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

Question

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 should be 0x60, 0x0A. I want the Java code to recreate this checksum, however I cannot recreate it. I 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. Can someone please point me in the right direction in terms of the CRC formula.

Answer 1

Use the following code instead:

 // 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 need to cancel your return CRC in order to get the correct entity. I even tested it here:

http://ideone.com/PrBXVh

Using a Windows calculator or something, you can see that the first result (from the function call above) gives the expected value (although it is canceled).

Answer 2

I worked on modbus using Java 1.6, tried the above code and worked only partially? Agreed on some CRC, wrong on others. I examined it a little more and saw that I had a problem with the extension of the sign. I have masked high bits (see FIX HERE below) and now it works fine. NOTE. All CRC calculations do not match, MODBUS is slightly different:

  public static int getCRC(byte[] buf, int len ) { int crc = 0xFFFF; int val = 0; for (int pos = 0; pos < len; pos++) { crc ^= (int)(0x00ff & buf[pos]); // FIX HERE -- 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, crc has low and high bytes swapped, so use it accordingly (or swap bytes) val = (crc & 0xff) << 8; val = val + ((crc >> 8) & 0xff); System.out.printf("Calculated a CRC of 0x%x, swapped: 0x%x\n", crc, val); return val; } // end GetCRC 

Answer 3

Will CRC32 do, or should it be CRC16? If 32 is fine, have you tried using CRC32 in java.util.zip ?

 import java.util.zip.CRC32; byte[] buf = new byte[] { (byte) 0x01, (byte) 0x04, (byte)0x00, (byte)0x01,(byte)0x00, (byte) 0x01}; CRC32 crc32 = new CRC32(); crc32.update(buf); System.out.printf("%X\n", crc32.getValue()); 

Output:

 F9DB8E67 

Then you can do whatever extra calculation you want on top of that.