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Trail: Essential leJOS classes
Lesson: Controlling the Hardware

Controlling the Hardware: The IR Interface

The IR Interface located at the front of the RCX serves the purpose of receiving and sending infrared data from resp. to the LEGO® Tower or another RCX.
All changeable data like the firmware, programs or commands from the PC to the RCX are transmitted that way. Having the advantage of making the RCX independent of any cable stuff, the deranged communication between the IR tower and the RCX when the interface goes out of scope of the tower is the major drawback of such a solution.

The Serial class

The basic class for communication between the RCX to the tower or another RCX is josx.rcx.platform.Serial.
As usual with hardware related leJOS classes, Serial is static; hence a typical call would be

There are two methods available to configure the Serial class for the actual needs: which sets the sensitivity for transmitted signals up or down. The default setting is the latter one.
So if you encounter communication problems with your robot, be sure to set the long range at first.

Note that the Serial class can only be used on the RCX! If you want to send or receive data on the IR Tower, consult the according communication trail.

Sending and receiving data

Serial is intended to be used for transmission of byte data, so you won't be able to send other primitive Java types, let alone more complex types.
The method to read incoming data is

            public static int readPacket(byte[] aBuffer);
If incoming data is available (you can check this using Serial.isPacketAvailable()), these are filled into the argument buffer, which has to be instantiated before, and delivers the number of bytes actually read.

Note that the first incoming byte from the IR Tower always is the opcode. For a description of the protocol used see Kekoa Proudfoot's Opcode Reference.

The method to send data to the IR Tower or to another RCX is (not very surprisingly)

            public static boolean sendPacket(byte[] aBuffer, int aOffset, int aLen);
which sends aLen bytes in the argument buffer, starting from aOffset; it returns false, if the packet has already be sent.

Note that there is no confirmation of the addressee that the message was actually received.
Note also, that the IR Tower will reveice only responses to messages it has actually sent.
A much more elegant way for receiving data is using Java's event listener technology:
Your class to listen for incoming data event will implement the public void packetAvailable(byte[] aPacket, int aLength) method of the josx.platform.rcx.SerialListener interface:

            public class MySerialListener implements SerialListener {
                public void packetAvailable(byte[] aPacket, int aLength) {
                    // handle incoming data here
                } // stateChanged()
            } // class MySerialListener
Now you are able to add this class as a listener to the IR interface:

Every time the IR Interface receives data, the packetAvailable() method of the MySerialListener will be called, where the aLenght bytes of data are contained in the aPacket argument.

If you are unfamiliar with event listeners, feel free to consult the section of The Java Tutorial™ in question.

Examples that use the Serial and the SerialListener class

are the classes in examples/serial, examples/serial2rcx and examples/serial_listener_test sections of the leJOS tree.

The Serial API

may be found here.

The Serial class is a rather basic part of leJOS communication. There already exists a much more elaborate communication layer, contained in the josx.rcxcomm package. See the Trail on communication in the specialized section of this tutorial.

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