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Modbus ASCII

Build Modbus ASCII request and response frames wrapped in : ... CRLF, and compute the LRC checksum.

ASCII request generator

Slave Function

Start addr Quantity Write value

ASCII response generator

Slave Function

Data type Byte order Value

ASCII parser

Parsed fields appear here...

LRC checksum calculator

LRC result appears here...

How to use the Modbus ASCII tool

Generate an ASCII request

Set the slave address, function code, start address and quantity, then click Generate ASCII request. The tool builds the frame wrapped in a colon : prefix, LRC checksum and CRLF terminator. ASCII mode encodes each byte as two readable hex characters. Example: :010300000001FB: followed by CRLF.

Generate a response

Match the function code, choose a data type and byte order, enter the decimal value, then click Generate ASCII response to simulate slave data with a correct LRC.

Parse a captured frame

Paste an ASCII frame (with or without the leading colon and CRLF) and click Parse frame. The tool extracts the slave address, function code, data and verifies the LRC checksum.

LRC checksum

The LRC is the two's-complement of the sum of all payload bytes, sent as two hex characters. The standalone LRC calculator lets you verify any byte sequence.

Protocol note: Modbus ASCII is more readable than RTU but roughly twice as long on the wire, so it is slower — it is mainly used for manual debugging and systems with text-only links.