Arduino can read bytes from console, but not from java

Question

I want to receive multiple bytes on an Arduino Uno from a Java program. The arduino deals with the data as soon as it's received, and thus I dont need to store it, I use the Serial RX buffer as temporary storage until I actually read the bytes. When fully implemented, about 150 bytes will be sent every time, but i've modified the buffer size to take care of that. I use jSerialComm as my serial library for java

I'v put some arduino and java code below. The arduino code works perfectly when I send the bytes from the IDE's serial monitor, lighting up the led as intended. But as soon as I try to send the bytes with the java code, the RX onboard led blinks, but the yellow led never lights up and ExecuteMove() doesn't trigger. I tried to put a Thread.sleep() before attempting to close the port, but that doesn't help.

Arduino

int GREEN = 4;
int BLUE = 3;
int YELLOW = 2;

void setup() {
  pinMode(GREEN, OUTPUT);
  pinMode(BLUE, OUTPUT);
  pinMode(YELLOW, OUTPUT);
  Serial.begin(9600);
}

void loop() {
  byte rb = Serial.read();
  if(rb != 255){ //Documentation says it sould be -1, but I'v tested it and 
                   it's 255
    digitalWrite(YELLOW, HIGH);
    ExecuteMove(rb);
    delay(500);
    digitalWrite(YELLOW, LOW);
  }
}

void ExecuteMove(byte _move){ 
  Lights up the green LED if _move == 65, blue if 66 (Works perfectly)  
}

Java

public static void main(String[] args) throws IOException, 
                                              InterruptedException{

    SerialPort sp = SerialPort.getCommPort("COM3");
    sp.setComPortParameters(9600, 8, 1, 0);
    sp.setComPortTimeouts(SerialPort.TIMEOUT_WRITE_BLOCKING, 0, 0);

    if(sp.openPort()) {
        System.out.println("Port is open");
    }else {
        System.out.println("Port failed to open");
        return;
    }

    byte[] message = {65, 66, 65};
    for(int i = 0; i < message.length; i++) {
        sp.getOutputStream().write(message[i]); //Sends the message
        sp.getOutputStream().flush();
    }

    if(sp.closePort()) { 
        System.out.println("Port is closed"); 
    }else { 
        System.out.println("Failed to close port"); 
        return; 
    } 
}

As I already stated, the arduino code alone works perfectly with the monitor, but when I use the java code to send bytes, only the RX led lights up, but none of "my" LEDs do

Try to not close the Java program, adding a Thread.sleep( 2000 ) before the return of main, because it closes the serial comm. The monitor doesn't do that. — Aubin
– Aubin, Commented Apr 28, 2019 at 17:33
About that: if(rb != 255){ //Documentation says it sould be -1, but I'v tested it and it's 255 - the return value is int, not byte — KIIV
– KIIV, Commented Apr 28, 2019 at 17:34
@Aubin I tried that, but it doesn't work either (edited my post with the extra info) — NatMath
– NatMath, Commented Apr 28, 2019 at 23:07

NatMath · Accepted Answer · 2020-05-14 14:34:07Z

1

For anybody stumbling on this post and that seams to have a similar issue, it's because Windows sends a reset signal to the arduino when opening the port. Because it sends the data immediatly after, the Arduino deletes it from its buffer while resetting and can never read it. There are two main ways to correct this, first adding a Thread.sleep(5000); between opening the port and sending data. You can also add a 47μF capacitor between the RESET and GND pins.

Source:https://arduino.stackexchange.com/questions/22267/java-jssc-arduino-windows https://forum.arduino.cc/index.php?topic=96422.0

Hope this can help someone

edited May 14, 2020 at 14:34

answered May 12, 2019 at 20:43

NatMath

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