Studio 5k -Mapping IO vs Directly Addressing as Alias

In Rockwell Studio5k

I've noticed allot of time people will wait till the end of a ladder to map the IO they just used and then write it to a physical IO.

What's the pros or cons of doing this versus writing directly to the tag as an alias?

It operates the same from what I know.



Thanks!

Usually there are more operations on the tag before addressing the output and it also helps to locate all the outputs when making change or troubleshooting. I actually do not address the outputs till the end of the program as I usually have a routine/block addressing outputs and I do that so I can remap them anytime I want without having to search and find them all over the place. And if it is a reusable routine/block then it goes without saying no outputs should be addressed directly.

Logic io is updated asynchronously.
This technique is done to ensure that the update is synchronous to the plc scan, as well a tidier way of managing/structuring a program.
Also, an alias cannot be modified online...

There will be more detailed explanations, but that is a short summary.

I agree with making it easy to find/replace I/O in projects - I've done it before also.

The gist of another concept/possible reason....

The Logix 500 family of processors update their I/O once every scan, as the processor handles the I/O. The 5000 family processors do not handle I/O - they write to tags addressing I/O and the I/O modules update periodically (depending on your set RPI) reflecting the I/O tag value. Therefore, I/O can update during a scan. *Sometimes* you may not want I/O to update in the middle of a routine if your logic isn't yet done evaluating, so you may want to use a mapped (I/O) tag. The mapped I/O tag will not change states until the processor scans the routine you have it in, in other words the changing I/O state will have no effect on a mapped tag until scanned. I/O modules handling I/O tags and not the processor allows the processor to scan faster (less workload).

I'm sure Geospark will give a much more detailed explanation on this.

lostcontrol said:

Logic io is updated asynchronously.
This technique is done to ensure that the update is synchronous to the plc scan, as well a tidier way of managing/structuring a program.
Also, an alias cannot be modified online...

There will be more detailed explanations, but that is a short summary.

Click to expand...

Beat me to it....

When I am designing a new system and usually writing the program before all of the hardware is delivered I can simulate the program minus the real I/O. All my inputs and outputs are internal and when the system is ready to test I add the actual hardware to the rack and map the the real I/O to the internal tags. As others have stated all of your I/O can be in a couple routines and makes troubleshooting easier for anyone who follows in my footsteps. Since I started years ago as a tech I always try to design systems with them in mind from a troubleshooting standpoint. I use these same techniques when I teach any of our ControlLogix classes.

lostcontrol said:

logic io is updated asynchronously.
This technique is done to ensure that the update is synchronous to the plc scan, as well a tidier way of managing/structuring a program.
Also, an alias cannot be modified online...

There will be more detailed explanations, but that is a short summary.

Click to expand...

+1

Also, you can't edit aliases online. Mapping via logic let's you reassign I/O points without a download.