• We recently switched our forum platform. If you experience any issues please email support@crystalfontz.com

Crc-16 Code problems (Java)

NancyC

New member
Hi! I am trying to send out modbus messages with the last two bytes of the message being CRC check bytes.

I used the crc code below:
Code:
public class CCITTcrc 
{
	public static final int CRCINIT  = 0xffff;
	static final String HEXCHARS	= "0123456789ABCDEF"; 	
	
	static final int [] CRCTAB_CCITT =
	{
        0x0000, 0x1021, 0x2042, 0x3063, 0x4084, 0x50A5, 0x60C6, 0x70E7,
        0x8108, 0x9129, 0xA14A, 0xB16B, 0xC18C, 0xD1AD, 0xE1CE, 0xF1EF,
        0x1231, 0x0210, 0x3273, 0x2252, 0x52B5, 0x4294, 0x72F7, 0x62D6,
        0x9339, 0x8318, 0xB37B, 0xA35A, 0xD3BD, 0xC39C, 0xF3FF, 0xE3DE,
        0x2462, 0x3443, 0x0420, 0x1401, 0x64E6, 0x74C7, 0x44A4, 0x5485,
        0xA56A, 0xB54B, 0x8528, 0x9509, 0xE5EE, 0xF5CF, 0xC5AC, 0xD58D,
        0x3653, 0x2672, 0x1611, 0x0630, 0x76D7, 0x66F6, 0x5695, 0x46B4,
        0xB75B, 0xA77A, 0x9719, 0x8738, 0xF7DF, 0xE7FE, 0xD79D, 0xC7BC,
        0x48C4, 0x58E5, 0x6886, 0x78A7, 0x0840, 0x1861, 0x2802, 0x3823,
        0xC9CC, 0xD9ED, 0xE98E, 0xF9AF, 0x8948, 0x9969, 0xA90A, 0xB92B,
        0x5AF5, 0x4AD4, 0x7AB7, 0x6A96, 0x1A71, 0x0A50, 0x3A33, 0x2A12, 
        0xDBFD, 0xCBDC, 0xFBBF, 0xEB9E, 0x9B79, 0x8B58, 0xBB3B, 0xAB1A, 
        0x6CA6, 0x7C87, 0x4CE4, 0x5CC5, 0x2C22, 0x3C03, 0x0C60, 0x1C41, 
        0xEDAE, 0xFD8F, 0xCDEC, 0xDDCD, 0xAD2A, 0xBD0B, 0x8D68, 0x9D49, 
        0x7E97, 0x6EB6, 0x5ED5, 0x4EF4, 0x3E13, 0x2E32, 0x1E51, 0x0E70,
        0xFF9F, 0xEFBE, 0xDFDD, 0xCFFC, 0xBF1B, 0xAF3A, 0x9F59, 0x8F78, 
        0x9188, 0x81A9, 0xB1CA, 0xA1EB, 0xD10C, 0xC12D, 0xF14E, 0xE16F,
        0x1080, 0x00A1, 0x30C2, 0x20E3, 0x5004, 0x4025, 0x7046, 0x6067, 
        0x83B9, 0x9398, 0xA3FB, 0xB3DA, 0xC33D, 0xD31C, 0xE37F, 0xF35E,
        0x02B1, 0x1290, 0x22F3, 0x32D2, 0x4235, 0x5214, 0x6277, 0x7256,
        0xB5EA, 0xA5CB, 0x95A8, 0x8589, 0xF56E, 0xE54F, 0xD52C, 0xC50D,
        0x34E2, 0x24C3, 0x14A0, 0x0481, 0x7466, 0x6447, 0x5424, 0x4405,
        0xA7DB, 0xB7FA, 0x8799, 0x97B8, 0xE75F, 0xF77E, 0xC71D, 0xD73C,
        0x26D3, 0x36F2, 0x0691, 0x16B0, 0x6657, 0x7676, 0x4615, 0x5634,
        0xD94C, 0xC96D, 0xF90E, 0xE92F, 0x99C8, 0x89E9, 0xB98A, 0xA9AB,
        0x5844, 0x4865, 0x7806, 0x6827, 0x18C0, 0x08E1, 0x3882, 0x28A3,
        0xCB7D, 0xDB5C, 0xEB3F, 0xFB1E, 0x8BF9, 0x9BD8, 0xABBB, 0xBB9A,
        0x4A75, 0x5A54, 0x6A37, 0x7A16, 0x0AF1, 0x1AD0, 0x2AB3, 0x3A92,
        0xFD2E, 0xED0F, 0xDD6C, 0xCD4D, 0xBDAA, 0xAD8B, 0x9DE8, 0x8DC9,
        0x7C26, 0x6C07, 0x5C64, 0x4C45, 0x3CA2, 0x2C83, 0x1CE0, 0x0CC1,
        0xEF1F, 0xFF3E, 0xCF5D, 0xDF7C, 0xAF9B, 0xBFBA, 0x8FD9, 0x9FF8,
        0x6E17, 0x7E36, 0x4E55, 0x5E74, 0x2E93, 0x3EB2, 0x0ED1, 0x1EF0
	};

	short crcaccu;
	
//	public CCITTcrc(void)
//	{	}

	public int getint(byte b) 
	{
		if (b < 0)
			return b+0x100;
		else
			return b;
	}

	
	public int calc(byte[] buffer, int n) {
		int i,x;
	
		crcaccu=(short)CRCINIT;
		for (i=0; i<n; i++) {
			x=(byte)((crcaccu>>8)^buffer[i]);
			if (x<0)
				x+=0x100;
			crcaccu=(short)((crcaccu<<8)^CRCTAB_CCITT[x]);
		}
		buffer[n]=(byte)(crcaccu>>8);
		buffer[n+1]=(byte)crcaccu;

		
		return n+2;
	}
}

This is apparently not producing the correct crc bytes, and so I can't establish any communication with my slave device...but I have no idea where the code has gone wrong! If you recognize something I missed, or if you have similar code that I could use, please let me know! :)

Cheers,
-N
Looking for additional LCD resources? Check out our LCD blog for the latest developments in LCD technology.
 
Your calc() function is very difficult to understand. The "for()" loop statement looks malformed.

I didn't try to analyze your code. I gave up on CRC table-driven implimentations, to save memory on the table. Plus, I could never be sure the table wasn't corrupt when it wouldn't work.

Here is the CRC function that I use (in c++, not Java):
Code:
BOOL CComm::SendCrc(BYTE *pData, DWORD dwSize)
{
	DWORD a;
	WORD Crc;
        BYTE d, i, v, flag;

	Crc = 0xFFFF;
	for (a = 0; a < dwSize - 2; a++)
	{
		d = *(pData + a);
		v = 0x80;

		for (i = 0; i < 8; i++)
		{
			if (Crc & 0x8000) flag = 1;
			else flag = 0;

			Crc = Crc << 1;

			if (d & v) Crc++;

			if (flag) Crc ^= 0x1021;

			v = v >> 1;
		}
	}
	*(pData + a++) = Crc >> 8;
	*(pData + a++) = Crc & 0xFF;
	return Write((char *)pData, dwSize);
}
 
Top