Hello!
Sounds like you're doing something interesting, but there aren't many details. The task you're trying to do is quite simple for a PLC, but the solution can vary widely depending on the environment you want to put this in. Without info to narrow it down, here's a whole range of ideas...
I suggest finding a couple local control system integrators in your area (even if they're small 1 to 10 person shops), invite them over to take a look at your requirements, and ask them for a fixed price quote. They'll generally quote the hardware they're most familiar with, which is good...
No matter how you do it, you need to allocate one word of memory to store the value from the previous scan.
Assuming you're using the latest version of RSLogix 5k, I think you can actually write your own instruction that takes the watched value and a memory location as parameter.
Scott...
Looking at that document, there seems to be some parameters listed in page 131, though I admit it's a bit vague. It looks like you need to do at least 3 MSG transactions with the web server card just to send one UDP message (CreateSocket, OpenConnection, Write). I would assume that once the...
Consider the fact that you don't have to use OPC to communicate with the PLC. There are some 3rd party tools which communicate in the PLC's native protocol, and are much faster, not to mention simpler to use.
I suggest that you consider the In-Gear components from Cimquest...
I have found that the tutorials on this site are quite good to get people started: http://www.plcs.net/contents.shtml
Scott Whitlock
PLC Programming Articles: http://contactandcoil.com/