152.

  

Entry Objects for a Robot

The RCX was not designed for network visibility. It has no concept of identity or location. The closest it comes to this is when it communicates to other RCXs by the infrared transmitter ”then one RCX may have to decide whether it is the master, which it does by setting a local variable to "master" if it broadcasts before it receives, and the other RCXs will set the variable to "slave" if they receive before broadcasting. Then each waits for a random amount of time before broadcasting. Crude, but it works.

In a Jini environment, there may be many RCX devices. These devices are not tied to any particular computer, as they will respond to any infrared transmitter on the correct frequency talking the right protocol. All the devices within range of a transmitter will accept signals from the transmitter, although this can cause problems, because the source computers tend to assume that there is only one target at a time, and they can get confused by responses from multiple RCXs. The solution is to turn off all but one RCX when a program is being downloaded, to avoid this confusion. Then turn on the next , and download to it, and so on. Not very elegant, but it works.

An RCX may also be mobile ”it can control motors, so if it is placed in a mobile robot, it can drive itself out of the range of one PC and (maybe) into the range of another. There are no mechanisms to signal either passing out of range or coming into range.

The RCX is a poorly behaved animal from a network viewpoint. However, we will need to distinguish between different RCXs in order to drive the correct ones. An Entry class for distinguishing them should contain information such as this:

• An identifier for robot type, such as "Robo 1", "Acrobot 1", etc. This will allow the robot that the RCX is built into to be identified. The RCX will have no knowledge of its identifier ”it must be externally supplied.
• The RCX can be driven by direct commands or by executing a program already downloaded (there may be up to five of these). An identifier for each downloaded program should be available.
• The RCX will have some sort of location, although it may move around to a limited extent. This location information may be available from the controlling computer, using the Jini Location or Address classes.

There may be other useful attributes, and there are certainly issues to be resolved about how the information could be stored and accessed from an RCX. However, they stray beyond the bounds of this chapter.