/* CRC16 for MODBUS RTU protocol 
  -------------------------------------
  This  macro  transmits a message from
  the send dialog and appends two bytes 
  with CRC at the end
*/
main()
{
  var s; 
  s = crc()%0xFFFF;
  putx(s&0xFF);
  putx(shr(s,8)&0xFF);
  flush();
}

crc()
{
  var s,j,t,i;
  s = 0xFFFF;
  
  for(i=0; i<get_message_length(); i=i+1)
  {
   t = get_message_at(i);
   putx(t);
   s = t^s;
  
   for(j=0; j<8; j=j+1)
   {  
    if(s & 0x0001 == 1)
    {
     s = shr(s,1);
     s = s^0xA001;
    }
    else;
    { 
    s = shr(s,1);
    } 
   } 
  }
  return s;
}