Your First Brick of leJOS (for Linux)

The following instructions, which refer to the Linux platform, will help you to run your first leJOS program.
We presume that you are familiar with the Java language on an elementary level at least. If this is not the case, we encourage you to consult the first trails of the The Java Tutorial provided by Sun Microsystems.

Besides the LEGO MINDSTORMS RCX, the associated IR Tower and the leJOS environment (see below), you will need:

The Java^TM Platform, Standard Edition. We propose to download it from http://java.sun.com and to consult the associated installation instructions (make sure to download the SDK, not the JRE!).
Note: if there's a Java SDK installed on your machine already, you may use it - any version from 1.1 to 1.5 will work.

That's all you need except for the leJOS environment.

Download of the leJOS environment

You can find the actual version of the leJOS environment from the Download section of http://www.lejos.org.

Installation of the leJOS environment

After you have downloaded the leJOS environment tar file, extract it to an arbitrary directory.
Add the SDK's bin directory to your PATH environment variable.
Set the LEJOS_HOME evironment variable to the directory you installed lejos into:
export LEJOS_HOME=your lejos directory
Add leJOS's bin directory to your PATH environment variable:
export PATH=$PATH:$LEJOS_HOME/bin
Add the leJOS classes to your CLASSPATH environment variable:
export CLASSPATH=$CLASSPATH:.:$LEJOS_HOME/lib/classes.jar:$LEJOS_HOME/lib/pcrcxcomm.jar
Set the RCXTTY environment variable to your 'tower' device:
export RCXTTY=com1 or export RCXTTY=/dev/usb/legousbtower0.
You might need to adjust the execution permissions in the bin and in the release folder.
Open a shell and change to the directory you installed leJOS into; type ant. This will build the leJOS executables and libraries on your Linux machine.

leJOS-enable your RCX

To enable your RCX for the execution of leJOS programs, you will have to replace the original LEGO^® with the leJOS operating system. This is easily done:

Place the IR sensor of the RCX in front of the IR Tower
Turn on the RCX
Open a shell and change to the bin directory of your leJOS installation
call firmdl
the progress of the download is displayed in the command shell and on the display of the RCX; the RCX will double beep when the download is complete and display the battery voltage

If the download doesn't work, make sure that

your RCX is turned on
your RCX's IR port (the dark panel on the front) is facing towards the IR tower
your RCXTTY environment variable is set accordingly

The tower should blink green, when you start lejosfirmdl.

The original LEGO^® firmware isn't deleted from the RCX, but stored away. Hence you may re-activate it by just removing the batteries for some minutes and replacing it.
The leJOS firmware will be lost in this case, though, and will have to be downloaded again to be useable on the RCX.
Note that you have to install the leJOS firmware only once (unless removing the batteries); nevertheless it may be downloaded as often as you wish, but the leJOS programs you have installed will be lost in this case.

Create and Run your first leJOS program

There's a simple "Hello World" style example program available in the checkDistribution folder of the leJOS distribution:


import josx.platform.rcx.*;

public class DistributionSmokeTest
{
  public static void main (String[] args) throws Exception
  {
     LCD.clear();
     TextLCD.print ("SMOKE");
     Thread.sleep(1000);
     TextLCD.print ("TEST");
     Thread.sleep(1000);
  }
}

To run this program, you will:

Open a command shell and change to the check directory of your leJOS installation
Compile it using lejosjc:
lejosjc DistributionSmokeTest.java
Link and download it to the RCX using lejos:
lejos DistributionSmokeTest
the progress of the download is displayed in the command shell and on the display of the RCX; the RCX will double beep when the download is complete and displays a standing man
Press Run to start the program; The words "SMOKE" and afterwards "TEST" will be displayed on the RCX

If lejosc or lejos isn't found, make sure your PATH variable is set accordingly.
If the leJOS classes arent found, check your CLASSPATH variable.

Congratulations! You have run your first leJOS program on your RCX.

Troubleshooting

One of the steps above did not work for you? A look into the Common problems section might help.

Yet, if it doesn't, feel free to post your question to the lejos discussion list.

The leJOS Tutorial

Start of Tutorial > Start of Trail