LGF_CRC16: keeps giving the wrong CRC compared to Sunshine2k calculator

Hi All

I'm trying to get the LGF_CRC16 to work in a project, and it keeps giving the wrong CRC compared to the Sunshine2k calculator. Is there a reason it does not work for Modbus?

Regards

Alan

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Split from CRC 16 in SCL language.

IHCSA

Hi All

I'm trying to get the LGF_CRC16 to work in a project, and it keeps giving the wrong CRC compared to the Sunshine2k calculator. Is there a reason it does not work for Modbus?

 Some steps you can take to narrow the issue:

• Calculate the CRC for a single byte and check if the result matches. If not, check that you use the correct POLY.
• Calculate the CRC for a word and check if the result matches. If not, consider endianness and byte order.
• https://www.lammertbies.nl/comm/info/crc-calculation is another online calculator you can use.
  

I checked the LGF_CRC16 now. There are multiple ways to do the CRC calculation and I have only used one. But the method used in the LGF_CRC16 library seems to be fully opposite to the method I have used. It shifts Left?

I have used the CRC calculation method defined in Modbus documentation. Check attachment or the document linked below.

(Modbus.org > Technical Resources > Modbus specifications > Modbus serial line protocol and implementation guide.)

The Poly defined in that document is 0xA001, and this leads us to the Wikipedia article: https://en.wikipedia.org/wiki/Cyclic_redundancy_check#Polynomial_representations_of_cyclic_redundancy_checks

There it can be seen that each CRC calculation has different Polynomial, but the Poly can be Normal or Reversed.

CRC-16-IBM (used by modbus) Normal poly is 0x8005 but Reversed poly is 0xA001.

The Modbus specification uses the 0xA001 Poly, so the Reversed poly. With that Poly and with shift right (instead of left) the CRC calculation works. I can't help further with the LGF_CRC16 function. I have created my own CRC calculation function using the Modbus CRC calculation diagram. (I even created a separate CRC calculation to safety program using that diagram.)

Hi Elonen

Thank you for all the help.

Is there any chance you could send me the library containing your block, or the code inside your block?

Regards

Alan

Check the modbus CRC calculation specification diagram from the above message. The actual code is very short.

1. Initialize CRC to 16#FFFF
2. For each byte:
   1. Perform CRC XOR byte
   2. For each bit in the byte:
      1. Check the LSB (Least Significant Bit).
         1. If LSB is True, SHR (Shift Right) by one bit and then CRC XOR Poly
         2. If LSB is False, SHR by one bit.

You can use the LGF_CRC16 function as a baseline, it has similar program structure even though the operations are a bit different.