RS-485 termination and bias calculator
An RS-485 Modbus line works on the bench and drops frames on site because of missing bias, long stubs or too many nodes. Fail-safe bias resistors, cable length limit, stub length limit and unit-load budget for RS-485 networks.
Open the calculator in EM·LINKBelow is a worked example. The working calculator runs free in EM·LINK.
Worked example
Example inputs
Example result
Above the 200 mV fail-safe level, with 120 Ω termination at both ends.
| Check | Value | Limit |
|---|---|---|
| Idle bias | 228 mV | 200 mV min |
| Unit loads | 24 | 32 max |
| Length × rate | 1.54 × 10⁷ | 10⁸ |
| Length by rule | 5208 m | at 19200 bps |
| Max stub | 5.0 m | at 250 ns rise |
Bias draws 4.3 mA; 643 Ω is the largest bias that still gives 200 mV. At the 800 ns typical rise time the stub limit is 16 m, but size stubs for the fastest edge.
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The working calculator is RS-485 Termination & Bias in EM·LINK, our free browser tool. The button opens it directly.
How to read the result
Bias holds at least 200 mV differential on an idle bus, so receivers do not read noise as data.
Reduced-load transceivers at 1/2, 1/4 or 1/8 UL let one segment carry 64, 128 or 256 nodes.
How this is calculated
On an idle bus the bias resistors and the load between A and B form a divider. The load is the two 120 Ω terminators in parallel with every node input, 12 kΩ per unit load. The stub limit is a tenth of the distance an edge travels during the driver rise time. The example uses 250 ns, the minimum rise time in the MAX487 datasheet, a typical slew-limited transceiver for this class of bus.
The full derivation, step by step, is in the EM·LINK documentation.
Calculated per TIA/EIA-485. These tools are an engineering aid, not a certification.
Next step
Polling more than one device? Embedos Edge reads Modbus and pushes it to the cloud. Book a 20 min commissioning call.
Common questions
How long can an RS-485 cable be?
A practical rule is length times bit rate under 10^8. At 9600 baud that is well over 1 km; at 1 Mbps about 100 m.
Do I need bias resistors?
Yes, unless your transceivers have built-in fail-safe. Without bias an idle bus can float.
How many devices on one RS-485 segment?
32 standard unit loads, more with reduced-load transceivers.
Related tools
- Modbus register interpreter, once frames arrive cleanly and you need the values.
- CRC and checksum, to confirm whether a dropped frame was corrupted on the line.
- UART baud rate error, to rule out clock error at either end of the link.