Question

I am trying to use the RXTX library for blocking serial communication on Windows (XP and 7). I have tested the connection with Hyperterminal in both ends, and it works flawlessly.

I set up the connection with the following code: (exception handling and defensive checks omitted for clarity)

private InputStream inStream;
private OutputStream outStream;
private BufferedReader inReader;
private PrintWriter outWriter;
private SerialPort serialPort;
private final String serialPortName;

public StreamComSerial(String serialPortName) {
this.serialPortName = serialPortName;
CommPortIdentifier portIdentifier;
portIdentifier = CommPortIdentifier.getPortIdentifier(serialPortName);
CommPort commPort = null;
commPort = portIdentifier.open(this.getClass().getName(),500);
serialPort = (SerialPort) commPort;    serialPort.setSerialPortParams(4800,SerialPort.DATABITS_8,SerialPort.STOPBITS_1,SerialPort.PARITY_NONE);
inStream = serialPort.getInputStream();
outStream = serialPort.getOutputStream();
inReader = new BufferedReader(new InputStreamReader(inStream, Settings.getCharset()));
outWriter = new PrintWriter(new OutputStreamWriter(outStream, Settings.getCharset()));

When I use

outWriter.println("test message");
flush();

the message is recieved fine on the other end, but calling

inReader.readLine()

imidiately returns “java.io.IOException: Underlying input stream returned zero bytes”.

I then decided to try and implement my own blocking read logic and wrote this:

public String readLine() throws IOException {        
    String line = new String();
    byte[] nextByte = {-1};
    while (true) {
        nextByte[0] = (byte)inStream.read();
        logger.debug("int read: " + nextByte[0]);
        if (nextByte[0] == (byte)-1) {
            try {
                Thread.sleep(100);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            continue;
        }
        logger.debug("byte read: " + nextByte[0]);

        line = line + new String(nextByte);
        if (nextByte[0] == (byte)13) {  // 13 is carriage return in ASCII
            return line;
        }
    }
}

But this code goes in an infinite loop and “nextByte[0] = (byte)inStream.read();” assigns -1 no matter what is sent over the serial connection. In addition, the other end stutters quite badly and only lets me send a character every 1-3 sec. and hangs for a long time if I try to send many characters in a short burst.

Any help very appreciated.

*edit – using inStream.read(nextByte) instead of “nextByte[0] = (byte)inStream.read();” does not write to the nextByte variable, no matter what I send to it through the serial connection.

*edit2 – as my code works flawlessly with the SUN javax.comm lib and a win32com.dll I got from a friend, I have ceased trying to make it work with RXTX. I am not interested in unblocking communication, which seems to be the only way other people can make RXTX work.

Answer 1

Use RXTX-2.2pre2, previous versions have had a bug which prevented blocking I/O from working correctly.

And do not forget to set port to blocking mode:

serialPort.disableReceiveTimeout();
serialPort.enableReceiveThreshold(1);