Modbus RTU Online Tool

Modbus RTU message generation, parsing, and CRC checksum tool, supporting multiple data types and byte order configurations.

Bug report

Browser execution mode: Your data is processed in your browser and is not uploaded to the server.

Speed and Stability: Processing speed depends on your device and browser. For large batch work, the desktop version may be more stable.

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If the online tool fails to load or run, try the desktop tool.https://tools.yikeaigc.com/

Tool Usage

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Request Command Generator

Configure slave address, function code, register address, and write data to generate an RTU request frame with CRC.

Slave Address 0 is broadcast; most devices use 1-247 Function Code Start Address Enter the message address, usually starting from 0 Read Quantity Register read range: 1-125

Data Type Byte order Register Value Supports -32768 to 65535

Coil Status

Coil Value Separate with commas, spaces, or new lines; pack LSB first Write Value UINT32, INT32, and FLOAT32 use 2 registers per value

Results

Set parameters first, then click Generate Request Command.

Batch Request Parameters One per line. Read/single write: slave,function code,address,quantity or value; 15: slave,15,address,coil values...; 16: slave,16,address,type,byte order,values...

Bulk ResultsWaiting

After completing the request frame, you can continue to simulate the device response.

Response Message Generator

Generate read responses, write confirmations, or exception responses by function code. Ideal for building return frames before integration testing.

Slave Address Function Code Response Type Exception Code

Data Type Byte order

Coil/Discrete Return Value Pack into response bytes, LSB first Register Return Value

Start Address Quantity/Value Coil Status

Results

Set parameters first, then click Generate Response Frame.

Batch Response Parameters Read registers: slave,function code,type,byte order,value...; Write confirm: slave,function code,address,quantity or value; Exception: slave,function code,EX,exception code

Bulk ResultsWaiting

Message Parser

Enter a complete RTU frame to auto-detect fields, verify CRC, and parse register values by data type.

Complete RTU Frame Supports spaces, hyphens, colons, and commas; must include CRC field

Default Data Type Default Byte Order Data Start Byte Byte position in full frame, 0-based

Custom Parse Rules

NameStartTypeByte orderquantity

Parse ResultCRC is verified automatically

Enter a frame and click Parse.

CRC Check Calculation

Calculate the checksum using Modbus RTU CRC-16 rules and output the full frame with the low byte first.

Data frame without CRC

Full frame validation

Batch CRC calculation One data frame without CRC per line

Calculation resultsCRC low byte first

Enter data, then click Calculate CRC.

Bulk ResultsWaiting

Instructions

Software Usage Instructions

Request Command Generation: First set the slave address, function code, start address, and read quantity or write value, then click “Generate Request Command”. Function codes 01, 02, 03, 04, 05, 06, 15, and 16 are supported.
Write Data Configuration: For writing a single coil, you can select ON/OFF; for writing a single register, you can enter the register value; for writing multiple coils, you can enter a sequence of 0s and 1s; for writing multiple registers, you can select UINT16, INT16, UINT32, INT32, FLOAT32, and the byte order.
Response Message Generation: Select the response function code and response type to generate a normal response, write confirmation, or exception response. Read register responses support multiple data types and ABCD, DCBA, BADC, and CDAB byte orders.
Message Parsing: Enter a complete RTU hexadecimal message, and the tool will identify the slave address, function code, data area, and CRC check result. You can add custom rules to parse register values.
CRC Check: Enter a data frame without CRC to calculate CRC-16 and generate a complete message; enter a complete message with CRC to verify whether the checksum matches.
Batch Processing: Request generation, response generation, and CRC calculation all support line-by-line batch processing. After processing, you can copy the results or download a TXT file.
Save Settings: Commonly used slave addresses, data types, byte orders, and parsing start positions can be retained by clicking “Save Settings”, making it easier to continue using them later.

FAQ

A: The CRC-16 result of Modbus RTU is placed in the message with the low byte first and the high byte second. For example, if the calculated value is 0xCDC5, it is written as C5 CD at the end of the message.

A: 03 is used to read holding registers, commonly used for parameters, set values, or read/write data; 04 is used to read input registers, commonly used for read-only data such as sensor measurements.

A: These options are used for the byte arrangement of 32-bit data. The device manual usually specifies the byte order for floating-point numbers or 32-bit integers; if the documentation does not specify it, you can switch between different byte orders in the parser and check whether the values match the on-site measurement range.

A: A Modbus register is 16 bits wide, while FLOAT32 is a 32-bit floating-point number, so it requires two consecutive registers, which is 4 data bytes.

A: Common causes include message entry errors, missing or extra bytes, reversed CRC byte order, communication interference, or the device using a non-standard checksum method. You can first use the CRC function to recalculate and compare the last two bytes.

A: The format for read and single write is “slave, function code, address, quantity or value”, for example 1,03,0,10. The format for writing multiple registers is “slave,16,address,type,byte order,value...”, for example 1,16,0,FLOAT32,ABCD,3.14.