Documentation sections

USB Termination & Signal Integrity Calculator

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

Slug usb-termination. Calculate USB bus termination resistor values, differential impedance, cable propagation delay and the USB 2.0 minimum rise time for USB 2.0 and USB 3.0

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 usb-termination
Name	Unit	Range	Default	Description
usbVersionnumber	none	0 … 1	0	USB VersionOne of: 0 USB 2.0 (High Speed, 480 Mbps) · 1 USB 3.0 (SuperSpeed, 5 Gbps)
lineImpedancenumber	Ω	50 … 150	90	Differential impedance of the data pair, trace or cable: USB 2.0 and USB 3.0 both specify 90 Ω (45 Ω odd-mode per line)
cableLengthnumber	m	at least 0.01	2	USB cable length in meters

Outputs

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

Outputs of usb-termination
Field	Unit	Description
terminationResistorheadline	Ω	Termination Resistor
differentialImpedance	Ω	Differential Impedance
propagationDelay	ns	Cable Propagation Delay
signalRiseTime	ps	USB 2.0 Table 7-8: the high-speed driver rise time is at least 500 ps (T_HSR), a minimum, not a typical value. USB 3.0 specifies no transmitter rise time, so none is reported for it.
impedanceError	Ω	How far the line impedance is from the 90 Ω nominal: \|Z₀ − 90 Ω\|
impedanceTolerance	Ω	The deviation from 90 Ω the chosen USB version allows: ±13.5 Ω (USB 2.0, ±15 %) or ±7 Ω (USB 3.0)
impedancePercent	%	The deviation as a percentage of the tolerance; above 100 % the line is outside the window
compliantImpedance	Ω	The nearest edge of the tolerance window: 90 Ω plus the tolerance at or above 90 Ω, minus it below

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": "usb-termination",
    "inputs": {
      "usbVersion": 0,
      "lineImpedance": 90,
      "cableLength": 2
    }
  }'

import rftools

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

result = client.calculate("usb-termination", {"usbVersion": 0, "lineImpedance": 90, "cableLength": 2})
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": "usb-termination",
    "inputs": {
      "usbVersion": 0,
      "lineImpedance": 90,
      "cableLength": 2
    }
  }),
});
console.log(response.status, await response.json());

package main

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

func main() {
	body := []byte(`{
	  "slug": "usb-termination",
	  "inputs": {
	    "usbVersion": 0,
	    "lineImpedance": 90,
	    "cableLength": 2
	  }
	}`)
	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": "usb-termination",
  "values": {
    "terminationResistor": 90,
    "differentialImpedance": 90,
    "propagationDelay": 10,
    "signalRiseTime": 500,
    "impedanceError": 0,
    "impedanceTolerance": 13.5,
    "impedancePercent": 0,
    "compliantImpedance": 103.5
  },
  "warnings": [],
  "errors": [],
  "provenance": {
    "method": "calculator:usb-termination",
    "version": "api@3f2c1a9b04de",
    "formulaRef": null,
    "assumptions": [],
    "validRange": {
      "status": "inside",
      "bounds": {
        "usbVersion": {
          "min": 0,
          "max": 1,
          "unit": ""
        },
        "lineImpedance": {
          "min": 50,
          "max": 150,
          "unit": "Ω"
        },
        "cableLength": {
          "min": 0.01,
          "max": null,
          "unit": "m"
        }
      },
      "outside": [],
      "model": null
    },
    "computedAt": "2026-09-24T14:05:11.003Z",
    "inputs": {
      "usbVersion": 0,
      "lineImpedance": 90,
      "cableLength": 2
    },
    "seed": null,
    "elapsedSeconds": 0.003
  }
}

{
  "detail": [
    {
      "type": "float_parsing",
      "loc": [
        "body",
        "inputs",
        "usbVersion"
      ],
      "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 pageUSB Termination & Signal Integrity Calculator, at these values