Computing stuff tied to the physical world

Hacking around in software

In Software on Sep 24, 2011 at 00:01

Here’s a web page I’ve been designing recently – one of many planned for the JeeMon system I’m working on:

Screen Shot 2011 09 20 at 13 00 26

A couple of notes:

  • there are three “master” tables on this page: Devices, Interfaces, and Drivers
  • devices are a 1:N mapping between drivers and interfaces
  • … except that some devices are remote, and not tied to a local interface
  • clicking on any row of these three tables displays details in the rightmost column
  • drivers are listed but not managed on this page, that’s a software admin task

I’m describing this here for a number of reasons (other than showing off what I’ve been spending my days on).

First of all, you can see that this more or less covers what needs to be set up to in a home monitoring and/or home automation system. I like to see everything at a glance, i.e. on a single page and with as little navigation as possible. With this setup, I hope to keep it all essentially on one page. Whereby the right-side column may vary widely, depending on the device / interface / driver.

But it doesn’t really need to be about home automation at all. A very similar setup could be used to hook up to devices which I’m experimenting with in the lab, or devices which act like measuring instruments, or control some aspect of an experiment. Anything related to Physical Computing, really.

The other thing that interests me is the “degree of variety” needed to cover all the cases. Many devices will simply collect or send out one or more values, but not all devices are like that.

The RF12demo sketch is essentially a gateway from and to an entire group of RFM12B-based nodes. Each node can have its own driver. The ookRelay2 sketch is similar: it collects data from quite different devices, each of them sending out their specific packets with their specific types of measurements. It differs from RF12demo, in that it contains decoders for all the devices it knows about. There are no separate KS300 drivers, etc (although perhaps there should be, since some code is currently duplicated in the CUL driver).

The autoSketch driver is yet another beast. It listens on a serial interface for a line of the form “[…]” and when it sees one, tries to load a driver with the name in brackets and hand over control to it. This is the reason why just about all my sketches start off with the following boilerplate code:

    void setup() {

When associated with the autoSketch driver, this will automatically load a “mySketch” driver, if present. Which in turn means that all you have to do is plug in the JeeNode (JeeLink, Arduino, etc) and JeeMon will automatically detect it and start communicating with it. IOW, plug-and-play – with a simple implementation.

This is why there’s a “Use autoSketch for new interfaces” checkbox. I could have called it “Enable plug & play”.

But although this web page is functional in my test setup, it’s far from finished. The hardest part I want to get right, is to make the page completely dynamic. Right now, a click on a row will update the details via Ajax, but nothing will happen when there is no user interaction. What I want is that the page automatically adjusts when a JeeLink is plugged in (on the web server side). What I really want, is to generalize that mechanism to everything shown on any web page. The challenge is to do this without tedious periodic polling or complete table refreshes, just “pushing” changes in an event-driven manner. Events on the server side, that is.

The visual layout, styling, and behavior of a page like this has become very simple with today’s tools:

  • jQuery is the core JavaScript library
  • jQuery UI is used as basic style engine (the tabbed group on the right, for example)
  • jQuery TableSorter adds support for (surprise!) table sorting via their headers
  • Knockout is used to manage a lot of the user-facing behavior in the browser
  • 960 grid regulates the page structure flow in CSS
  • there is a small amount of custom CSS and custom Javascript (about 20 lines each)
  • the entire page is implemented as a single Tcl source file (175 lines so far)

There are a few trivial utility functions in there, which are bound to migrate to a more general module once I better understand the trade-offs and benefits of re-use. And the rest is gobbledygook which looks like this:

Screen Shot 2011 09 23 at 22 19 41

This is the notation I came up with and described in my Playing with indentation post a short while back. This particular code describes the HTML text returned for drivers in the “details” column, via Ajax.

This notation gets expanded to HTML, but the real value of it is that describing the HTML on a page is fun, because it’s now trivial to insert grouping levels (i.e. mostly <div>’s) and rearrange stuff until it’s just right.

  1. Hi

    Have you looked at websockets for your async updates.

    Also worth looking at is comet, and nginx’s push capability (I can’t remember what server you use)

    • Yes, thanks. I’m using Server-Sent Events ( It’s the simplest mechanism I’ve found so far (thinking about very low-end Linux here). I do have a nginx front-end facing the internet, but the embedded web server I use is Wibble (a Tcl package). So the problem is not really the communication layer, but getting the data structures right to support automatic change propagation.

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