Comms cable selector
Pick a comms cable and check the run length.
Inputs
Worked example with the default inputs. The live calculator replaces it when the page loads.
- Interface
- RS-485
- Environment
- Fixed installation
- Run length
- 100 m
- Sheath material
- PVC
- The cable is screened
- Yes
- Transmission speed
- 187.5 kbps
- Bus cable family
- Standard, robust, FRNC, food, underground or marine
Run length
Inside the limit
- Length limit
- 1,000m
- Cable construction
- 1 pair or more, 2 cores, larger than 0.34 mm2, required screen
- Screen
- Required
- Screen check
- Pass
- Termination at each end
- 120Ω
- Unit loads per line
- 32
- Environment check
- Pass
- Category check
- Not applicable
- Note
- A PROFIBUS DP segment at 187.5 kbps on standard bus cable may run 1000 m, from a cable maker network manual. The held table falls from 1000 m at 9.6 kbps to 100 m from 3 Mbps up. TIA-485 itself sets no maximum line length, and a separate report gives a nominal 1.2 km on 24 AWG cable.
This result is a design aid. Verify it against the current standard.
Calculation steps
- InterfaceRS-485Industrial communications sources 2019-2024
- Run length100m
- Length limit1,000mIndustrial communications sources 2019-2024
- Length checkPass
- Construction1 pair or more, 2 cores, larger than 0.34 mm2, required screenIndustrial communications sources 2019-2024
- Screen requiredrequiredIndustrial communications sources 2019-2024
- Screen checkPass
- EnvironmentFixed installationIndustrial communications sources 2019-2024
- Environment checkPass
- Category checkNot applicable
- Transmission speed187.5 kbps
- Printed speed row187.5 kbpsIndustrial communications sources 2019-2024
- Bus cable familystandard bus cableIndustrial communications sources 2019-2024
- Termination at each end120ΩIndustrial communications sources 2019-2024
- Unit loads per line32Industrial communications sources 2019-2024
- Bus attachments per segment32Industrial communications sources 2019-2024
Questions
Each answer describes what this tool calculates. The result is a design aid. Verify against the current standard.
What does the comms cable selector do?
It names the cable construction for a 24 V DC, 4-20 mA, RS-485, Ethernet or PROFINET link, and says whether the run length is inside the limit for that interface. It also checks the sheath, the screen and the bending radius against the environment.
How long can an RS-485 segment be?
The tool reports PROFIBUS DP segment lengths from a cable maker manual: 1000 m up to 187.5 kbps on standard bus cable, 400 m at 500 kbps, 200 m at 1.5 Mbps and 100 m from 3 Mbps up. Flexible and torsion cable runs shorter.
What does the RS-485 limit of 32 count?
Unit loads, not devices. One unit load draws 1 mA under a common-mode stress of 12 V. A receiver above 12 kilohm input resistance presents only part of one, so a line can carry more than 32 receivers.
What termination does RS-485 need?
The termination resistance equals the characteristic impedance of the line. The held source works to 120 ohm at each end, so match it to the cable you use. A line short enough to treat as lumped-parameter needs no termination.
What length limit applies to 24 V DC and 4-20 mA?
Neither carries a published length. A 24 V DC run is limited by volt drop, so use the AmpSize voltage drop tool at 24 V. A 4-20 mA loop is limited by the loop burden and the transmitter supply voltage.