Calculating CRC for int32 data format using the Modbus protocol

Question

I am connecting to a device using the modbus protocol. I need to obtain 3 values from the machine. The first value is of the data format int16 and when I send an example byte array:

static byte[] hz = new byte[] { (byte) 0x01, (byte) 0x03, (byte) 0x00,
        (byte) 0x33, (byte) 0x00, (byte) 0x01 };

and use a CRC calculation method I obtained from a previous question I asked on the subject.

    // 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];          // 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;  
    }

I can recieve a response. However, the other two values are of the int32 data format and do not return a reply when I use this method. To help troubleshoot I am using a program called Realterm. to fire off the commands as well. I use it to append a Modbus 16 CRC to the end of the byte stream and send it, this works for all three and returns the desired reply. Is this a case of the data format not working with this specific calculation formula? Whats the difference between CRC16 and Modbus16?

Answer 1

Modbus16 is a CRC16. CRC calculations have several parameters:

• the bit width, in this case 16
• the polynomial, in this case 0xA001
• the initial value,in this case 0xFFFF
• the bit order
• whether the final CRC is inverted with an XOR.

There are quite a number of CRC16s defined, with different values for these parameters, and this appears to be one of them. See the Wikipedia article on Cyclic Redundancy Checks for more informaton.