Also, to use picocom as suggested in the Wireless Sensor book, download and compile from source. The version available in Ubuntu is stale (1.4 vs. 1.6) and doesn’t support the –echo command.

Finally, note that the ATDL address for the two chips should be that of the other chip. You are specifying the destination address.

Got my two ZigBee’s to talk to one another!

I don’t want to debate whether past efforts (e.g. Basic Stamp) have been accessible or inaccessible, but a newer effort has certainly garnered a lot of attention in recent months: Arduino. Not that Arduino is new; if anything, they are merely refining their designs these days.

While I’ve had a couple of Arduinos for over a year now, I haven’t really done anything with them lately. The processors in Arduinos are limited, 8-bit Atmels that harken back to an earlier, more limited age of computing. One could argue that is what makes them an good basis for an introduction to electronics and programming, but I don’t think that is what makes them so special, as there are plenty of similar efforts. Instead, I think that Arduino has hit critical mass. For some reasons – economic or artistic, a cottage industry has grown up around them, and they seem to be (by popular mention) the dominant hardware hacking platform of the day. This largely online community is now largely self-sustaining.

I’ve made a couple of Arduino shields (daughterboards), but lately they have lain idle. Robert Fauldi‘s recently book, Building Wireless Sensor Networks, renewed my interest in using them in conjunction with ZigBee radios, low power transmitter/receivers that are a perfect match to the Arduino philosophy. There are many ZigBee shields that allow you to connect an Arduino board to the radio, and Fauldi’s book can walk you through not only that, but several example uses of increasing complexity.

In the weeks to come I’ll be documenting my own experiments. Stay tuned!

First, I assume that you’ve installed a fairly recent version of Erlang and that erl and erlc are executable from your prompt.

1. Edit hotswap.erl

   Bump the version number in -define(VERSION, 3).

   Edit the message in the io:format to something distinctive.

2. Compile the module

   erlc hotswap.erl

3. Start a Erlang shell. Here I set use two command-line options, sname and setcookie. Sname sets the “shortname” of the node; setting the node’s name is necessary if I want to connect to it later (and we do, so we can update the code running in it). Setcookie is a security mechanism used in connecting to nodes. Setting it like this on the command-line is an unrecommended practice but necessary if you don’t have a cookie file.

   Note that the values for sname and the cookie are arbitrary.

   erl -sname node1 -setcookie foo

4. In the shell, start the server and test. You should get a response back formatted according to your io:format line. After you’ve tested this do not exit the Erlang shell. We will use it again in a minute.

   Pid = hotswap:start().

   Pid ! {echo, "foo"}.

5. Open another command-line shell/terminal.
6. Repeat steps 1 & 2. Try to edit the io:format string to make it visually distinctive from the current value.
7. Connect to the current (remote) Erlang Shell. It’s called a remote shell even it isn’t on another machine because it is running on a separate node. By default, if you start an Erlang shell you’re starting a new node.

   erl -sname node2 -remsh node1@MACHINENAME -setcookie foo

   where MACHINENAME should be replaced with your host’s name.

8. In the connected shell perform the code swap

   code:load_file(hotswap).

9. Test the change in the original Erlang shell. The first echo will likely show the existing message, while the second will show the new message.

   Pid ! {echo, "foo"}.

   Pid ! {echo, "foo"}.

So, in the words of Bill Murray, it’s got that going for it.

“This is one reason the source code of Arc itself is so short, incidentally. I did the same thing to it. But my first priority was making applications shorter, not the language. There are features, most notably Prolog-style pattern-matching, that seem to promise great savings in length, but turn out only to be useful for writing a few basic sequence operations like append, remove, and so on. Prolog is a great language for writing append; after that it’s all downhill.”

Ouch! Still, his arguments make for a good read of language design motivation and issues.

package fibonnaci;

object RecursiveFibonnaci {

    // Fibonnacci using if/else.  Note implicit return
    def fib(n: int): int = {
        if (n == 0)  0
        else if (n == 1) 1
        else fib(n-2) + fib(n-1)
    };
}

That’s a simple example of the Fibonnaci function with and .