Access Platform
System Properties
  Demo 1
Run Ext. Programs
  Demo 2
Java Comm API
Port Classes
Port Operations
  Demo 3   Demo 4   Demo 5
Serial Port IO
Demo: Serial Port
  Code Listings
    Demo 6
Parallel Port
Port I/O Apps
Exercises

Perhaps you are developing a package of instruments and sensors to distribute to remote locations. A small weather station kit would be an example. With the techniques and tools discussed so far in JavaTech, you could use Java to develop much of the software that is needed to do analysis of the data and to build a client/server system to provide the data to distant users via the Internet.

But to make the system work, you would also need software to access the instrument hardware. You might also need to control a modem to do dial-up connections if there was no local network available.

To do these things you will need to access the platform's communications ports, such as those for serial lines, that are connected to the instruments. You could use the techniques in Chapter 22 to connect via C hardware drivers, but a much easier alternative is to take advantage of the available communication classes and talk to these ports directly from your Java programs.

The Java Communications API consists of the javax.comm package. This package does not come with the core Java development kits but instead is included in the set of optional Java Extensions packages that provide various useful services but are not available for all platforms.

With the Communications API you can obtain a set of objects representing the RS232 serial ports and IEEE 1284 parallel ports on a platform. With these you can obtain exclusive ownership of a port, read and write to the port either synchronously and asynchronously, and receive events from the port that indicate some state change in the port such as the arrival of data.

The serial and parallel port classes provide low level I/O capabilities. The serial port class methods even give access even to individual pins. You can open an input and/or output stream to the port. You must write your own protocol handler, or obtain one from a third party, to deal with a particular device such as a modem or printer.

Other types of ports (see below) like USB are coming to dominate desktop PC systems. RS232 serial ports and especially the 1284 parallel ports are used less and less and are often absent on new consumer machines. However, for technical applications we expect that the RS232 serial port will remain common for many more years. There are thousands of new and old types of devices that use RS232 for communications and it will be a long time before they completely disappear.

In the following sections we will look at sample codes for carrying out basic I/O tasks with the serial and parallel ports.

Installing javax.comm

The J2SE installation does not include the javax.comm set of files. You must download and install it separately. The Sun Java Communications site currently provides files suitable for Windows platforms and Solaris. See the resources section below for links to sites that provide javax.comm for Linux .

For example, for MS Windows follow this procedure:

1. Download and unpack the javacomm20-win32.zip file.
2. Place the win32com.dll in <jdk>\jre\bin directory (or the \jre\bin subdirectory of your current J2SDK directory.)
3. Place the comm.jar in <jdk>\jre\lib\ext.
4. Place the javax.comm.properties in <jdk>\jre\lib.
5. Do not alter the CLASSPATH.

where <jdk> refers to the directory path for your JDK, e.g. C:\Program Files\Java\jdk1.5.0.

For help with installation problems, see the readme.html that comes with the javax.comm files and the Java Comm API FAQ.

Other Types of Ports

Unfortunately, the javax.comm package does not currently support access to other types of ports such as USB and Firewire. Below we discuss the USB and 1-Wire systems for which independent Java software packages are available.

USB

The specifications for a javax.usb package is currently under development by the Java Community Process. There is third party software available for some platforms such as the jUSB: Java USB open source software that works with USB ports on Linux platforms.

Note that indirect access to USB and other ports that communicate with audio and video devices comes via the Java Media Framework (java.sun.com/products/java-media/jmf/), which is another Java Extensions API not available on all platforms. While they don't allow for low level interaction with the ports, the JMF classes do provide for considerable control over the communications with these devices in the context of audio and video applications.

1-Wire & iButton

1-Wire® is a proprietary protocol of Dallas Semiconductor (now part of Maxim-IC) that needs only one wire plus ground to do serial transmission and also to supply power. The system works only over short ranges but provides for a simple two-contact interface.

A network of 1-Wire nodes via twisted pair wire can be configured in a master and slaves arrangement since each node has a unique address. This report discusses 1-Wire circuits with Maxim-IC chips for measuring various environmental parameters such as humidity, temperature, barometric pressure, wind direction, solar radiance ,

The iButton, also from Maxim-IC, consists of a computer chip, ROM, EPROM, and EEPROM in a stainless-steel case. The lid provides the data line connection and the base the ground. It communicates using the 1-Wire protocol and physical contact between the can and a reader. Two speeds are available: 16kbps and overdrive mode at 142kbps.

The primary application of iButtons is to provide a compact authentication device. An ID number is etched in the can and can be stored in the EPROM. A number of different versions of iButtons are now available such as those that act as temperature sensors.

The 1-Wire API for Java provides classes for communicating over a 1-Wire interface and for talking to iButton devices.

The TINI system also includes a 1-Wire interface to provide for remote web access to individual devices connected to a 1-Wire network.

References and Web Resources

• These books give more details on the javax.comm capabilities:
  • Java I/O by Elliotte Rusty Harold, 1999, O'Reilly. Author's site at www.ibiblio.org/javafaq/books/javaio/
  • The Java Cookbook: Solutions and Examples for Java Developers by Ian F. Darwin, 2001, O'Reilly. Author's site at javacook.darwinsys.com
• Installing Java Tools - PracticalEmbeddedJava.com - good advice on installing javaxcomm
• Java Communications API at java.sun.com/products/javacomm/
• javax.comm specifications at java.sun.com/products/javacomm/javadocs/
• javax.comm for Linux
  • Kevin Hester
  • Java Comm Serial API How-To for Linux
  • RXTX
• USB
  • jUSB: Java USB at jusb.sourceforge.net
    • Specifications for javax.usb API
  • RCX Java API (current address - www.escape.com/~dario/java/rcx/) - Provides classes and interfaces for handling and supporting communication with the Lego Mindstorms robots via serial and USB ports.
  • The Java Community Process(SM) Program - JSRs: Java Specification Requests - detail JSR# 80 - javax.usb specifications in development
• iButton & 1-Wire interface:
  • Overview of 1-Wire Technology and Its Use - Maxim-IC
  • App Note 74: Reading and Writing iButtons via Serial Interfaces - Maxim-IC - technical details of the 1-Wire interface.
  • Tech Brief 1: 1-Wire Net Design Guide - Maxim-IC
  • 1-Wire™ Weather Station - Texas Weather Instruments
  • iButton Overview
  • iButton: 1-Wire API for Java - classes for using 1-Wire communications and talking to iButton devices.
  • iButton Apps Search at Maxim-IC
• Serialio.com - this company offers commercial classes to run Java serial I/O on over 26 platforms.
  • USB-Serial adapter
• RXTX : serial and parallel I/O libraries supporting Sun's CommAPI

Most recent update: Nov. 3, 2006