MickeyBob wrote:
I want to develop a web application that displays real-
time data. I'm thinking that a typical web form would
have between 25 to 75 values that needed to be updated
every second or so. The original data source would be an
OPC (i.e., OLE for Process Control) server (preferred) or
a database (e.g., SQL Server).
I've surfed all over the web trying to figure out 1) can
this be done, 2) should it be done, and 3) what's the
best way to do it. Some of the options I've found are...
1. Client side scripts calling web service on a periodic
basis
2. Implement an active channel
3. Custom control that links directly to server or
database
We successfully implemented live OPC data in a Web page using an OPC
Client ActiveX control, and the results are impressive (0.1 second
updates for groups counting over a 100 incrementing items). The
bottleneck is not the OPC DCOM link, not our ActiveX, but the HTML
document object that needs to be continuously updated. So a bit of
consideration in designing the web form, and performance measurement to
vaildate each design choice is the key to success. Things aren't too
well documented in this respect, so expect some amount of frustration there.
For instance we had to revert to a mix of VBScript and Javascript to
support all the goodies we wanted (events, reading and writing of OPC
array items, ... )
We chose this approach for
(1) its performance (you can't rival this with web services),
(2) the feasibility to do all this on both Windows CE and NT (the Web
pages are displayed both locally on the WinCE automation device, as
remotely on regular Windows 2000/XP clients)
(3) its support for remote control without development overhead,
(4) the match with the developers available for this project,
and (5) its maintainability (OPC server, Web pages and ActiveX control
are all on the OPC device, so you only need to upgrade files there).
My input.
If you need more information, please let me know,
Koen.
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