Issue 107 June 1999
A Web-Based Chart Recorder
WEB
BROWSER 101
When
a browser is given a URL, it first breaks down the URL
into its component parts [2]. The protocol specifies
how the browser should get the requested document, the
server part specifies where it should get the document,
and the document part specifies what document to get.
The
browser sends a request (usually HTTP) to the server
asking for the document [3]. After the browser has the
document, it looks inside it to see if it contains embedded
images or applets. It then generates requests for each
of these embedded elements.
When
it has all of the document and the embedded pieces,
it renders the result onscreen. So, if an HTML document
has some text and two embedded pictures, it requires
three HTTP transactions to properly render the document:
one request for the base document and a separate request
for each of the embedded pictures.
DEVELOPMENT
ENVIRONMENT
The
beauty of this project is the lack of PC software. After
the application is downloaded into the embedded system’s
flash memory, the only PC software that is necessary
is a standard web browser.
The
embedded application was developed using the GNU/EGCS
Coldfire cross-compiler running on a Windows workstation
[4]. The standard Netburner web server and RTOS were
used as is.
Some
of my code samples are specific to the Netburner environment,
but the concepts are the same for any embedded TCP/web
server product.
Using
the Netburner environment, the development steps are:
•
copy one of the development examples into a new directory
• add the desired HTML pages
• add the desired program functionality
• compile and compress the code and HTML files
into a flash image
• download the flash image to the ColdFire board
using the Ethernet link
• restart the Coldfire board
• test the result
• repeat steps 2–7 until done
After
the code is deployed, it’s also possible to update
the running code over the network without having any
physical access to the unit. I believe that good, rapid
development is an iterative process and the speed of
your tools is important.
Using
the EGCS/GNU tools and Ethernet for the flash download,
the entire recompile (compress and download process)
takes about 30 s (on a P-II 450).
