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Accessing Devices Using a Web Service
Manufacturing News Center
Abstract--January 22, 2004-- The old days when embedded devices
and factory floor machines had only minimal interaction with humans,
the on/off button and little else, are gone. Today the ability to
access the device from anywhere is expected. This is a significant challenge
when we are sweating over every dollar required in hardware. With a
little bit of knowledge and a relatively small piece of software, we
can provide a Web service for this type of interaction.
Introduction:
There are at least four questions we need to address to examine the
issue of remote access:
- Who needs access? The manufacturer of the machine, the user of the
machine or both, other?
- What will they do? Gather status/historical information, perform maintenance,
updating or troubleshooting.
- How will access be provided? Proprietary protocols, open protocols
or a mixed set.
- When will access be allowed? User controlled physical access, open
access with appropriate security measures or something else.
In general the answers to these questions are:
- Both manufacturer and the user will need access.
- The user will access the machine primarily to gather status/historical
information (i.e. listeners).
- The manufacture will use their access for all the stated reasons (i.e.
controllers).
- We will use the most ubiquitous and cost effective transport possible,
TCP/IP on top of Ethernet (probably of the 10BaseT variety).
Lean
Manufacturing. A Plant Floor Guide.
I strongly favor a two-channel architecture. This is:
- Use a TCP/IP socket interface with an unknown TCP port and a proprietary
application message protocol to provide control. This is both the most
secure and the best performance solution possible. In addition it is
a small footprint/low overhead approach that can be supported even on
a slim platform.
- Use open technology (HTTP and XML) to address data gathering/surveillance
requirements. This is a “read only” channel thereby eliminating most
security and performance issues. (In the parlance of the time this is
a bare bones web service.)
A typical factory floor machine controller might look like:
The second channel is what will be explored in more depth in this paper.
The normal means of achieving this capability is by using a full function
Web Server with extensions to provide an interactive exchange between
the client and the server, e.g. Microsoft’s IIS with ASP, or Macromedia’s
ColdFusion with JSP etc. This is a complicated solution that can prove
to be costly and difficult to administer and secure. For an embedded application
it is like driving a finishing nail with a ten-pound sledgehammer. Luckily
there is a straightforward solution that does not require the sledgehammer.
Even a rather cursory knowledge of HTTP and XML enables a software engineer
to develop a secure application specific solution with the smallest possible
resource demands. The basic facts are:
- The transport for HTTP is a TCP/IP socket that uses TCP port 80 on
the server.
- HTTP is a simple text based protocol, most of which we can ignore.
- XML is a text based mark-up language that is easy to understand and
create. It does not have to be any more complicated then we want it
to be.
We need a TCP/IP stack, a socket interface to the stack and a relatively
small piece of software, the Web Service Interface, and we are there.
We will travel down the road of developing this interface one step at
a time:
- Understanding HTTP requests.
- Understanding HTTP responses.
- Understanding an XML document.
- Defining requests and responses.
- Implementing a simple Web service.
Read the remaining info of the article by Terry Ess at Dev Articles
http://www.devarticles.com/c/a/Web-Services/Accessing-Devices-Using-a-Web-Service/1/
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