How to Choose the Right Cable for RS485 – A Complete Guide for Installers and Designers
The RS485 protocol is one of the most widely used solutions in industrial communication, building automation, and HVAC systems. Its key advantages are reliability, resistance to interference, and the ability to transmit data over long distances. However, for an installation to function correctly, selecting the right RS485 cable is essential.
In this article, we explain what to consider when choosing an RS485 cable, both for indoor and outdoor installations.
Characteristic Impedance – The Foundation of RS485 Transmission
One of the most important technical parameters is the characteristic impedance of the cable.
The RS485 standard requires an impedance of 120 Ω. This ensures stable data transmission and minimizes the risk of signal reflections and interference.
Always choose an RS485 cable with 120 Ω impedance, compliant with EIA-485 requirements.
Cable Length vs. Conductor Cross-Section
The length of the RS485 cable directly affects the required conductor cross-section:
Up to 200 m:
Conductors with a cross-section of about 0.22 mm² are usually sufficient.
Above 200 m:
A larger cross-section (e.g., 0.5 mm² or more) is recommended to reduce signal attenuation.
The RS485 standard allows a maximum bus length of up to 1200 m, provided that proper cable parameters are maintained.
Transmission Speed vs. Maximum Cable Length
The higher the transmission speed, the shorter the allowable cable length. Example values:
| Transmission speed | Maximum cable length |
|---|---|
| 100 kb/s | up to 1200 m |
| 1 Mb/s | up to 400 m |
| 10 Mb/s | approx. 10–15 m |
Twisted Pair – Protection Against Interference
Because RS485 uses differential transmission, twisted pair cables are strongly recommended.
This provides:
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higher resistance to electromagnetic interference (EMI)
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stable signal transmission in demanding environments
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reduced crosstalk between pairs
Use twisted-pair RS485 cables, preferably shielded versions.
Outdoor Installation? Choose Weather-Resistant Cable
If the RS485 installation is planned outdoors, choose cables with:
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UV-resistant outer sheath
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resistance to moisture, frost, oils, and chemicals
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durable materials such as PE or PUR
These types of cables ensure long-term mechanical and environmental durability, even in extreme conditions.
Summary – Which Cable Should You Choose for RS485?
When selecting a cable for an RS485 installation, consider the following factors:
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Characteristic impedance of 120 Ω
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Cable length and appropriate conductor cross-section
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Transmission speed – higher speeds require shorter cable runs
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Interference level – shielding is essential in industrial environments
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Environmental conditions – outdoor installations require UV- and moisture-resistant cables
Loop Resistance (R<sub>loop</sub>)
When selecting a cable, check the DC loop resistance at 20°C in the technical datasheet.
For a 0.22 mm² conductor, it is approximately 180 Ω/km, which at 1200 m results in a significant signal level drop.
For cable runs longer than 500 m, we recommend using 0.5 mm² conductors only (loop resistance approx. 78 Ω/km).
Capacitance per Unit Length
This parameter becomes more important than conductor cross-section at high transmission speeds (Mb/s).
Cables dedicated for RS485 applications (e.g., with PE insulation) typically have low capacitance of around 40–60 pF/m.
Using a standard control cable with PVC insulation and higher capacitance (120–150 pF/m) can significantly reduce the transmission range.
By choosing the right cable, you ensure stable data transmission, compliance with standards, and reliable system operation.
Need help selecting the right cable for your application?
Contact our Cable Design Department at konstrukcja@technokabel.com.pl – we will be happy to assist you.