//Serial buffer
#define serial_buffer_len  80
char serial_buffer[serial_buffer_len];
String user_input = "";

void setup() { 
	Serial.begin(9600);
}

void loop() {
	if (Serial.available() > 0){
		if (read_serial_input(Serial.read(), serial_buffer, serial_buffer_len) > 0) {
			user_input = String(serial_buffer);
		}
		Serial.println("serial user input:" +user_input);	
	}
}

// funtion to read user serial input
int read_serial_input(int read_ch, char *buffer, int len) {
    static int pos = 0;  
    int rpos;   
    if (read_ch > 0) {
        switch (read_ch) {
            case '\r': // Ignore CR
                break;
            case '\n': // Return on new-line
                rpos = pos;
                pos = 0;  // Reset position index ready for next time
                return rpos;
            default:
                if (pos < len-1) {
                    buffer[pos++] = read_ch;
                    buffer[pos] = 0;
                }
        }
    }
    return 0;
}