Web Interface
As we said before, to communicate with the sketch from the outside of Arduino’s “domain”, we took advantage of the “other half” of RandA, that is, Raspberry Pi and the software tools made available by the Linux world. In particular, among them is the Apache Tomcat program: it is a web server, or better a web application server since in addition to managing all the server’s functions, it enables the execution of programs (written in Java and identified as servlets in this context), on the basis of accurate calls remotely made by the client. Without delving deeper into details (the whole documentation needed to examine the various concepts in depth can be found on the Internet), we shall only say that, among other things, the usage of these programs enables the management of Raspberry Pi’s serial communication, and therefore the communication with Arduino. In our case we will take advantage of the “SerialIO” servlet, supplied during RandA’s software installation, since it is already used by other web pages (Arduino Console, Arduino I/O management, etc.) that are included in the installation. This servlet accepts the requests coming from the client (we will shortly see how) and filters them on the basis of the “cmd” parameter, in order to call the corresponding function that in turn will send the information on Raspberry Pi’s serial port, towards Arduino. Following the communication, it waits the (possible) reply coming from the serial port (and therefore, from Arduino) and sends it back to the client. To better understand the details (if you know some Java), you may naturally edit the servlet’s source (SerialIO.class) that can be found in the /home/apache-tomcat-7.0.47/webapps/RandA/WEB-INF/classes/ArduIO folder. Just keep in mind that, if you want to modify it at leisure, it is needed to deploy it again on Tomcat (talking of which, we refer you to the guide, that can be found on https://tomcat.apache.org/tomcat-7.0-doc/appdev/deployment.html).
In figure we can see the general plan of the data stream and the actors taking part in it, from the web page to Arduino’s micro. In it we may notice the SerialIO servlet’s interface function, between the web server and Raspberry Pi’s serial port (described by /dev/ttyS0).
In the same picture, we may now “move” ourselves towards the left to reach the client-side, that is to say the web page to be displayed remotely. This page is divided in two parts: a global one made in html, which is used, more than anything, for the purpose of building the introduction (that is to say, the graphics), and a javascript code section that deals with the events management (basically, “clicking” on the keys) activated by the user.
The mechanism that has been used is a very simple one: for each key in the graphical interface a javascript function is associated, and it will be activated every time the button is pressed. This function may carry out in local processing (on the client machine). In our case, in which we want to send commands to the RandA board, the function will in turn call the ajax() function that, with appropriate parameters, establishes a talk with the server; or better, with the SerialIO servlet. More specifically, the client sends a request to the server that (as we have seen) will activate a function on the servlet: this one will in turn convert it into serial communication to Arduino. The latter’s reply, by following the course in reverse order, will then be intercepted and returned by the same ajax() function (under the form of a characters string), so to be possibly displayed and to give to the user the result required, or at least an answer concerning the operation’s outcome.
