Industrial protocols

Modbus RTU CRC-16 calculator

Calculate the Modbus CRC-16 for a frame entered in hexadecimal, and show it low byte first as it is sent.

Calculate

RTU payload before CRC
Example: server 0x11, function 0x03, start offset 0x006B, quantity 0x0003.

Worked example

CRC bytes in wire order

76 87 

16-bit CRC value
0x8776
Payload length before CRC
6 bytes
Complete RTU frame
11 03 00 6B 00 03 76 87

The low-order CRC byte is transmitted first. Validate the function and data fields independently.

Method and assumptions

Formula

CRC starts at 0xFFFF; XOR each byte, shift eight times using polynomial 0xA001; transmit CRC low byte first

Enter the server address through the final data byte, without an existing CRC. Spaces and common byte separators are accepted.

Limits of this result

  • A matching CRC proves only that the entered bytes reproduce the checksum; it does not validate the function, address, quantity or device response.
  • This tool is for Modbus RTU binary frames, not Modbus ASCII LRC or Modbus TCP MBAP framing.

Technical sources

  1. MODBUS over Serial Line Specification and Implementation Guide V1.02Modbus Organization · 2006-12-20
  2. SCADAPack E Modbus frame diagnostics exampleSchneider Electric · modified 2025-08-18

Put the value to work

Read the related engineering guidance, or carry this task into System Builder to identify the signal path and EpiSensor products.

Continue in System Builder Read: What Is SCADA?

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