June 2006, Issue 191

Measurement System for Weight and Dimensions
Renesas M16C Platform Design Contest 2005

HOST SOFTWARE

Let’s take a look at how the Weasure works in a typical shipping application. I created a program that takes a package’s measurements and a couple of additional parameters (parcel origin and destination). It then looks up the prices of various shipping options on UPS’s web site (see Photo 4). I wrote the application in Python language because it has excellent tools for interacting with other Internet services and processing Internet-based data. Because applications in Python are portable, it should run on almost any host supporting serial port access and a web browser (Windows, Mac OS, Linux, etc.). Refer to the “Python Programming” sidebar (p. 18) for more information.

My first thought when implementing the host application was to use one of the available user interface libraries for Python to open up a dialog box to collect the user information. But because the result from the UPS site comes back as a nicely formatted web page, I came up with a much quicker solution. I used a local web page form to collect the information because I knew I’d be using a web browser to display the results anyway. Thus, rather than writing code to format and display dialog boxes, I simply used my favorite web page editor to create a web form page. The host application then became a simple web server, collecting parameters and displaying results via my web browser.

When the application runs, it launches the web browser and displays a form like the one shown in the foreground of Photo 4. The fields in the Package Size table are read via the Weasure’s serial line. When you click the Update Size and Weight button, the web server rereads the measurement data and updates the page. If you fill out the Package Origin and Package Destination fields and then click on the Get Rate button, the program will send the data to the UPS web site and return a web page like the one shown in the background.

Most of the application’s data is kept in a Python dictionary structure called UPSPostData. This represents the data posted to the UPS web site, with fields named as required by the UPS Calculate Cost and Time form. Some of these fields are constant (e.g., currency and measurement units) and some you must supply (e.g., package origin and destination). The Weasure supplies the rest of the fields.

The program uses an HTML template file for the web form. The ReplaceHTMLTemplateValues routine replaces fields delimited with anchor tags with the values in the UPSPostData dictionary read from the serial port by the UpdateMeasurements routine. (A serial port library isn’t part of the normal Python language distribution, but an open source solution is available at http://sourceforge.net/projects/pyserial/.)

When the application starts, it launches the web browser to a URL on your PC’s local host TCP address (127.0.0.1:8000). It then starts a web server listening in on that port that’s implemented in the LocalHTTPServer routine. Within that routine, the do_GET subroutine handles the browser’s initial request to display the updated template page. The do_POST method handles the requests coded into the HTML template for either updating the measurements or fetching the UPS data when you click the form’s buttons. The FetchUPSRatePage routine sends the request to the UPS web site and takes the response from there and routes it through the local server to your browser.

The complete source code is available on the Circuit Cellar FTP site. Using the extensive Internet tool kit in Python’s library and getting your web browser to handle the user interface, the host application is extremely compact—only 160 lines of code!