Issue
151 February 2003
Working
the ’Net
by
Tom Cantrell
Shouldn’t
every engineer know a little more about the process
of network-enabling his or her widget? Tom definitely
thinks so. With Tom on your side, you’ll be working
the ’Net to your advantage in no time.
Start
•
Web Site Lite • COM
Spi with Me
• Freshen Up • Burning
Down the 'Net • Have It Your
Way • Sources
and PDF
WEB
SITE LITE
I
suspect that when the marketing department bangs on
the table for networking, they usually mean to say,
"web browser enabling." In other words, the
ability to interrogate, control, upgrade, and otherwise
interact with an embedded product using a standard web
browser.
These
days, practically all embedded products have a user
interface, whether it’s a full-featured, touch-screen
display or a DIP switch and blinking LED. The user interface
even can be a PC with some kind of software making the
connection, ranging from a simple terminal program to
a custom-programmed GUI.
The
advantage of the web-based approach is compelling. In
short, it puts the user-interface ball squarely in the
user’s court, which turns out to be a win-win for everyone.
Users get to choose their favorite browsing platform,
which is typically a PC but is inexorably expanding
to include PDAs, smart phones, web TVs, hand-held games,
and ultimately just about anything with a display and
input device.
Meanwhile,
the equipment supplier is relieved of the burden of
having to make provisions for (and support) proprietary
software and hardware at the user end. The engineering
spec is simple: add an Ethernet jack and cough up a
web page. All that’s left is to figure out the simplest,
quickest, and cheapest way to do it.
NetMedia
targets this scenario with their SitePlayer, which is
notable for its laser-like focus. All nonessential baggage
is tossed overboard in pursuit of the singular goal
of talking to a web browser. There’s no socket-level
programming, e-mail, execution of user code (although
the web site alludes to a programming language in the
works), or anything else. Thanks to it’s single-minded
purpose, SitePlayer gets by with a remarkably tiny bill
of materials. It’s literally made up of two chips: a
standard Philips flash memory ’51 and a commodity RealTek
Ethernet MAC chip.
That’s
not to say such a streamlined approach doesn’t come
with some compromises. For example, because your SitePlayer-based
web site is limited to the ’51’s 48-KB on-chip flash
memory, leave your bloated eye candy at home. Nevertheless,
with care you can come up with up with an attractive
user interface that doesn’t bust the bit budget (see
Photo 2).
 |
| Photo
2—If you keep your design tight, you can get SitePlayer
to serve up an appealing interface, even with only
48 KB of flash memory to play with. |
Of
course, serving up a mere static web page isn’t that
hard nor is it very useful. You’ll generally want to
display or input dynamic data. For instance, a web-enabled
thermostat would allow the user to enter a desired temperature
as well as display the actual current temperature.
SitePlayer
makes an otherwise static page dynamic by embedding
special tags in the HTML. The tags, denoted by a caret
(^), refer to variables that pass one-way or the other
(or both ways) between the SitePlayer web server and
your embedded device server. For instance, in the thermostat
example, your web page could simply display the following
string:
The
current temperature is ^temp
As
SitePlayer dishes up the web page, it will notice the
^temp variable and substitute the current value of the
temperature variable.
SitePlayer
knows about the ^temp variable by virtue of a SitePlayer
definition file (extension .spd) that you create. Along
with various configuration parameters (i.e., the IP
address, whether or not to use DHCP or a password, etc.),
the .spd file defines the location and data type for
each variable.
On
the device server side (i.e., RS-232), the embedded
system (e.g., thermostat) updates the ^temp variable
using a simple protocol (command and data) that accesses
SitePlayer’s internal memory. In the thermostat example,
you would send the current temperature via the RS-232
to the location in SitePlayer corresponding to the ^temp
variable as defined in the .spd file. Now, the next
time the page is refreshed the current temperature will
be displayed.
Of
course, as soon as you’ve crafted a simple character-oriented
interface, the call will come down from on high to spiff
it up with graphics for a more glamorous first impression.
The SitePlayer allows you to, for example, replace a
simple numeric variable with its virtual seven-segment
LED equivalent. Photo 3 shows a web page in which var
is defined as a byte displayed as a decimal number of
up to three digits (i.e., 0–255).
|
|
Photo
3—A key SitePlayer technique is to parse the numeric
value of a variable to create a file name that references
a graphic file, in this case a seven-segment LED.
Meanwhile, the Serial Tester utility facilitates
PC-based debugging by allowing easy access to variables
and monitoring the associated RS-232 command and
response packets. |
SitePlayer
allows you to pick off individual digits using a colon
notation (:). Examples include ^var:3, ^var:2, and ^var:1.
In turn, the digits are grafted onto the file name calling
out a particular LED image file. For instance, if the
value of the variable is 234, ^var:1_LED.jpg will bring
in the file 4_LED.jpg, which is the bitmap image of
an LED showing the number four. A variety of useful
image files (e.g., seven-segment LEDs, sliders, and
knobs) come with the SitePlayer software.
As
usual, the complexity (if not the devil) is in the details.
Using Word as my web page authoring tool, I couldn’t
find a way to generate the colon syntax. Thus, I had
to edit the HTML directly, something I imagine is required
for some of the other fancy constructs as well. Also,
I was unable to come up with a scheme to blank leading
LED zeros. There may be a way for users to kludge around
it, but I would say this is a feature that should be
added to the tools.
