online parser: crc presentation

[a.pythe]

When playing the online Rapid SCADA Modbus Parser, I’ve noticed queer CRC presentation at the bottom of the player.

As of Modbus Serial Line Protocol and Implementation Guide V1.02:

“2.5.1.2 CRC Checking …
When the CRC is appended to the message, the low-order byte is appended first, followed by the high-order byte. A detailed example of CRC generation is contained in Appendix B.”

In Appendix B there is an example of proper relations between the byte sequence and integer presentation of a CRC tail of a Modbus message. Unlike register address, which is written to the message as a big-endian integer, CRC is written as a little-endian integer, so byte sequence must appear there “in reverse”.

Instead, the online Rapid SCADA Modbus Parser treats CRC bytes as big-endians which seam inexact.

[manjey73]

In Modbus, the lowest CRC byte always goes first, then the highest.
A feature of the CRC calculation in the Modbus protocol is that if we send a packet along with the CRC to the calculation, we should get 0

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