses from slave transmitters back to the master receiver. This permits simultaneous transmission and reception, although the communication protocol must prevent multiple slave drivers from driving the return pair at the same time.

For the specific Analog Devices topology shown, the master-to-slave pair is terminated at the farthest slave receiver, and the slave-to-master pair is terminated at the master receiver. This arrangement should be described as applying to the illustrated topology rather than as a universal rule for every four-wire RS-485 system.

Figure 2. Example Four-Wire RS-485 Full-Duplex Master/Slave Topology. Diagram source: Analog Devices

Termination placement can depend on the protocol and network architecture. For example, the Modbus Serial Line specification states that in a four-wire Modbus system, each balanced pair is terminated at both ends of the bus. Therefore, when implementing a protocol-specific four-wire network, follow the applicable protocol specification and equipment documentation rather than assuming that the termination arrangement in Figure 3 applies to every system. Keep the polarity and pair assignments consistent throughout the network.

Multipoint Bus with Short Stubs

Figure 3. Multipoint Bus with Short Stubs. Diagram source: Analog Devices

A correct multipoint network uses one continuous trunk cable with a termination resistor at each physical end. Intermediate devices connect to the trunk through short branch wires called stubs. Keep each stub as short as practical because it acts as an unterminated transmission-line section and can create reflections. There is no universal maximum stub length because the safe length depends mainly on the transceiver’s rise time, cable propagation delay, and network data rate. Do not place termination resistors on the intermediate stubs.

Incorrect Star-Wired Network

Figure 4. Incorrect Star-Wired Network. Diagram source: Analog Devices

In a star network, several long cable branches start from one central point. Each branch becomes a separate transmission path and creates an impedance discontinuity. If every branch is terminated, the combined resistance can overload the driver. If only two branches are terminated, the remaining branches have unterminated ends that reflect signals. Replace the star with a linear bus or use active RS-485 repeaters or hubs that create separately terminated bus segments.

RS-485 Termination Case Studies

Practical Modbus RTU Termination Case Study

Figure 5. Modbus RTU Termination. Diagram source: DRAGO Automation

The diagram shows a linear Modbus RTU trunk with termination enabled only at its two physical ends. Intermediate slaves remain unterminated, while one polarization network at the master establishes a defined idle-bus state. The common conductor provides a voltage reference between the connected devices.

This arrangement avoids the excessive driver loading caused by terminating every device. It also reduces the reflections that can occur when intermediate branches or cable ends are left incorrectly terminated.

Check each device manual before connecting the data lines because manufacturers do not always use A and B labels consistently. Verify the terminal functions instead of relying only on the letters printed on the equipment. Verify the installed termination by following the resistance and waveform checks in Section 10.

Correct vs Incorrect Termination Case Study

Analog Devices tested a MAX3485 transceiver driving a 120 Ω twisted-pair cable. When the cable was terminated with a mismatched 54 Ω resistor, the measured waveform showed greater distortion. Replacing it with a matched 120 Ω resistor produced a cleaner and more stable signal.

The 54 Ω resistor created two problems. First, its resistance was much lower than the cable’s characteristic impedance, producing a strong inverted reflection. Second, it placed a heavier electrical load on the transceiver, requiring the driver to supply more current.

This test shows that using a lower termination resistance does not automatically improve communication. The resistor should match the cable impedance while remaining within the transceiver’s load-driving capability. Although the measured results apply to the tested MAX3485, cable, and operating conditions, they demonstrate the general importance of correct impedance matching in RS-485 networks.

Cable Length and Data Rate Case Study

Texas Instruments tested two THVD1550 transceivers using Cat5 cable, PRBS-7 test data, and 120 Ω termination at both cable ends. The differential receiver input produced the following results:

Cable length	Data rate	Total jitter
1,000 ft	1 Mbps	3.6%
2,000 ft	1 Mbps	10.8%
4,000 ft	0.5 Mbps	12.8%
4,000 ft	1 Mbps	42.8%

At 1 Mbps, jitter increased from 3.6% at 1,000 ft to 42.8% at 4,000 ft. Reducing the 4,000 ft link to 0.5 Mbps lowered jitter to 12.8%. This shows that longer cables generally require lower data rates to maintain timing margin.

These results apply only to TI’s specific transceivers, cable, termination, and test conditions. They demonstrate the general trend but are not guaranteed limits for every RS-485 network. This study also does not compare terminated and unterminated wiring because all tested cables used termination at both ends.

RS-485 Termination Resistance Troubleshooting

With the network powered off, measure the resistance across A and B. Interpret the result with the connected bias, protection, and transceiver circuitry in mind.

Measured resistance	Likely cause	Troubleshooting action
Approximately 120 Ω	Only one 120 Ω terminator may be active.	Check that both physical ends of the bus are terminated.
Approximately 60 Ω	Two 120 Ω terminators are connected in parallel, as expected for a normally terminated 120 Ω bus.	Confirm that they are installed at the two physical endpoints.
Approximately 40 Ω	Three 120 Ω terminators may be enabled.	Check intermediate devices for unintended or built-in termination.
Unexpectedly low resistance	Excess termination, an incorrect resistor value, a wiring short, or other connected circuitry may be loading the bus.	Isolate sections of the network and inspect wiring, termination settings, and connected components.
Approximately 60 Ω but ringing continues	The DC resistance is correct, but a signal-integrity problem remains.	Check cable impedance, stub length, topology, connectors, and termination location, then inspect the differential waveform with an oscilloscope.

Resistance testing is a static check. Use waveform inspection when communication errors or ringing remain despite an expected resistance reading.

Frequently Asked Questions [FAQ]

1. Why might an RS-485 network stop working after replacing the transceiver but keeping the same data rate?

The replacement transceiver may have a faster signal rise time. Faster edges create stronger transmission-line effects, so reflections that were harmless with the previous device may now cause ringing and false receiver transitions.

2. How should RS-485 termination be handled when repeaters or isolated hubs are used?

Each repeater or hub port normally creates a separate electrical bus segment. Every segment should be treated as its own RS-485 network and terminated at its two physical ends according to the equipment manufacturer’s instructions.

3. Can surge-protection components affect RS-485 signal quality?

Yes. TVS diodes, filters, connectors, and PCB traces add capacitance and impedance changes to the bus. Components with excessive capacitance can slow signal edges or increase distortion, especially at high data rates.

4. Can an unpowered RS-485 device disturb an active network?

It can if the transceiver does not provide powered-off high-impedance protection. Internal protection paths or enabled termination may continue loading the bus. Check the datasheet for powered-off bus behavior before connecting devices that may lose power independently.

5. Does the physical placement of the termination resistor matter?

Yes. Place the resistor as close as practical to the cable connection at the physical end of the bus. Long PCB traces or wires between the cable and resistor act like an unterminated stub and can reduce the termination’s effectiveness.

6. Are ordinary through-hole resistors suitable for high-speed RS-485 termination?

They may work at moderate speeds, but their lead length adds unwanted inductance. Surface-mount resistors with short PCB traces generally provide better high-frequency performance and more accurate termination in fast RS-485 networks.