RS485 Cable Length and Baud Rate Guide
Check whether an RS485 network will work: maximum cable length for your data rate, number of devices on the segment and the termination and biasing it needs.
Typical length limits (twisted pair, 24 AWG class)
| Data rate | Approx. maximum length |
|---|---|
| 9.6 to 93.75 kbit/s | 1200 m |
| 187.5 kbit/s | 1000 m |
| 500 kbit/s | 400 m |
| 1.5 Mbit/s | 200 m |
| 12 Mbit/s | 100 m |
RS485 Design Rules
Up to 32 unit loads per segment without repeaters
The table shows the Profibus DP segment lengths, a widely used reference for RS485 networks. Use shielded twisted pair with 120 Ω characteristic impedance.
Worked example
At 19200 bit/s the rule allows the full 1200 m. At 500 kbit/s it drops to about 200 m by the rule (Profibus allows 400 m with type A cable).
Frequently Asked Questions
What is the maximum RS485 cable length?
About 1200 m at low data rates; it decreases as speed increases.
How many devices can be on one RS485 segment?
32 unit loads; with 1/8 UL transceivers that is up to 256 devices.
Where do I put the termination resistors?
One 120 Ω resistor at each end of the trunk, never at stubs or in the middle.
Learn more on the blog
RS232 vs RS485 in industrial automation
A practical, field focused guide on instrumentationblog.in that explains the theory behind this tool and how engineers apply it on real plants.
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