Documentation sections

Modbus RTU Frame Timing Calculator

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

Slug modbus-frame-timing. Calculate Modbus RTU character time, 3.5-character inter-frame gap, total frame duration, and maximum frame rate.

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 modbus-frame-timing
Name	Unit	Range	Default	Description
baudRatenumber	bps	300 … 115200	9600	Modbus RTU baud rate
dataBytesnumber	bytes	1 … 252	8	Number of data bytes between the function code and the CRC
parityBitsnumber	bits	0 … 1	1	Parity BitsOne of: 0 None · 1 Even or odd (1 bit)
stopBitsnumber	bits	1 … 2	1	Stop bits (1 or 2 — use 2 stop bits when parity=none)
charGapnumber	ms	0 … 100	0	The longest pause the sender leaves between two characters of a frame; a USB adapter or an RTOS driver can leave one

Outputs

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

Outputs of modbus-frame-timing
Field	Unit	Description
charTime	ms	Time to transmit one character: (1+8+parity+stop) / baud_rate
t35headline	ms	Modbus RTU inter-frame silence: 3.5 × character time, a fixed 1.75 ms above 19 200 Bd
frameTime	ms	Total frame transmission time (address + function + data + CRC), with the pauses between characters
maxFrameRate	frames/s	Maximum frames per second including inter-frame gap
t15	ms	A pause longer than this inside a frame makes the receiver discard it: 1.5 character times, a fixed 750 µs above 19 200 Bd
charGapChars	char	The sender's pause between characters, in character times
t15Chars	char	The 1.5T limit in character times: 1.5 up to 19 200 Bd, more above it, where the limit is a fixed 750 µs

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": "modbus-frame-timing",
    "inputs": {
      "baudRate": 9600,
      "dataBytes": 8,
      "parityBits": 1,
      "stopBits": 1,
      "charGap": 0
    }
  }'

import rftools

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

result = client.calculate("modbus-frame-timing", {
    "baudRate": 9600,
    "dataBytes": 8,
    "parityBits": 1,
    "stopBits": 1,
    "charGap": 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": "modbus-frame-timing",
    "inputs": {
      "baudRate": 9600,
      "dataBytes": 8,
      "parityBits": 1,
      "stopBits": 1,
      "charGap": 0
    }
  }),
});
console.log(response.status, await response.json());

package main

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

func main() {
	body := []byte(`{
	  "slug": "modbus-frame-timing",
	  "inputs": {
	    "baudRate": 9600,
	    "dataBytes": 8,
	    "parityBits": 1,
	    "stopBits": 1,
	    "charGap": 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": "modbus-frame-timing",
  "values": {
    "charTime": 1.1458333333333333,
    "t35": 4.010416666666666,
    "frameTime": 13.75,
    "maxFrameRate": 56.30498533724341,
    "t15": 1.71875,
    "charGapChars": 0,
    "t15Chars": 1.5
  },
  "warnings": [],
  "errors": [],
  "provenance": {
    "method": "calculator:modbus-frame-timing",
    "version": "api@3f2c1a9b04de",
    "formulaRef": "Modbus Application Protocol V1.1b3, Modbus.org",
    "assumptions": [],
    "validRange": {
      "status": "inside",
      "bounds": {
        "baudRate": {
          "min": 300,
          "max": 115200,
          "unit": "bps"
        },
        "dataBytes": {
          "min": 1,
          "max": 252,
          "unit": "bytes"
        },
        "parityBits": {
          "min": 0,
          "max": 1,
          "unit": "bits"
        },
        "stopBits": {
          "min": 1,
          "max": 2,
          "unit": "bits"
        },
        "charGap": {
          "min": 0,
          "max": 100,
          "unit": "ms"
        }
      },
      "outside": [],
      "model": null
    },
    "computedAt": "2026-09-24T14:05:11.003Z",
    "inputs": {
      "baudRate": 9600,
      "dataBytes": 8,
      "parityBits": 1,
      "stopBits": 1,
      "charGap": 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 pageModbus RTU Frame Timing Calculator, at these values