Documentation sections

RS-485 Termination & Bias Resistor Calculator

POSThttps://rftools.io/api/py/v1/calculate

Slug rs485-termination. Calculate RS-485 bus termination resistors, bias resistors, maximum baud rate for cable length, propagation delay, and bias current consumption

Inputs

Send every one of these in inputs, in the units shown. Most are required: a call without one is refused (400) and costs nothing; one the calculator can do without takes the default shown. An input outside its range is still computed and flagged in the provenance.

Inputs of rs485-termination
Name	Unit	Range	Default	Description
baudRatenumber	kbps	at least 0.1	115.2	RS-485 communication baud rate in kbps
cableLengthnumber	m	at least 0.1	100	Total bus cable length in meters
supplyVoltagenumber	V	3 … 5.5	5	Logic supply voltage for the RS-485 transceivers
eSeriesnumber	none	0 … 96	0	Rounds every part to the nearest value in the series; the response and the export are the rounded design's.One of: 0 Ideal values · 12 E12 (10%) · 24 E24 (5%) · 48 E48 (2%) · 96 E96 (1%)

Retired, still accepted and ignored with a warning: numNodes (Number of Nodes, 2026-10-12).

Outputs

Returned in values, by key. A value that is unbounded or undefined is null.

Outputs of rs485-termination
Field	Unit	Description
terminationResistorheadline	Ω	Match to cable characteristic impedance (~120Ω for twisted pair); 121 Ω in E48 and E96
biasResistor	Ω	Failsafe bias resistors to maintain idle state differential voltage > 200 mV; rounded to the part series when one is chosen
maxBaudRate	kbps	Max Baud Rate for Cable Length
propagationDelay	ns	Cable Propagation Delay
loadCurrent	mA	Bias Load Current
baudRateUtilisation	%	The entered baud rate as a share of the maximum for this cable length (rate × length ≤ 10⁸ bit·m); above 100 % the cable is too long for the rate
failsafeVoltage	mV	The A–B voltage the bias resistors hold on the idle bus across the two terminators in parallel, Vcc·(R_term/2)/(2·R_bias + R_term/2); TIA-485-A receivers need at least 200 mV to read it as a defined state (TI SLLA272C)

Calling it

Calculate: one call, the request and response in full, and its errors.
Batch: this calculator with others, or a sweep, in one request.
Solve for a target: the value of one input that brings an output to a target.

curl -X POST https://rftools.io/api/py/v1/calculate \
  -H "X-API-Key: $RFTOOLS_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "slug": "rs485-termination",
    "inputs": {
      "baudRate": 115.2,
      "cableLength": 100,
      "supplyVoltage": 5,
      "eSeries": 0
    }
  }'

import rftools

client = rftools.Client()  # reads RFTOOLS_API_KEY, or the key `rftools login` saved

result = client.calculate("rs485-termination", {
    "baudRate": 115.2,
    "cableLength": 100,
    "supplyVoltage": 5,
    "eSeries": 0,
})
print(result.values)
print(result.warnings, result.provenance.formula_ref)

const response = await fetch("https://rftools.io/api/py/v1/calculate", {
  method: "POST",
  headers: {
    "X-API-Key": process.env.RFTOOLS_API_KEY,
    "Content-Type": "application/json",
  },
  body: JSON.stringify({
    "slug": "rs485-termination",
    "inputs": {
      "baudRate": 115.2,
      "cableLength": 100,
      "supplyVoltage": 5,
      "eSeries": 0
    }
  }),
});
console.log(response.status, await response.json());

package main

import (
	"bytes"
	"fmt"
	"io"
	"net/http"
	"os"
)

func main() {
	body := []byte(`{
	  "slug": "rs485-termination",
	  "inputs": {
	    "baudRate": 115.2,
	    "cableLength": 100,
	    "supplyVoltage": 5,
	    "eSeries": 0
	  }
	}`)
	req, err := http.NewRequest("POST", "https://rftools.io/api/py/v1/calculate", bytes.NewReader(body))
	if err != nil {
		panic(err)
	}
	req.Header.Set("X-API-Key", os.Getenv("RFTOOLS_API_KEY"))
	req.Header.Set("Content-Type", "application/json")
	res, err := http.DefaultClient.Do(req)
	if err != nil {
		panic(err)
	}
	defer res.Body.Close()
	out, _ := io.ReadAll(res.Body)
	fmt.Println(res.Status, string(out))
}

application/json

{
  "slug": "rs485-termination",
  "values": {
    "terminationResistor": 120,
    "biasResistor": 540,
    "maxBaudRate": 1000,
    "propagationDelay": 500,
    "loadCurrent": 4.385964912280701,
    "baudRateUtilisation": 11.52,
    "failsafeVoltage": 263.1578947368421
  },
  "warnings": [],
  "errors": [],
  "provenance": {
    "method": "calculator:rs485-termination",
    "version": "api@3f2c1a9b04de",
    "formulaRef": null,
    "assumptions": [],
    "validRange": {
      "status": "inside",
      "bounds": {
        "baudRate": {
          "min": 0.1,
          "max": null,
          "unit": "kbps"
        },
        "cableLength": {
          "min": 0.1,
          "max": null,
          "unit": "m"
        },
        "supplyVoltage": {
          "min": 3,
          "max": 5.5,
          "unit": "V"
        },
        "eSeries": {
          "min": 0,
          "max": 96,
          "unit": ""
        }
      },
      "outside": [],
      "model": null
    },
    "computedAt": "2026-09-24T14:05:11.003Z",
    "inputs": {
      "baudRate": 115.2,
      "cableLength": 100,
      "supplyVoltage": 5,
      "eSeries": 0
    },
    "seed": null,
    "elapsedSeconds": 0.003
  }
}

{
  "detail": [
    {
      "type": "float_parsing",
      "loc": [
        "body",
        "inputs",
        "baudRate"
      ],
      "msg": "Input should be a valid number, unable to parse string as a number",
      "input": "abc"
    }
  ]
}

An input sent as text. Nothing was counted.

{
  "detail": "More than 120 requests in 60 seconds from this address.",
  "errorKind": "rate_limited",
  "limit": 120,
  "windowSeconds": 60,
  "upgradeUrl": "https://rftools.io/pricing/"
}

Too many requests in a minute, with Retry-After.

Same inputs on the calculator pageRS-485 Termination & Bias Resistor Calculator, at these values