Add 0x before sending RS232 messages

Question

Hi, I have an issue using comwrt function, the message I d like to send is like 0103040F000F unfortunately comwrt function convert each character as  hexadecimal ascii so the message sent looks like 30 31 33 ...... and I actually like to send the exact message 0103040F000F  on port so I have tried to use comwrtbyte function sending my datas byte per byte 01, 03 but I cannot send A to F values literally I must put a 0x before each, but I don't know how to put the whole thing in a variable,
int SendMessage (char *in)
    int len = 0;
    int i=0;
    int val=0;
    int ByteToSend;
    char *CharByteToSend;
    char Hexa[3]; Hexa[2]=0;
    len = strlen(in);
    CharByteToSend=(char*)malloc(5*sizeof(char));
    CharByteToSend[0]='0';
    CharByteToSend[1]='x';
    do                                 
        strncpy(CharByteToSend+2, in+i, 2);  CharByteToSend[4] = 0;
        ByteToSend = atoi(CharByteToSend);
        ComWrtByte(1, ByteToSend);
        i++;
        *in++;
    while(i<len);
    return 0;

Answer 1

Hi Olivier,
I am attaching the code fragments I use for CRC calculation and control in Modbus protocol: take a look and see if they give you the correct results. These have been used with several instruments (Siemens S7 plc, CHINO and CAL Controls temperature regulators and so on) so they should be ok for you too.
Before any instruction:
#define CRC16  0xA001     // To calculate CRC tables
static unsigned char crc_table_1[256];  // Table for CRC1 calculation
static unsigned char crc_table_2[256];  // Table for CRC2 calculation
At program start (or before you begin preparing Modbus messages with their CRC):
//                             Initialize tables for CRC calculation
void InitCRC(void)
 unsigned char i;
 unsigned int mask, crc, mem;
 for (mask = 0; mask < 256; mask++) {
  crc = mask;
  for (i = 0; i < 8; i++) {
   mem = (unsigned int)(crc & 0x0001) ;
   crc /= 2;
   if (mem)
    crc ^= CRC16 ;
  crc_table_2[mask] = (unsigned char) (crc & 0xff); // lobyte
  crc_table_1[mask] = (unsigned char) (crc >> 8);   // hibyte
 return;
Calculate CRC when preparing a message to send out:
//          Computes the CRC and append to the buffer with the data to be transmitted
// Parameters:    buf = message buffer
//                size = lengh of message buffer excluding the crc (crc will be appended to buf
//                         so you must pass a buffer with extra lenght)
// Return values: none
void CalcCRC(unsigned char * buf, unsigned char size)
 unsigned char car, i;
 unsigned char crc0, crc1;
 crc0 = 0xff;
 crc1 = 0xff;
 for (i = 0; i < size; i++) {
  car  = buf[i];
  car  = * ((unsigned char *) buf + i);
  car ^= crc0;
  crc0 = (unsigned char) (crc1 ^ crc_table_2[car]);
  crc1 = crc_table_1[car];
 * (buf + size)     = crc0;  // CRC low
 * (buf + size + 1) = crc1;  // CRC high
 return;
When checking CRC on a received message:
//           CRC check and match with the CRC found on the received data buffer
// Parameters:    buf = message buffer
//                size = lengh of message buffer excluding the crc
// Return values: 0 if CRC ok
//                1 if CRC bad
//                If CRC is bad the data packet is corrupted, so it is necessary to discard it.
int CheckCRC(unsigned char * buf, unsigned char size)
  unsigned char car, i;
  unsigned char crc0, crc1;
  unsigned int  crc_calc, crc_read;
  crc0 = 0xff;
  crc1 = 0xff;
  for(i = 0; i < size; i++) {
    car  = buf[i];
    car  = * ((unsigned char *) buf + i);
    car ^= crc0;
    crc0 = (unsigned char) (crc1 ^ crc_table_2[car]);
    crc1 = crc_table_1[car];
  crc_calc  = (crc1 << 8) + crc0;
  crc_read = * (buf + size) + 256 * (* (buf + size + 1));
  return (crc_read != crc_calc);
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