thgood

I’m a software engineer who works with our PLC team, so please bear with me!    We log data from the PLC into our ERP application.  The PLC team provides an L5X file which my application parses for tag names, we then choose individual tags to poll within our user interface.  The application automatically generates the Eweb XML view for the chosen tags, the point here is that it doesn’t require any PLC programming.   This works great for polled data like temperatures read every 5 minutes via Eweb, but it doesn’t work for event-driven data like alarms or detecting state change for a tag. I’m looking for a way for my application to register (subscribe) as a listener for an event without requiring PLC programming for each additional event we want to observe. Upon state change the PLC or middleware just needs to send some type of outbound message, for example, post via http (rest), connect to a socket server or send a message via MQTT within a couple of seconds... We have 8 Eweb cards, all 1756 series controllers at multiple locations and we currently poll them from a central server, so the hardware overhead is minimal.  Is there some functionalized code/ladder logic that could act as a standard event listener within the PLC controller?  For instance, could my application write tags I’d like to monitor to a string array via eweb write, then have some standard PLC code iterate through the array looking for changes in the underlying tags?  Since we’ve got polling working so well, we’d like to avoid a separate device / local middleware server just for events, but perhaps its required? All recommendations welcome as I don’t think I have a grasp of what’s possible or practical!