Comms cable selector
Pick a comms cable and check the run length.
Reads AS/NZS 3000:2018 and Industrial communications sources 2019-2024. Tables checked 26 Sept 2026.
Try a job 8
Each job fills in example values. Replace them with your values.
- Add an instrument loopA cable for the signal over a 150 m route.
- Select a comms cableA PROFINET cable for a 60 m run through a drag chain.
- Check an RS-485 runWhether 400 m at 115.2 kbps is in the limit.
- Check a long Modbus trunkWhether 1200 m at 9.6 kbps is in the limit.
- Check an Ethernet run past 90 mWhether a 110 m Ethernet run is in the limit.
- Pick a 24 V field supply cableA cable for a 24 V d.c. supply over 80 m.
- Pick a screened cable beside a driveA screened signal cable for a noisy area.
- Pick an outdoor Ethernet cableAn Ethernet cable for an outdoor run.
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-485
Industrial communications sources 2019-2024
- Run length100m
- Length limit1,000m
Industrial communications sources 2019-2024
- Length checkPass
- Construction1 pair or more, 2 cores, larger than 0.34 mm2, required screen
Industrial communications sources 2019-2024
- Screen requiredrequired
Industrial communications sources 2019-2024
- Screen checkPass
- EnvironmentFixed installation
Industrial communications sources 2019-2024
- Environment checkPass
- Category checkNot applicable
- Transmission speed187.5 kbps
- Printed speed row187.5 kbps
Industrial communications sources 2019-2024
- Bus cable familystandard bus cable
Industrial communications sources 2019-2024
- Termination at each end120Ω
Industrial communications sources 2019-2024
- Unit loads per line32
Industrial communications sources 2019-2024
- Bus attachments per segment32
Industrial communications sources 2019-2024
About this calculator
How this is calculated
The tool picks the cable construction for a control or data link. It checks your run length against the limit for the interface. No AmpSize figure here comes from a printed Standard. The figures come from maker data and course notes, which the tool labels as secondary sources.
- Interface. You pick 24 V d.c., 4-20 mA, RS-485, Ethernet or PROFINET. For RS-485 you also pick the data rate and the bus cable family.
- Length limit. The tool reads the longest run for that interface. For RS-485 the limit falls as the data rate rises.
- Length check. It compares your run with the limit. It reports the run as inside or over the limit.
- Construction. It gives the pairs, the cores and the smallest core size. It says whether a screen is required, and checks the screen you enter.
- Environment. It checks the sheath and the bend radius for a fixed or a flexing installation. For Ethernet it checks the category.
- Termination. For RS-485 it gives the termination at each end and the unit loads per line.
| Step | Source | What it decides |
|---|---|---|
| Length limit | Industrial communications sources 2019 to 2024 | The longest run for the interface |
| Construction and screen | Industrial communications sources 2019 to 2024 | The cable build |
| Termination | Industrial communications sources 2019 to 2024 | The resistor at each end |
Worked example
- RS-485 at 187.5 kbps over 1200 m. The limit is 1,000 m, so the run is over the limit. The screen and environment checks pass. Add a repeater or shorten the run.
Limits
- The figures are secondary source and maker data. They are not a Standard. Confirm each one with the device maker.
- The tool does not size power conductors. Use the DC cable size calculator for a 24 V supply.
- The tool does not check segregation from power cables. Use the segregation checker.
- The termination value suits typical cable. Match it to the cable you use.
- The 32 unit loads on an RS-485 line are not a device count.
See also: Control panels and PLC wiring, 24 V DC instrument loop voltage drop and Loop power.
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.
Clauses and tables this tool reads
- AS/NZS 3000:2018 Table 3.3Nominal minimum cross-sectional area of conductors
- Industrial communications sources, interface limitsLength, screen and construction limits for five industrial interfaces
Every clause and table, with how each was verified
Tables last checked on 26 Sept 2026.
Related tools
- Aerial clearance lookupLook up aerial conductor clearances and spacing.AS/NZS 3000
- Site cord length calculatorThe longest a cord or cable may run on a construction site.AS/NZS 3012
- Loop power budget for 4-20 mA loopsCheck the voltage a 4-20 mA loop leaves the transmitter.AS/NZS 3008.1.2AS/NZS 3008.1.1
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