• Purpose:
  • Serial communications between Data Terminal Equipment (DTE) and Data Communications Equipment (DCE)
• Original Application:
  • Communications between a terminal (the DTE) and a modem (the DCE)

• The Standards Organization:
  • The Electronic Industries Association (EIA)
• The Timeline:
  • 1960 - Original adoption
  • 1969 - Third version (RS-232C)

• The (Almost) Simplest System:
  • One wire (i.e., circuit) is used to transmit a sequence of bits by raising and lower the signal
  • Another wire is used as a ground
• Problems with this System:
  • Bits can easily be lost
  • One lost bit contaminates the remainder of the transmission

• Reducing Errors with Timing:
  • Have a specific, carefully measured, amount of time between data bits
• Reducing Errors with Markers:
  • Send a specific bit pattern between data "words"

• DB-25 Connector:
  • 
• DB-9 Connector:
  • 

• Data Words:
  • 7 or 8 bits
  • Least significant bit first
• Markers:
  • 1 start bit before the word
  • 1 parity bit and 1 stop bit after the word
• Possible Parity Bits:
  • Mark - always 1
  • Space - always 0
  • Even - make the number of 1s even
  • Odd - make the number of 1s odd
  • None

• Clear to Send (5, 8)
• Data Carrier Detect (8, 1)
• Data Set Ready (6, 6)
• Data Terminal Ready (20, 4)
• Signal Ground (7, 5)
• Transmitted Data, DTE to DCE (2,3)
• Received Data, DCE to DTE (3,2)
• Request to Send (4, 7)
• Ring Detector (22, 9)

• Voltages:
  • Low (logical 1) is between -5 and -15 volts
  • High (logical 0) is between +5 and +15 volts
• Transmitted Data:
  • Transmits when low on CTS, DTR, DSR, DCD
• Received Data:
  • Transmits when low on RTS, DTR, DSR, DCD

• Supports:
  • RS232 serial ports
  • IEEE1284 parallel ports
• Allows:
  • Port enumeration
  • Port ownership
  • Synchronous and asynchronous communications

• Makes extensive use of the observer pattern
• Is delivered as a .jar file and a native dynamic link library (see the javacomm site)

Reading NMEA Sentences from a Serial GPS Receiver

javaexamples/navigation/SerialGPSLogger.java

        import java.io.*;
import java.util.*;
import javax.comm.*;

/**
 * Read and print the information transmitted from 
 * a serial GPS receiver
 *
 * Notes: (1) The .jar file containing the javax.comm package must
 *            be in the classpath and the dynamic link library
 *            containing the native library must be in the
 *            path.
 *
 *        (2) Logging does not start until the start() method is called.
 *
 * @author  Prof. David Bernstein, James Madison University
 * @version 1.0
 */
public class SerialGPSLogger implements Runnable, SerialPortEventListener
{
    private boolean            keepRunning;
    private DataInputStream    dis;
    private InputStream        is;
    private LinkedList         queue;
    private SerialPort         serialPort;
    private Thread             controlThread;
    
    
    
    
    /**
     * Explicit Value Constructor
     *
     * @param comport  The serial port (e.g., "COM1")
     */
    public SerialGPSLogger(String comport)
    {
       CommPortIdentifier portID;
       Enumeration        portList;
        
       // Get the list of ports
       portList = CommPortIdentifier.getPortIdentifiers();
        
       // Loop through the ports looking for the requested port
       while (portList.hasMoreElements())
       {
          portID = (CommPortIdentifier)portList.nextElement();
            
          if ((portID.getPortType() == CommPortIdentifier.PORT_SERIAL) &&
              (portID.getName().equals(comport))                      )
          {
             try
             {
                // The arguments are the owner of the port and the
                // timeout in milliseconds
                serialPort = (SerialPort)portID.open("SerialGPSLogger", 
                                                     2000);
                    
                is  = serialPort.getInputStream();
                dis = new DataInputStream(is);
                    
                serialPort.setSerialPortParams(4800,
                                               SerialPort.DATABITS_8,
                                               SerialPort.STOPBITS_1,
                                               SerialPort.PARITY_NONE);
             } 
             catch (PortInUseException piue)
             {
                System.err.println("The serial port is in use!");
                System.exit(1);
             } 
             catch (IOException ioe) 
             {
                System.err.println("Unable to open an InputStream "+
                                   "for the serial port!");
                System.exit(2);
             } 
             catch (UnsupportedCommOperationException uscoe) 
             {
                System.err.println("Unable to set the parameters "+
                                   "of the serial port!");
                System.exit(3);
             }
             break;  // Out of the while loop
          }
       }
        

       queue         = new LinkedList();
       controlThread = new Thread(this);
    }
    
    
    
    
    
    
    /**
     * Take serial point information off of the queue and
     * distribute it to the observers (required by Runnable)
     */
    public void run() 
    {
       String          data;
       

       while (keepRunning)
       {
          // Log all observations that are in the queue
          while (queue.size() > 0)
          {
             data = (String)queue.removeFirst();
             System.out.println(data);
             System.out.flush();
          }

          // Sleep for 10 milliseconds
          try 
          {
             controlThread.sleep(10);
          }
          catch (InterruptedException ie)
          {
             // Resume
          }
       }
    }


    
    
    
    /**
     * Handle events generated by the SerialPort 
     * (required by SerialPortEventListener)
     *
     * @param event   The event generated by the SerialPort
     */
    public void serialEvent(SerialPortEvent event) {
        
       String     line;
        
        
       switch(event.getEventType())
       {

          case SerialPortEvent.BI:   // Break Interrupt

          case SerialPortEvent.CD:   // Carrier Detect
            
          case SerialPortEvent.CTS:  // Clear to Send
            
          case SerialPortEvent.DSR:  // Data Set Ready
            
          case SerialPortEvent.FE:   // Framing Error
            
          case SerialPortEvent.OE:   // Overrun Error
            
          case SerialPortEvent.PE:   // Parity Error
            
          case SerialPortEvent.RI:   // Ring Indicator
            
          case SerialPortEvent.OUTPUT_BUFFER_EMPTY:
            
             break;
            
          case SerialPortEvent.DATA_AVAILABLE:
            
            
             try 
             {
                while ( ((line = dis.readLine()) != null) &&
                        (!line.equals(""))                  ) 
                {
                   queue.addLast(line);
                }
             } 
             catch (IOException ioe) 
             {
                
                // Do nothing
             }
            
             break;
            
             // End of cases
       }
    }
    
    
    
    /**
     * Start the logging process
     */
    public void start()
    {
       keepRunning = true;
       controlThread = new Thread(this);
       controlThread.start();

       try
       {
          serialPort.addEventListener(this);
          serialPort.notifyOnDataAvailable(true);
       } 
       catch (TooManyListenersException tmle)
       {
          keepRunning = false;
          System.err.println("Too many listeners on the serial port!");
          System.exit(4);
       }

    }


}