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CRC / Checksum Generator

Calculate CRC-16 or XOR checksums from hex payloads.

Data Input

Checksum Result

Calculated CRC (Hex) 0x0000
Decimal 0
Binary 0000000000000000
info
Disclaimer of Liability: This calculator is provided for educational and estimation purposes only. By using this tool, you agree to discharge AutomationView of any liability for direct, indirect, or consequential damages resulting from its use. It is your strict responsibility to independently verify all calculations, validate the results against official manufacturer documentation, and ensure compliance with all applicable safety and engineering standards before implementing any parameters in a production environment.

What is a Modbus RTU CRC Checksum?

In industrial serial communications, particularly Modbus RTU (RS-485/RS-232), ensuring data integrity is paramount. A Cyclic Redundancy Check (CRC) is an error-detecting code appended to the end of a message frame. By recalculating the CRC on the receiving end, a PLC or Master device can instantly determine if electrical noise or interference corrupted the data payload during transmission.

Modbus CRC-16 Parameters

To successfully generate or verify a Modbus RTU checksum, you must use the exact mathematical parameters dictated by the protocol specification. Variations in the initial value or polynomial will result in a mismatched checksum and a rejected frame.

  • Polynomial: 0x8005 (Normal) or 0xA001 (Reversed for LSB-first processing).
  • Initial Value: 0xFFFF. Note that initializing to zero is a common mistake that will break the calculation.
  • Final XOR Value: 0x0000.
  • Byte Order: The resulting 16-bit CRC must be transmitted in Little-Endian format, meaning the Low Byte is sent first, followed by the High Byte.

Implementing CRC Logic in PLCs

While many modern drivers and gateways handle Modbus automatically, engineers developing custom serial drivers in C/C++ or Structured Text (ST) for embedded controllers must implement the CRC algorithm manually. The most efficient approach is the “Bitwise Method.” This involves iterating through each byte of the payload, applying an XOR operation, and shifting the bits to the right while applying the reversed polynomial (0xA001) whenever the least significant bit is set.

Troubleshooting Serial Communications

If your device is constantly timing out or returning “CRC Error” faults, verify two critical elements. First, ensure you are calculating the CRC over the entire message frame, including the Slave ID and Function Code, but excluding the CRC bytes themselves. Second, confirm the byte transmission order; swapping the High and Low CRC bytes is the single most common reason for failed Modbus RTU polling.