Hi, I have read many post about how to calculate the CRC code of a modbus message, but no one solved the problem. To help other programmers i'll share the solution I made.
I work with an Leveltroll500 & Aquatroll200 probes and an Ultrasonic Flow Meter model: TUF-2000M (from china) that works with modbus protocols through rs485 communication standar.
I got alot of experience working with these equipments and the attached code works well to check the message.
void setup() {
// put your setup code here, to run once:
Serial.begin(19200);
}
void loop() {
// put your main code here, to run repeatedly:
String raw_msg = "01030000000AC5CD"; //like Modbus menssage: 01 03 00 00 00 0A C5 CD
String raw_msg_data = raw_msg.substring(0, raw_msg.length()-4); //Modbus message without crc code
String raw_msg_crc = raw_msg.substring(raw_msg.length() - 4, raw_msg.length()); //only Modbus message crc code
String calculated_crc = "";
Serial.println("Modbus message: " + raw_msg);
Serial.println("Modbus data: " + raw_msg_data);
Serial.println("Modbus CRC: " + raw_msg_crc);
calculated_crc = ModRTU_CRC(raw_msg_data); //Modbus message without crc code. Reply the calculated crc code
Serial.println("---------------------------------");
Serial.println("Modbus CRC Calculated: " + calculated_crc);
while(1);
}
// Compute the MODBUS RTU CRC
String ModRTU_CRC(String raw_msg_data) {
//Calc raw_msg_data length
byte raw_msg_data_byte[raw_msg_data.length()/2];
//Convert the raw_msg_data to a byte array raw_msg_data
for (int i = 0; i < raw_msg_data.length() / 2; i++) {
raw_msg_data_byte[i] = StrtoByte(raw_msg_data.substring(2 * i, 2 * i + 2));
}
//Calc the raw_msg_data_byte CRC code
uint16_t crc = 0xFFFF;
String crc_string = "";
for (int pos = 0; pos < raw_msg_data.length()/2; pos++) {
crc ^= (uint16_t)raw_msg_data_byte[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)
//Become crc byte to a capital letter String
crc_string = String(crc, HEX);
crc_string.toUpperCase();
//The crc should be like XXYY. Add zeros if need it
if(crc_string.length() == 1){
crc_string = "000" + crc_string;
}else if(crc_string.length() == 2){
crc_string = "00" + crc_string;
}else if(crc_string.length() == 3){
crc_string = "0" + crc_string;
}else{
//OK
}
//Invert the byte positions
crc_string = crc_string.substring(2, 4) + crc_string.substring(0, 2);
return crc_string;
}
//String to byte --> Example: String = "C4" --> byte = {0xC4}
byte StrtoByte (String str_value){
char char_buff[3];
str_value.toCharArray(char_buff, 3);
byte byte_value = strtoul(char_buff, NULL, 16);
return byte_value;
}