RS-485 Termination Calculator
Calculate RS-485 and RS-422 differential termination values, reflection estimates, effective bus loading, termination current, resistor power, and split termination resistor pairs.
COM-007 is a termination and load calculator. It does not design failsafe bias networks, guarantee cable distance, simulate stubs, model eye diagrams, or replace cable, transceiver, topology, EMC, grounding, surge, and isolation review.
Engineering tool
RS-485 Termination Calculator
Calculate RS-485 termination resistor match, effective bus load, termination current, resistor power, split termination and option comparisons.
Use the cable datasheet value. 100 Ω to 120 Ω is common, but not universal.
RS-485 termination summary
Ideal Match
Ideal Match: the selected standard termination equals the entered cable impedance.
Result console
- Ideal termination
- 120Ω
- Selected termination
- 120Ω
- Signed reflection coefficient
- 0Γ
- Reflection coefficient
- 0|Γ|
- Return loss
- Infinite / Idealperfect match
- VSWR
- 1:1
- Resistance error
- 0.0000%
- Match quality
- Ideal Match
- Nearest standard resistor
- 120Ω
- Lower standard resistor
- 120Ω
- Higher standard resistor
- 120Ω
Result is valid for the selected RS-485 termination model.
Formula reference
RS-485 Termination Formulas
The calculator uses impedance matching, equal parallel load, termination power, and split termination formulas.
Ideal Termination: RT ≈ Z0Reflection Coefficient: Γ = (RT - Z0) / (RT + Z0)Return Loss: RL = -20 log10(|Γ|)VSWR: VSWR = (1 + |Γ|) / (1 - |Γ|)Equal Parallel Terminators: Reffective = RT / NCurrent per Terminator: Ieach = Vdiff / RTPower per Terminator: Peach = Vdiff² / RTTotal Current: Itotal = N × Vdiff / RTTotal Power: Ptotal = N × Vdiff² / RTSplit Termination: Rsplit = RT / 2Approximate Common-Mode RC Reference: fc,approx = 1 / (2πRcm,approxCmid)Variable definitions
- RT
- termination resistance across the differential pair, in ohms
- Z0
- differential characteristic impedance of the cable, in ohms
- Γ
- reflection coefficient produced by resistive mismatch
- RL
- return loss, in dB
- VSWR
- voltage standing wave ratio
- N
- number of active full-value terminators
- Reffective
- equivalent differential load seen by the driver
- Vdiff
- differential voltage across the termination
- Ieach
- current through each terminator
- Itotal
- total differential load current from all terminators
- Peach
- power dissipated in each terminator
- Ptotal
- total termination power
- Rsplit
- each resistor in a two-resistor split termination
- Cmid
- optional midpoint capacitor used only for approximate common-mode RC reference
- fc,approx
- approximate midpoint RC corner frequency, not a differential termination cutoff frequency
Worked Examples
Ideal 120 Ω termination
Z0 = 120 Ω and RT = 120 Ω gives Γ = 0, VSWR = 1:1, and Return Loss = Infinite / Ideal.
100 Ω on 120 Ω cable
Γ = (100 - 120) / (100 + 120) = -0.090909. |Γ| ≈ 0.090909, Return Loss ≈ 20.8279 dB, and VSWR ≈ 1.2:1.
150 Ω on 120 Ω cable
Γ = 30 / 270 ≈ 0.111111. Return Loss ≈ 19.0849 dB and VSWR ≈ 1.25:1.
Single 120 Ω terminator
With N = 1, Reffective = 120 / 1 = 120 Ω.
Double 120 Ω terminators
With N = 2, Reffective = 120 / 2 = 60 Ω. This is the common two-end RS-485 bus load reference.
Three 120 Ω terminators
With N = 3, Reffective = 120 / 3 = 40 Ω. The calculator warns about excessive driver loading and topology review.
Termination current
For Vdiff = 2 V and RT = 120 Ω, Ieach = 2 / 120 ≈ 16.667 mA and Peach = 4 / 120 ≈ 33.333 mW.
Double-end total power
For Vdiff = 2 V, RT = 120 Ω, and N = 2, total current ≈ 33.333 mA and total power ≈ 66.667 mW.
5 V differential reference
For Vdiff = 5 V and RT = 120 Ω, Peach ≈ 208.333 mW. With 2× margin, the target is about 416.7 mW, so 0.5 W is the nearest common rating.
Split termination
Target RT = 120 Ω gives ideal split resistors of 60 Ω + 60 Ω.
Standard split pair
Using 60.4 Ω + 60.4 Ω gives 120.8 Ω total, about +0.6667% from a 120 Ω target.
Zero differential voltage
For Vdiff = 0 V, current and power are both zero. This is valid and should not produce an error.
Engineering Notes
- RS-485 is a balanced differential physical layer used for robust industrial and embedded communication links.
- Termination resistors are used to match the cable differential characteristic impedance and reduce reflections.
- Typical RS-485 cable impedance is often near 120 Ω, but the actual value should come from the cable datasheet.
- Termination resistors belong at the physical ends of the main bus trunk.
- A linear bus usually uses two terminators, one at each end.
- Do not install a full-value terminator at every node.
- Two 120 Ω terminators form about a 60 Ω differential DC load.
- The driver must be able to support the termination load and any other network loading.
- Correct termination cannot fix poor topology, long stubs, star wiring, or connector discontinuities.
- Baud rate and electrical edge speed are not the same parameter; reflections are strongly related to edge speed.
- Split termination can improve some common-mode high-frequency behavior, but it requires careful layout and capacitor selection.
- Termination matching does not replace oscilloscope or differential-probe validation.
Common Mistakes
- Confusing termination resistors with failsafe bias resistors.
- Installing 120 Ω termination at every RS-485 node.
- Installing termination only in the middle of the bus.
- Ignoring actual cable characteristic impedance.
- Assuming every RS-485 cable is exactly 120 Ω.
- Assuming low baud rate always removes the need for termination.
- Ignoring signal edge speed and stub length.
- Forgetting that two 120 Ω terminators create a 60 Ω load.
- Ignoring driver current capability and termination power.
- Choosing 120 Ω for each split resistor instead of half-value split resistors.
- Assuming split termination automatically solves EMC issues.
- Applying RS-485 termination assumptions to RS-232 links.
Termination vs Failsafe Bias
Termination Resistors
- Match the line characteristic impedance.
- Reduce signal reflections.
- Usually connect across A and B.
- Usually sit at the physical bus ends.
Failsafe Bias Resistors
- Set a known idle differential state when all drivers are high impedance.
- Usually connect to supply and ground.
- Add DC load to the bus.
- May be unnecessary with receivers that include internal failsafe behavior.
Topology Guidance
A linear or daisy-chain bus with terminators at the two physical ends is the usual first-pass RS-485 topology. Star wiring, long stubs, multiple branches, intermediate-node terminators, and connector discontinuities can create reflections or excessive loading. Some special topologies can be engineered, but they require datasheet review, signal-integrity analysis, and measurement rather than simple calculator approval.
Support reference
FAQ
What termination resistor should I use for RS-485?
Use a termination resistance close to the differential characteristic impedance of the cable. 120 ohms is common for RS-485 twisted pair, but the cable datasheet is the correct source.
Why is 120 ohms commonly used for RS-485?
Many RS-485 twisted-pair cables have differential characteristic impedance near 120 ohms, so a 120 ohm termination is a common first-pass value.
Where should RS-485 termination resistors be placed?
A linear RS-485 bus usually places one termination resistor across A and B at each physical end of the main trunk.
Does RS-485 need termination at both ends?
Many linear buses use two end terminators, but short, low-speed, or point-to-point links should still be checked against edge rate, cable length, topology, and transceiver guidance.
Why do two 120 ohm terminators create a 60 ohm load?
Two equal 120 ohm terminators are in parallel from the driver's differential perspective, so the effective differential load is 120 / 2 = 60 ohms.
Should every RS-485 node have a termination resistor?
No. Installing full-value terminators at every node can overload the driver and usually indicates incorrect topology. Termination belongs at the physical bus ends.
What is split termination?
Split termination replaces one full-value resistor with two half-value resistors in series across the differential pair, with the midpoint optionally AC-coupled to a reference for common-mode noise control.
What is the difference between termination and failsafe bias?
Termination matches the cable impedance to reduce reflections. Failsafe bias establishes a known idle bus state when all drivers are high impedance. They use different networks and formulas.
Can RS-485 work without termination?
Some short or slow networks can work without termination, but this depends on cable length, edge rate, receiver margin, topology, and measurement results.
Does baud rate determine whether termination is needed?
Baud rate matters, but reflections are strongly related to electrical edge speed, cable length, topology, and discontinuities. Baud rate alone is not enough.
How do I calculate RS-485 termination resistor power?
For each terminator, use P = Vdiff squared divided by RT. Total termination power is that value multiplied by the number of active terminators.
Can I use a 100 ohm resistor on a 120 ohm RS-485 cable?
It may work in some systems, but it creates a nonzero mismatch. This calculator estimates reflection coefficient, return loss, and VSWR so you can compare the effect.
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Future Engineering Guide Topics
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In_DevelopmentRS-485 Biasing and Failsafe Design
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In_DevelopmentEngineering Disclaimer
This calculator provides first-pass RS-485 termination, loading, reflection, and power estimates. It does not guarantee signal integrity, EMC compliance, surge immunity, isolation safety, cable length, failsafe bias behavior, common-mode performance, or topology suitability. Validate the final network with the selected cable, transceivers, layout, grounding, operating voltage, and measurements.