Whats Better - aliasing in driver, or putting addresses into the tag in the client?

SCADA_Dude

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Feb 2008
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I am just curious on some opinions from the experts out there.

I have seen some people put tag addresses in the HMI tag database and let the driver route the data, and some people enter the tags/addresses into the driver and use the tagname as an alias to the data in the OPC server/driver.

I wont say which one I prefer because I dont want to influence others' comments. But I will say I think one method is far superior than the other. Others I work with are not in agreement with my opinion on which way is better. :p

Opinions please!
 
I generally prefer not to have my HMI's doing a lot of thinking, therefore I prefer whenever possible to map straight to the tag in the PLC.

I do however understand how building a tag database in the HMI can be beneficial. It just seems to me this would be a slower process.
 
I almost always build the tag database in the HMI, even when the HMI application supports direct referencing to the comms server/PLC.

Several reasons, some of which being that usually, the HMI can provide limits, is self documenting (assuming you take the time), can be forced to read a value in a specific fashion (sometimes, you really DO want to read a floating point as a simple 32 bit integer, for example).

Another reason, is that I often display the same information on several screens... if I want to put a bit of filtering on some value, I can just change the PLC reference in one place for all screens, rather than multiple places.
 
I don't see how you could absolutely say one way is better than the other. Much depends on the packages you're using and the architecture. That said, I strongly favor the latter when dealing with larger projects. With small projects it's simpler to cut the middleman.

When dealing with more than one HMI interface, you're going to want your setup to behave as much of a "black box" that allows changes in one place as RDRast suggested. This allows data caching, helps against race conditions, and in general is done better in software than PLCs can handle.

That "black box" approach can happen at multiple "layers". As RDRast suggested, it could be using a central tag database in one HMI package so that multiple objects refer to that tag database instead of direct PLC registers. Taking a step back to a distributed system, centralizing addresses in the OPC Server consolidates that farther for all client computers requesting those tags. That's not to say that a good OPC Server wouldn't automatically dynamically combine requests - but your latter suggestion forces it.

There are limited cases where it might be more efficient to go to the PLC directly. Writes come to mind. Or if you had a tag that needed to be read with different rounding/scaling and could guarantee that it wasn't shared. Even that's questionable - making special exceptions for minor things tends to complicate things.

For "real world" scenarios, my preference depends on which packages have the best .CSV import/export tools to crank out the project. Minimize your workload and set it up so that you can make changes in the fewest number of locations. That is, you really don't want to have to change tags all over the place that all point to the same thing.
 
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