RS-485 Termination Resistor
R = Z_cable
Quick answer
This calculator solves R_termination = Z_cable for Termination Resistance. With Cable Impedance 1.03 kΩ, the result is 1.03 kΩ. It solves in 2 directions, so you can enter the values you have and get the one you need.
Calculator
Formula
Description
RS-485 bus termination prevents signal reflections that degrade data integrity at high baud rates or long cable runs. A termination resistor equal to the characteristic impedance of the cable is placed at each end of the bus. Standard twisted-pair cables used in RS-485 networks typically have a characteristic impedance of 100-120 ohms. Without proper termination, reflected signals can cause bit errors, especially at baud rates above 100 kbaud or cable lengths above 10 meters.
Variables
- R — Termination resistance in ohms (Ω)
- Z — Characteristic impedance of the cable in ohms (Ω)
Practical Notes
Only the two physical ends of the bus should be terminated. Intermediate nodes (stubs) should not have termination resistors. For short, slow buses, termination may be omitted. Some RS-485 transceivers have built-in fail-safe biasing, but external termination is still recommended for robust communication.
Symbols and units
| Symbol | Meaning | Default unit | Accepted units |
|---|---|---|---|
| R | Termination Resistance | Ω | mΩ, Ω, kΩ, MΩ |
| Z | Cable Impedance | Ω | mΩ, Ω, kΩ, MΩ |
Step by step
- 1. What you know: Cable Impedance 1.03 kΩ
- 2. The relation: R_termination = Z_cable
- 3. Substitute: Z = 1.03 kΩ
- 4. Result: Termination Resistance = 1.03 kΩ
Nearest standard component values
| Series | Nearest value | Error |
|---|---|---|
| E12 | 1 kΩ | 2.9% |
| E24 | 1 kΩ | 2.9% |
Worked examples
Minimum range
Given: Cable Impedance 2.2 Ω
Result: Termination Resistance = 2.2 Ω
Low range
Given: Cable Impedance 22 Ω
Result: Termination Resistance = 22 Ω
Typical
Given: Cable Impedance 1.03 kΩ
Result: Termination Resistance = 1.03 kΩ
High range
Given: Cable Impedance 10.3 kΩ
Result: Termination Resistance = 10.3 kΩ
Maximum range
Given: Cable Impedance 103 kΩ
Result: Termination Resistance = 103 kΩ
Solving for each variable
Solve for Termination Resistance
Cable Impedance 2.2 Ω → 2.2 Ω
Solve for Cable Impedance
Termination Resistance 1 Ω → 1 Ω
How Termination Resistance changes with Cable Impedance
| Cable Impedance (input) | Termination Resistance (result) |
|---|---|
| 0.227 kΩ | 0.227 kΩ |
| 0.5675 kΩ | 0.5675 kΩ |
| 1.135 kΩ | 1.135 kΩ |
| 1.703 kΩ | 1.703 kΩ |
| 2.27 kΩ | 2.27 kΩ |
| 3.405 kΩ | 3.405 kΩ |
| 4.54 kΩ | 4.54 kΩ |
| 9.08 kΩ | 9.08 kΩ |
| 15.89 kΩ | 15.89 kΩ |
| 22.7 kΩ | 22.7 kΩ |
The same result in other units
| Unit | Value |
|---|---|
| mΩ | 1030000 |
| Ω | 1030 |
| kΩ | 1.03 |
| MΩ | 0.00103 |
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<p><a href="https://vbatpower.com/formulas/rs485-termination/">RS-485 Termination Resistor</a> Calculator by vbatpower.com</p>Frequently asked questions
How do you calculate RS-485 Termination Resistor?
Use R_termination = Z_cable. With Cable Impedance 1.03 kΩ, Termination Resistance works out to 1.03 kΩ.
What is the formula for RS-485 Termination Resistor?
The formula is R_termination = Z_cable. RS-485 bus termination prevents signal reflections that degrade data integrity at high baud rates or long cable runs.
How do you find Cable Impedance when you know the other values?
Rearrange for Cable Impedance: with Termination Resistance 1 Ω, Cable Impedance is 1 Ω.
What units does RS-485 Termination Resistor use?
Termination Resistance is shown in mΩ, Ω, kΩ, MΩ. Values are held in SI base units internally, so you can mix prefixes without converting by hand.
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