CRC-16 calculaion with qChecksum

[ChrisW67]

I need to calculate crc-16 for this string:
0A0105000000102347455420444154414F524445520D0A01
Then answer should be: F65B(hex)
I can get this answer using online calculator http://www.zorc.breitbandkatze.de/crc.html
just click button crc-16 and paste line below and click compute
%0A%01%05%00%00%00%10%23%47%45%54%20%44%41%54%41%4 F%52%44%45%52%0D%0A%01
this will give the correct answer F65B, but I want to integrate it in to my qt code.

There are numerous sites with CRC code in C that you can use if the Qt CCITT variant is not to your liking.

Regardless of which of the 16-bit CRC variant you use, it would certainly help if you fed the online CRC calculator and the Qt function the same input. Your code snippet feeds the Qt function a 48 byte string of characters, which is what your first sentence says you want to do. It seems obvious from your percent-encoded input to the online calculator that what you intended to do was send 24 bytes, the result of interpreting the original string as a hexadecimal representation of the bytes. Even using the same function this leads to different results:

char *chars = "0A0105000000102347455420444154414F524445520D0A01";
QByteArray bytes = QByteArray::fromHex(chars);
quint16 crcChars = qChecksum(chars, strlen(chars));
quint16 crcBytes = qChecksum(bytes.data(), bytes.length());
qDebug() << strlen(chars) << "chars" 
  << hex << crcChars << "hex" 
  << dec << crcChars << "decimal";
qDebug() << bytes.length() << "bytes"
  << hex << crcBytes << "hex" 
  << dec << crcBytes << "decimal";

char *chars = "0A0105000000102347455420444154414F524445520D0A01";
QByteArray bytes = QByteArray::fromHex(chars);
quint16 crcChars = qChecksum(chars, strlen(chars));
quint16 crcBytes = qChecksum(bytes.data(), bytes.length());
qDebug() << strlen(chars) << "chars" 
  << hex << crcChars << "hex" 
  << dec << crcChars << "decimal";
qDebug() << bytes.length() << "bytes"
  << hex << crcBytes << "hex" 
  << dec << crcBytes << "decimal";

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48 chars 49fc hex 18940 decimal 
24 bytes 55ae hex 21934 decimal

48 chars 49fc hex 18940 decimal 
24 bytes 55ae hex 21934 decimal

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What I find interesting is that Qt's answers doesn't seem to match other implementations of the same CCITT CRC-16: 0x52A6 (string) and 0xD2C2 (binary).