在C＃中计算CRC16

Question

I'm trying to port an old code from C to C# which basically receives a string and returns a CRC16 of it...

The C method is as follow:

#define CRC_MASK 0x1021 /* x^16 + x^12 + x^5 + x^0 */

UINT16 CRC_Calc (unsigned char *pbData, int iLength)
{
    UINT16 wData, wCRC = 0;
    int i;

    for ( ;iLength > 0; iLength--, pbData++) {
        wData = (UINT16) (((UINT16) *pbData) << 8);
        for (i = 0; i < 8; i++, wData <<= 1) {
            if ((wCRC ^ wData) & 0x8000)
                wCRC = (UINT16) ((wCRC << 1) ^ CRC_MASK);
            else
                wCRC <<= 1;
        }
    }
    return wCRC;
}

My ported C# code is this:

private static ushort Calc(byte[] data)
    {
        ushort wData, wCRC = 0;
        for (int i = 0; i < data.Length; i++)
        {
            wData = Convert.ToUInt16(data[i] << 8);
            for (int j = 0; j < 8; j++, wData <<= 1)
            {
                var a = (wCRC ^ wData) & 0x8000;
                if ( a != 0)
                {
                    var c = (wCRC << 1) ^ 0x1021;
                    wCRC = Convert.ToUInt16(c);
                }
                else
                {
                    wCRC <<= 1;
                }
            }
        }
        return wCRC;
    }

The test string is "OPN"... It must return a uint which is (ofc) 2 bytes A8 A9 and the #CRC_MASK is the polynomial for that calculation. I did found several examples of CRC16 here and around the web, but none of them achieve this result since this CRC calculation must match the one that the device we are connecting to.

WHere is the mistake? I really appreciate any help.

Thanks! best regards

Gutemberg

UPDATE

Following the answer from @rcgldr, I put together the following sample:

_serial = new SerialPort("COM6", 19200, Parity.None, 8, StopBits.One);
        _serial.Open();
        _serial.Encoding = Encoding.GetEncoding(1252);
        _serial.DataReceived += Serial_DataReceived;
        var msg = "OPN";
        var data = Encoding.GetEncoding(1252).GetBytes(msg);
        var crc = BitConverter.GetBytes(Calc(data));
        var msb = crc[0].ToString("X");
        var lsb = crc[1].ToString("X");
        //The following line must be something like: \x16OPN\x17\xA8\xA9
        var cmd = string.Format(@"{0}{1}{2}\x{3}\x{4}", SYN, msg, ETB, msb, lsb);
        //var cmd = "\x16OPN\x17\xA8\xA9";
        _serial.Write(cmd);

The value of the cmd variable is what I'm trying to send to the device. If you have a look the the commented cmd value, this is a working string. The 2 bytes of the CRC16, goes in the last two parameters (msb and lsb). So, in the sample here, msb MUST be "\\xA8" and lsb MUST be "\\xA9" in order to the command to work(the CRC16 match on the device).

Any clues?

Thanks again.

UPDATE 2 For those who fall in the same case were you need to format the string with \\x this is what I did to get it working:

protected string ToMessage(string data)
    {
        var msg = data + ETB;
        var crc = CRC16.Compute(msg);
        var fullMsg = string.Format(@"{0}{1}{2:X}{3:X}", SYN, msg, crc[0], crc[1]);
        return fullMsg;
    }

This return to me the full message that I need inclusing the \\x on it. The SYN variable is '\\x16' and ETB is '\\x17'

Thank you all for the help!

Gutemberg

Answer 1

The problem here is that the message including the ETB (\\x17) is 4 bytes long (the leading sync byte isn't used for the CRC): "OPN\\x17" == {'O', 'P', 'N', 0x17}, which results in a CRC of {0xA8, 0xA9} to be appended to the message. So the CRC function is correct, but the original test data wasn't including the 4th byte which is 0x17.

This is a working example (at least with VS2015 express).

private static ushort Calc(byte[] data)
{
    ushort wCRC = 0;
    for (int i = 0; i < data.Length; i++)
    {
        wCRC ^= (ushort)(data[i] << 8);
        for (int j = 0; j < 8; j++)
        {
            if ((wCRC & 0x8000) != 0)
                wCRC = (ushort)((wCRC << 1) ^ 0x1021);
            else
                wCRC <<= 1;
        }
    }
    return wCRC;
}