Modbus RTU Frame Decoder
Paste a Modbus RTU frame captured with a serial sniffer or shown in a gateway log, and read it field by field, including exception responses and the CRC check.
Decoded fields
| Field | Value |
|---|---|
| Slave address | 1 |
| Function code | 3 = Read Holding Registers |
| Byte count | 4 |
| Register 1 | 100 (0x0064) |
| Register 2 | 200 (0x00C8) |
| CRC | BA 7A (valid) |
RTU Frame Layout
Exception: function code + 0x80, then 1 byte exception code
Worked example
01 03 04 00 64 00 C8 BA 7A is a response from slave 1 to FC 03 with 4 data bytes: register 1 = 100, register 2 = 200, CRC valid.
Frequently Asked Questions
How do I know if a frame is a request or a response?
For read functions a request is always 8 bytes (address, FC, start, quantity, CRC); a response contains a byte count after the function code.
What does function code 0x83 mean?
It is an exception response to function code 03; the next byte is the exception code.
What is exception code 02?
Illegal data address: the register requested does not exist in the device.
Learn more on the blog
Modbus RTU troubleshooting
A practical, field focused guide on instrumentationblog.in that explains the theory behind this tool and how engineers apply it on real plants.
Read Full Article