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Modbus Packet Builder for RTU and TCP Frames

Build, parse, and CRC16-check Modbus RTU and TCP (MBAP) frames in your browser. Covers common function codes FC01–FC16 for coils and registers used in industrial IoT debugging.

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ModbusIoTProtocol
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Use cases

  • Build Modbus RTU/TCP request frames to debug PLCs and sensors
  • Parse raw hex captured from a serial link field by field
  • Batch-calculate CRC16-Modbus checksums
  • Use frame layouts as a teaching reference

How to use

  1. Build a request: choose RTU or TCP, pick a function code, fill parameters, then generate
  2. Parse a frame: paste hex bytes to inspect fields and verify CRC
  3. CRC check: enter payload bytes and append CRC16

Example input and output

RTU request

01 03 00 00 00 0A C5 CD

Parsed

Address=1, FC03 read holding registers, start=0, quantity=10, CRC OK

FAQ

Which function codes are supported?

FC01–FC04 (reads), FC05/FC06 (single writes), and FC15/FC16 (multiple writes), covering most common Modbus use cases.

What CRC byte order is used?

Modbus RTU uses CRC16 with polynomial 0xA001 and appends the low byte first (little-endian).

Can it parse exception responses?

Yes. It detects exception responses where the high bit of the function code is set and shows the exception code.

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