Java Arduino Serial Communication

[Scott Eric Catalano]

Hello Group!

I am having an issue that I believe is simple to fix however I cannot wrap my head around it. I have a java program that connects to an arduino I can issue commands from the java program and the arduino responds accordingly. My issue is when the arduino starts up or resets I believe it is sending a white space or something to the serial port and my java picks that up and is disrupts the program. Arduino code and Java code posted below. I may have to send some sort of message from the arduino to clear the buffers and then start clean. I am not sure. Thanks in advance!

// ARDUINO CODE

void setup() {
//Start the serial port at 9600 baud rate.
Serial.begin(9600);
pinMode(13, OUTPUT);
}

//MAIN LOOP
void loop() {
//Assign the input value to a variable.
char input;

//Read in the characters being sent from Java.
input = Serial.read();

//Turn on the LED.
if(input == '1'){
digitalWrite(13, HIGH);
Serial.print("ON");
}

//Have the LED blink 10 times.
if(input == '2') {
for(int i = 0; i < 10; i++) {
digitalWrite(13, HIGH);
delay(1000);
digitalWrite(13, LOW);
delay(100);
Serial.print("OX");
}
//Serial.println("OX");
}

//Turn off the LED.
if(input == '3') {
digitalWrite(13, LOW);
Serial.print("OFF");
}
}

And below is the Java code that is for a button. When the button is pressed it sends a command to turn on an LED and it blinks....the Arduino is suppose to send data back in the form of "OX" when the Java code receives this code "OX" it prints to the console and states: LED BLINKING however it does not get the "OX" because there is something stuck in the buffers.

//Listener for the blink button.
		btnBlink.addActionListener(new ActionListener() {
			public void actionPerformed(ActionEvent e) {
				btnOn.setBackground(null);
				btnBlink.setBackground(Color.yellow);
				btnOff.setBackground(null);
				//Turns on the #13 pin LED and flashes it 10 times.
				if(comPort.isOpen() == true) {
					
					InputStream inPut = comPort.getInputStream();
					BufferedReader readBuffer = new BufferedReader(new InputStreamReader(inPut));
					//Send a 2 to the Arduino and update the user.
					System.out.println("STATUS: LED BLINKING!!");
					lblStatus.setText("Status: LED Blinking");
					outPut.print("2");
					outPut.flush();
byte[] buffer = new byte[2];
  try {
    String message = "";
    
        int len = inPut.read(buffer);
        if (len > 0) {
            message = new String(buffer);
            System.out.println(message);
        }
		
        len = 0;
        String messages[];
        messages = message.split("\r\n");
        
            if (message.contains("OX")) {
                btnBlink.setBackground(Color.yellow);
				outPut.flush();
            } else {
					System.out.println("ERROR!!");
		}
	

} catch (Exception ep) { ep.printStackTrace(); }

} else {
					//Update the status/console if the Arduino hasn't been connected.
					System.out.println("CONNECT ARDUINO");
					lblStatus.setText("Status: Connect Arduino");
				}
				
			}
			
		});

[Tim Holloway]

Sorry. I can't read that stuff. Your code indentation is all over the map, and it's hard to tell what's dependent on what.

It's often the case that communications interfaces get random junk when the device is first powered up. That's usually something that hardware design can help with.

When talking to a PC, though, there can be 2 issues where you have stale data left over from previous connections. First, at the OS level, depending on the TTY characteristics set and what options were applied when the port was opened. And in Java, your choice of what stream/reader you use can further complicate things, because if they are maintaining buffers, those buffers might need to be flushed explictly.

It can be messy and annoying. You might get better results if you use some sort of framing characters around your data and simply discard anything that's not within a frame.

[Scott Eric Catalano]

@Tim H.

That's exactly what is happening....the start up and/or reset of the Arduino is throwing junk into the serial stream and that is what needs to be taken care of. How do I go about doing that in getting clean data to read from? I don't have an issue getting feedback from the Arduino on the second or third command issued from my Java code as that flushes the buffers and I get a clean read.

[Scott Eric Catalano]

Ok I was able to get it working correcting using the below code. The message is sent from the Arduino to the java program is is printed and acknowledged on the console. Now I am having issues trying to to the same thing each time a button is pressed. How ever the process just hangs. My question is why doesn't it hang in the first instance and hangs in the second instance? Thanks again!

// JAVA CODE

if(comPort.isOpen() == false) {	
					try {
						//Open the USB port and initialize the PrintWriter.
						comPort.openPort(1000);
						comPort.clearDTR();
						comPort.clearRTS();
						Thread.sleep(10000);
						outPut = new PrintWriter(comPort.getOutputStream(), true);
						inPut = comPort.getInputStream();
						comPort.getOutputStream().flush();
					String line = t.nextLine();
					if(line.equals("CONNECTED")) { // MESSAGE FROM ARDUINO ON STARTUP
					System.out.println("MESSAGE RECEIVED:" +line);
					outPut.flush();
					} else {
					System.out.println("ERROR...CLOSING PORT...SHUTTING DOWN...EXITING PROGRAM!!");
					comPort.closePort();
					System.exit(0);
					}

Code that sends the response from the Arduino to the Java program:

// SETUP
void setup() {
//Start the serial port at 9600 baud rate.
delay(5000);
Serial.begin(9600);
while (!Serial) {
; // wait for serial port to connect. Needed for native USB port only
}
//Set pin 13 for output.
pinMode(13, OUTPUT);
Serial.println("CONNECTED"); // MESSAGE SENT TO JAVA
}

Here is the code that is used in the Arduino when a button in Java is pressed and the response back:

if(input == '1'){
digitalWrite(13, HIGH);
Serial.print("ON");
}

My question is why does it work in one section and not in the other as it just hangs.

                     String line2 = t.nextLine();
					if(line2.equals("ON")) { // MESSAGE FROM ARDUINO ON STARTUP
					System.out.println("MESSAGE RECEIVED:" +line2);
					outPut.flush();

Its the same code. Or is it? Thanks for the help!

[Norm Radder]

the process just hangs.

What statement is the execution hanging on?

Is some read statement blocking waiting for input?

[Audy Iltis]

Hi..as per my understanding there can be 2 issues where you have stale data left over from previous connections. First, at the OS level, depending on the TTY characteristics set and what options were applied when the port was opened. And in Java, your choice of what stream/reader you use can further complicate things