GreenGodot - 1 year ago 183
Java Question

# Getting the CRC checksum of a byte array and adding it to that byte array

I have this byte array:

``````static byte[] buf = new byte[] { (byte) 0x01, (byte) 0x04, (byte)0x00, (byte)0x01,(byte)0x00, (byte) 0x01};
``````

Now, the CRC checksum of this byte array is supposed to be 0x60, 0x0A. I want the Java code to recreate this checksum, however I cant seem to recreate it. I have tried crc16:

``````static int crc16(final byte[] buffer) {
int crc = 0xFFFF;

for (int j = 0; j < buffer.length ; j++) {
crc = ((crc  >>> 8) | (crc  << 8) )& 0xffff;
crc ^= (buffer[j] & 0xff);//byte to int, trunc sign
crc ^= ((crc & 0xff) >> 4);
crc ^= (crc << 12) & 0xffff;
crc ^= ((crc & 0xFF) << 5) & 0xffff;
}
crc &= 0xffff;
return crc;

}
``````

and convert them using Integer.toHexString(), but none of the results match the correct CRC. Could someone please point me in the right direction in terms of CRC formula.

``````// Compute the MODBUS RTU CRC
private static int ModRTU_CRC(byte[] buf, int len)
{
int crc = 0xFFFF;

for (int pos = 0; pos < len; pos++) {
crc ^= (int)buf[pos] & 0xFF;   // XOR byte into least sig. byte of crc

for (int i = 8; i != 0; i--) {    // Loop over each bit
if ((crc & 0x0001) != 0) {      // If the LSB is set
crc >>= 1;                    // Shift right and XOR 0xA001
crc ^= 0xA001;
}
else                            // Else LSB is not set
crc >>= 1;                    // Just shift right
}
}
// Note, this number has low and high bytes swapped, so use it accordingly (or swap bytes)
return crc;
}
``````

You may have to reverse your return CRC to get the right endianness, though. I even tested it here:

http://ideone.com/PrBXVh

Using windows calculator or something you can see that the first result (from the above function call) gives the expected value (albeit reversed).

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