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What is AIRcable OS?

AIRcable Diagram        AIRcable OS is the operating system and programming environment that runs on AIRcable's line of programmable Bluetooth devices known as "motes", or "mesh nodes". In the words of AIRcable themselves:

       "Revolutionary in design and function, the AIRcable OS products unite three cutting-edge technologies -- programmable, autonomous mesh nodes (motes), with data loggers and industry-standard Bluetooth® 2.0 compatibility.

Based on our AIRcable operating system, these versatile devices gather data using intelligent sensors, run computations, transfer files, and communicate using Bluetooth technology. These units can be clustered together to create a low-power, highly flexible mesh communications network that can interact with other nodes and with other Bluetooth compatible devices such as PCs, cell phones, laptops, and handhelds."

Basically, AIRcable OS devices are tiny embedded computers (complete with read/write filesystem) coupled to a Bluetooth radio. You write a program for the device to run in BASIC, which is uploaded into it's filesystem over OBEX FTP. The BASIC software controls everything from the Bluetooth radio, to sensors that you can attach to the various I/O pins connected to the processor. Even though you are limited to BASIC, the AIRcable OS implementation is considerably more powerful than you might expect, supporting basic forms of multi-tasking, with rudimentary hardware and software interrupts. For example, there are interrupts that trigger when a client connects to the device or if a new Bluetooth device has been discovered. Your software can use these interrupts to stop what it is doing and immediately process a new set of instructions relative to that particular event.

Perhaps best of all, AIRcable OS wraps up all the complicated bits like transferring files over OBEX and pairing with other devices into simple to use BASIC commands. AIRcable OS does all the hard work of actually interfacing with the Bluetooth stack, leaving the programmer with a much simpler task. You don't have to know how the AIRcable device scans for discoverable devices, all you need to do is run "inquiry", and the discovered device's name and MAC pops up in a global variable ready for your program to process.

While a lot of people might be groaning at the thought of returning to BASIC after being spoiled by powerful languages like C/C++, using BASIC does have it's advantages. AIRcable has done a good job packing their OS with enough built-in functions to get almost any application rolling in a relatively short amount of time, so you don't need to hunt around for any additional libraries like you do on other embedded platforms.

You don't need any sort of compiler or cross-compiling development environment to get working with AIRcable OS, any basic text editor will work just fine. In fact, you can write software and program an AIRcable device from most modern cell phones. All you need is a basic text-editor application and support for OBEX FTP. In a pinch I have used my Android phone to pull down the current BASIC file from a running AIRcable, edit it, and push it back.

All in all, considering the type of work the AIRcable devices are expected to be doing, the programming environment is robust enough to get the job done while still being simple enough for even a person without much programming knowledge to understand.

AIRcable BASIC Structure

       I sometimes forget that not everyone started programming back when BASIC was about as advanced a programing language as the home user could expect to ever see like I did. I still have fond memories of getting my Apple II to conditionally flash lines of text for the first time. Today, a lot of people might not even know what a BASIC program looks like, let alone the little nuances that I would take for granted. Of course it doesn't help that BASIC is specifically avoided in most beginning programming courses, as it teaches some admittedly dangerous concepts that you need to be careful of. So with that in mind, let's take a step back and look at some basic...BASIC.

Code example

The first thing you need to understand is that every line in the program has to have a line number proceeding it. You will want to be careful when you first start writing BASIC programs, as the logical thing to do is simply to number each line in sequential order. That is fine, until you realize that you need to add in an additional function between lines 6 and 7...of a 200 line program. It is therefore extremely important, especially when you are just learning the programming language and syntax, to number lines at least by 5's, and ideally by 10's. So now you have the capability of adding 4 - 9 additional lines between each of the original lines if the need arises, and it almost certainly will. The only reserved line number is zero, which is used for comments in the code. These comment lines will be ignored by the AIRcable device, but are extremely useful for documenting the code and helping yourself remember why you did certain things or how some function is supposed to work.

After the line number, the actual code begins. If you are looking at AIRcable's example programs, you will probably notice that almost all lines start with something like "A =", the function of which might not be immediately obvious. What that part does it save the output of the following command to the variable A. Not that "A" has any specific importance, it could be any variable you wish. What this allows you to do is examine the contents of "A" after a line has been executed to see what was returned. In the following example, we see how the return variable from "exist" is evaluated. If "exist" returns anything other than a "0" (meaning the file exists), the program branches execution to another line.

Return example

Return variables are the only way to use commands like "exist", as they have to return some information to the program. We don't have the luxury of a screen to dump output to, or a standardized system for return values, so we need to capture them as they happen. But even for commands that don't necessarily need to return data to function, return variables can be useful in debugging. The AIRcable OS's shell allows you to check the current contents of any variable as the program is running, which can be very helpful when trying to track down that one command that doesn't seem to be working the way you expected.

Finally, the last part of the line (shown as red in the examples) is the argument to the command. In the first example you can see we have executed the "slave" command with the argument of 5, which means the AIRcable device will wait for an incoming connection for 5 seconds. Almost all commands have at least one argument, so make sure you check the AIRcable documentation to see what the appropriate argument is for each command. Without the correct argument, the code isn't going to work and you might not realize at first what the problem is.


       AIRcable OS has a number of limitations imposed on the complexity of the code you can write, which you are going to want to get familiar with before you go any farther. In this image from AIRcable's documentation, we can see the vital statistics of the programming environment.


As you can see, you are limited to a maximum number of 2047 lines. This is not that big of a deal in terms of raw source code, but keep in mind that in some cases you might want to store data in the program as individual lines. It is much easier and faster to load lists of information from inside the program itself than from an external file, so it could be that your code plus data goes over the 2047 line limit. Take note of that 32 character line length limit as well, complicated expressions could go over if you are not careful.

The variables are probably what most people are going to be concerned with. You are limited to 25 total variables, the letters A through Y. There are no variable names, you just have to use the available letters. 25 individual variables isn't too bad for most applications, but the fact that you can't name them is going to be a painful change of pace for many programmers. You need to make sure you are commenting your code carefully so you can remember what does what. The variable length limitation can be tricky as well. For example, parsing NMEA sentences from GPS devices needs some special handling.

Perhaps the biggest limitation under AIRcable OS is speed. AIRcable devices execute code at an average speed of 10 lines per second (maximum is 250 lines per second). It is hard to put into words how slow this really is compared to even a very old x86 computer, but let's just say that things you expect to be instantaneous on modern hardware could very easily grind AIRcable OS to all but a complete halt. I've found out the hard way, I wrote a program that had a buggy subroutine which was executing constantly. The hardware was so bogged down I had trouble getting the OBEX FTP service to even respond when I tried to send a fixed version.

Tips and Tricks

       While most of the this is pretty straight forward for anyone with any BASIC experience, there are a few AIRcable OS specific concepts I would like to point out.

First is the immensely useful debug function. If you put the line "Z = 1" in your program's @INIT routine, AIRcable OS will output every line of your program to the UART upon execution. This is invaluable when trying to figure out why your program isn't behaving quite as you intended it to. The debug mode even shows the line numbers, so you will always know exactly where the program is at any given time.

You will probably notice that all programs have an @ERASE routine at the very beginning. This is very important, and you need to remember to include it when you are writing your code. What this does is clear out the old program code before loading your new code into memory. If you don't have this line, your new and old code can get jumbled together, which is almost certainly not going to go well.

While AIRcable OS is pretty limiting with your variables, it does let you do some interesting things with your strings. Under AIRcable OS, strings (other than the $0 system string) are actually lines of code in your program; any changes you make to these strings are written immediately into system memory, which essentially means you can create software that dynamically rewrites itself on the fly. Your code can "evolve" in real-time without rebooting the AIRcable device, which could have some interesting applications in mesh networking.

On the other hand, this also means you always need to allocate enough lines in your program to store any data you might collect during execution. If you are logging something to strings starting at line 350, you need to make sure you don't accidentally write your program too large and cross over into the data area. Otherwise, your code would start getting deleted as soon as data began getting logged (and would surely crash soon after).

Finally, a little note which took me awhile to figure out myself. To enable reading and writing of the AIRcable device's internal filesystem to your program, you need to modify the "security" line in your config.txt file to read: "@0006 = 0000 0000 0000 0000". This turns off all of the filesystem safeguards, which seems like it would be unsafe, but this is the method that AIRcable support advises.

Uploading Code

       Assuming you have come to terms with using BASIC and the AIRcable OS-specific peculiarities that entails, it's time to get code uploaded into your device.

AIRcable devices have internal storage that can hold up to 4 files, two of which are required by AIRcable OS to function. One is "AIRcable.bas", which is the program the firmware will load when it boots, the other is "config.txt", which is a plain text configuration file that holds various system parameters such as PIN and device name. AIRcable OS will load only "AIRcable.bas", so you need to make sure your filename is correct. It is possible to load a second .bas file from inside of "AIRcable.bas", but we will get into that later.

Files are uploaded into the AIRcable device via OBEX FTP. You can configure a few parameters for the OBEX FTP service from the command line, but by default FTP will be running for 2 minutes after each boot. All you need to do is power up your device, and run a few commands like:

bash:~# hcitool scan
Scanning ...
	00:50:C2:7F:XX:XX	AIRcable Mini
bash:~# obexftp -b 00:50:C2:7F:XX:XX -p ./AIRcable.bas
Browsing 00:50:C2:7F:XX:XX ...
Sending "./AIRcable.bas"...\done
Uploading code is pretty slow, and the transfer may appear to stop early on due to the AIRcable device erasing it's flash storage to load the new code. Just let it run, a large program can take up to 30 seconds to transfer completely.


       As an example, here is a program I wrote that allows you to use your AIRcable device to scan for discoverable Bluetooth devices and log them to a file autonomously. This program makes a good example as it shows a large array of AIRcable OS functions, from reading and writing files to conditional statements. I tried to be as descriptive as possible in the comments, so it shouldn't be too hard to follow along.

To use this code, simply upload it to your AIRcable OS device as described in the previous section. Once your device reboots, it will immediately begin scanning the area for discoverable devices and log them to the file "found.log". Entries will be written until the file reaches 1000 bytes in size, to prevent filling the device's storage. However, I am still a little unclear as to how much space is actually available on the AIRcable devices, so it is possible this value could be raised considerably.

A single blink on the LED signifies the program has detected a discoverable device, and a double blink immediately afterwards means it is a new device and has logged it to the file. Scanning is a bit slow, and the storage space is limited, but this gives you an idea of the sort of things that are possible.


0 REM AIRscanner Version 1.0
0 REM Simple Bluetooth scanner written for AIRcable OS

0 REM Set LED on
6 A = pioset 20
0 REM Baud rate for serial line
7 A = baud 96
0 REM Start of result table
8 L = 900
0 REM Result Index
9 M = 0
0 REM Debug toggle
10 Z = 0

@IDLE 15
0 REM Booted OK, turn off LED
15 A = pioclr 20
0 REM Delete old log, should only be done once
16 A = delete "found.log"
17 ALARM 1

0 REM Use ALARM to keep INQUIRY running forever
20 A = inquiry 10
21 ALARM 1

0 REM Single LED blink to show device found
25 A = pioset 20;
26 A = pioclr 20
0 REM Compare result to current table
27 GOSUB 50
0 REM Check table size
28 GOSUB 100

0 REM Search result table
50 FOR C = 0 TO M;
0 REM If $0 matches string in table, go to RETURN
51 B = strcmp $(L+C);
52 IF B = 0 THEN 55;
0 REM If we get here, then there was no match, so write to table
54 GOSUB 60

0 REM Write current hash value to table
60 $(L+M) = $0
0 REM Increment table index
61 M = M + 1
0 REM Double flash to indicate new result
62 A = pioset 20;
63 A = pioclr 20
64 A = pioset 20;
65 A = pioclr 20
66 GOSUB 110

0 REM Check size of table, every 100 devices it will start back from 0.
100 IF M > 100 THEN 102
0 REM If under 100, do nothing
0 REM Getting a little big, let's reset
102 M = 0

0 REM File handling code
110 A = exist "found.log"
111 IF A <> 0 THEN 114
112 F = open "found.log"
113 GOTO 115
0 REM File exists, so we start at end
114 F = append "found.log"
0 REM Add newline
115 PRINTV "\n"
0 REM Find bytes length, write to file, then close
116 B = strlen $0
0 REM Before we write, check the file size
117 Q = size
0 REM If file is too large, we just have to let the results go :(
118 IF Q > 1000 THEN 120
0 REM File size is OK, write the data then close the file
119 F = write B
120 F = close