To address the discrepancy in CRC verification between written and returned data in Modbus RTU communication, a systematic approach is essential. Here’s how I would tackle this issue:
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Understand Modbus RTU Frame Structure: Begin by thoroughly understanding the structure of a Modbus RTU frame, which includes device address, function code, data field, and CRC.
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Verify CRC Calculation Methodology: Ensure both the master and slave devices use the same 16-bit CRC calculation method. Differences in hardware/software implementations or initialization can cause discrepancies.
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Check Data Field Lengths: Confirm that the data fields for read and write operations are of the same length. Any difference here will affect CRC results.
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Review Device Address Handling: Both devices must include the device address in CRC computation. Verify there’s no mismatch or incorrect handling during transmission/reception.
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Inspect Function Codes: Ensure function codes used in read and write operations are correct and consistent, as they influence CRC calculation.
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Analyze Data Transmission Integrity: Look for data corruption due to noise by capturing frames with tools like Wireshark to check for bit errors affecting CRC.
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Examine Implementation Details: Review the firmware of both devices for any bugs or inconsistencies in CRC computation, especially regarding byte order and endianness.
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Test with Sample Data: Use specific test cases to simulate write/read operations, comparing expected vs. actual CRCs to pinpoint issues.
By following these steps, I can systematically identify where the CRC mismatch occurs, ensuring both devices compute CRCs correctly and consistently in accordance with Modbus RTU standards.