Formula and Inputs
The practical bus limit is evaluated from baud rate, cable length, node count, and maximum stub length against signal-integrity margins.
VoltCheck24 Engineering Calculator
RS-485 Termination and Cable Distance Calculator
Estimate RS-485 and Modbus cable distance, baud-rate limits, node loading, stub length, and 120 ohm termination requirements.
How to use this calculator
- Collect the values below from the equipment nameplate, data sheet, or field measurement.
- Enter values using one consistent unit system and use worst-case operating conditions.
- Review the result, margin, and engineering note before selecting hardware.
- Cross-check the result with the related calculator and save the review for your project record.
Input checklist
- Baud rate and cable length
- Node count and cable type
- Maximum stub length
- Trunk-end termination plan
Industrial Use Cases
Use this tool when designing Modbus RTU, drive networks, remote I/O, meters, and multi-drop industrial serial buses.
Engineering Note
Install termination only at the two physical ends of the trunk, keep stubs short, and follow the cable manufacturer’s impedance and shielding recommendations.
FAQ
Where should 120 ohm termination resistors go?
Place them at the two ends of the main bus trunk, not at every device or branch.
What is the most common RS-485 wiring mistake?
Long T-shaped stubs, incorrect reference wiring, and termination placed in the middle of the bus are frequent causes of intermittent faults.
Preliminary engineering reference only. Confirm final selections against current standards, project specifications, and manufacturer data.