从 java 中的串口读取时获取垃圾值

Question

I am reading data from serialport using jSerialComm package. I am getting following data

A total of 22 bytes are being received and I am getting these three garbage bytes too. The readable data is correct but what these garbage characters are happening?

Following is my code.

public static void main(String[] args) {
    SerialPort serialPort = SerialPort.getCommPort("/dev/ttyUSB0");
    if(serialPort.openPort())
    {
        System.out.println("Port Opened Successfully...");
    }
    else
    {
        System.out.println("Unable to open port....");
        return;
    }
    serialPort.setComPortParameters(1200, 8, 1, 0);
    try
    {
        while(true)
        {
            while(serialPort.bytesAvailable() != 0)
            {
                Thread.sleep(1000);
                byte[] readBuffer = new byte[serialPort.bytesAvailable()];
                int numRead = serialPort.readBytes(readBuffer, readBuffer.length);
                String data = new String(readBuffer);
                System.out.println("Read "+numRead+ " bytes." + readBuffer);
                System.out.println(data);
            }
        }
    }
    catch(Exception e)
    {
        e.printStackTrace();
    }
    serialPort.closePort();
    System.out.println("done...");
}

Answer 1

SerialPort.readBytes seems to be not thread safe, thus yielding "garbage" when called during character reception.

I suggest to use the snippet from the author's example which worked fine for me:

// Get a new instance of SerialPort by opening a port.
SerialPort port = SerialPort.open("COM2");

// Configure the connection
port.setTimeout(100);
port.setConfig(BaudRate.B115200, Parity.NONE, StopBits.ONE, DataBits.B8);

// You have the choice, you can either use the Java NIO channels
// or classic Input/Ouput streams to read and write data.
//DEL SerialChannel channel = port.getChannel();
InputStream istream = port.getInputStream();

// Read some data using a stream
byte[] byteBuffer = new byte[4096];
// Will timeout after 100ms, returning 0 if no bytes were available.
int n = istream.read(byteBuffer);

// *** Use n bytes of byteBuffer ***

//DEL ...

port.close();

//DEL : removed from the original code for clarity

Answer 2

You did not explain the protocol but I suggest to look it up. Presumably these are control characters or like a comment suggests binary data. You create a String from a byte buffer without an encoding so this also depends on your environment/ JVM's default encoding.

Try treating the first and the last two bytes as specified in the protocol in use for your project. It might also be related to jSerialComm not removing serial signalling, eg handshake, EOT and such.

If you're reverse-engineering the protocol instead maybe also try another library like RxTx to see if the bytes stay the same.

To inspect the bytes safely use for example BigInteger to print out a Hex-String instead:

BigInteger bigInteger = new BigInteger(1, bytes);
System.out.printf("%0" + (bytes.length << 1) + "x", bigInteger);

Answer 3

Use this code block and it will run perfectly;

    serialPort.addDataListener(new SerialPortDataListener() {
        @Override
        public int getListeningEvents() {
            return SerialPort.LISTENING_EVENT_DATA_RECEIVED;
        }

        @Override
        public void serialEvent(SerialPortEvent serialPortEvent) {
            if (serialPortEvent.getEventType() != SerialPort.LISTENING_EVENT_DATA_RECEIVED) {
                return;
            }
            byte[] newData = serialPortEvent.getReceivedData();
            String data = new String(newData);
            System.out.println(data);

        }
    });