simple question rslogix alias tags

I don't have a controller here to check this. so my question is can I write to an alias tag in an rs logix 5000 controller from a panel view. The base tag would be an input from a flex I/O at the bit level. and I would like to use a momentary PB from the panel view to write to the alias. the real world input is a momentary PB and is only used as xic in the program. would the real world input over write the panel view PB?
Thanks, Steve

bce123 said:

I don't have a controller here to check this. so my question is can I write to an alias tag in an rs logix 5000 controller from a panel view. The base tag would be an input from a flex I/O at the bit level. and I would like to use a momentary PB from the panel view to write to the alias. the real world input is a momentary PB and is only used as xic in the program. would the real world input over write the panel view PB?
Thanks, Steve

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I don't have a ControlLogix handy to test it either. However, I am sure it would be overwritten by the next I/O Update. Just create a new Tag, and put it in parallel with the I/O Point.

Stu....

Well maybe I should have said: 20 or 30 or 100..... tags
just to clairfy the real world input would be n.o contacts would this still be the case?
Or if there were two panel view PBs?

bce123 said:

Well maybe I should have said: 20 or 30 or 100..... tags
just to clairfy the real world input would be n.o contacts would this still be the case?
Or if there were two panel view PBs?

Click to expand...

100 Tags would be a lot of work allright.

The Input doesn't really care what device is connected, or even what state it is in. If the Input point is Energized it will put a one in the I/O Points Tag Memory Location. If it is De-Energized, it will put a zero in the I/O Points Memory Location. The Alias Tag will get its Value from the I/O Point Base Tag. I am not sure exactly when an Alias Tag gets updated, but everything happens pretty fast in the ControlLogix PLCs.

Depending on how the PanelView Buttons are setup, i.e Momentary, you could have two PanelViews pointed at the same Tag. I don't like to do that, because if you get a stuck button on an PanelView, how do you tell which one it is.

Stu....

The value of the base tag and the Alias tag in the tag database are both updated to match the physical value of the Input point at the RPI rate.

If you wrote a "1" to the Alias or the base tag, it would be overwritten by the value of the Input in a few milliseconds. There would be no way to predict how long the tag would be at the HMI-written value.

I think you might be implying that you want to write to an I/O tag for testing or checkout purposes. It's technically possible to programmatically (or using RSLogix 5000) inhibit an I/O connection so that the Input data doesn't get applied to the Tag Database and the Output data doesn't get applied to the module. That would let you write to the Input data table from an HMI.

Unless you have a very good reason to do otherwise, I recommend doing HMI "Inputs" and actual inputs using parallel branches.

Unless you have a very good reason to do otherwise, I recommend doing HMI "Inputs" and actual inputs using parallel branches

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Depending on how the PanelView Buttons are setup, i.e Momentary, you could have two PanelViews pointed at the same Tag. I don't like to do that, because if you get a stuck button on an PanelView, how do you tell which one it is.

Click to expand...

I agree
I'm really just trying to wrap my mind around this and its been a long day.
I am just trying to find an easy way to do this Because I have to add two panelviews. one now and one later I just have 4 hours to be online and do all the edits and test everything I have never tried it this way and in my mind it should work.I am just not sure my mind is working right now!
Thanks for the input guys. In the a.m. I going to run down the road and buy a compactlogix to try this on I will let you know how it turns out and no i wont bet a box of donuts on it. LOL
Best regards , Steve

sthompson said:

The Alias Tag will get its Value from the I/O Point Base Tag. I am not sure exactly when an Alias Tag gets updated, but everything happens pretty fast in the ControlLogix PLCs.

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An alias tag is not a separate tag in memory that has a value magically copied to it. An alias tag is a name that refers to the exact same memory location that the base tag refers to.

The address of the house where Bubba lives is #5 Elm street. "#5 Elm street" is like the base tag. "Bubba's house" is like the alias tag. They both refer to the same place, Bubba doesn't have two houses. When Bubba gets home the answer to the questions "Is Bubba at #5 Elm Street?" and "Is Bubba at Bubba's house?" are simultaneously in the affirmative.

So if you write to an alias for an input, you store a value in the input image memory address. When the module updates it will change it back, and in the CLX the update is asynchronous with the scan, it could change back right in the middle of a rung. HMI writes are also asynchronous - you're asking for unpredictable results.

asynchronous

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Oh independent of program scan
Still going to try it just to erase that thought in my brain if nothing else.
Just on the bench of course.

Thanks for the clarification Alaric. I suspected as much, but I don't like to make statements unless I am sure of my information.

Stu....

I don't have a ControlLogix handy to test it either

Click to expand...

just an update, I bought a compact logix l32e to try this on I know there a differences from the control logix. such as smarter faster I/O..... but would this give me the same results. as I am only looking at the flex I/O in this case?
Thanks, Steve
I will be using the local I/O to test since I don't have any flex I/O here at the moment.

It should be good enough for your testing purposes.

Stu....

The compact logix doesn't use smart IO modules, it uses 1769 micrologix modules. So a task in the compact logix scans the modules at a configured RPI. You may get slightly different results than you would get with a 1756 modules, but what you are trying still won't work.

but what you are trying still won't work.

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Alaric, I think you slightly misunderstand. At this point This is an experiment in which I am simply trying to understand why and how the asynchronous scans work. your analogy of how the alias and real tags associate is the best I have read even Bulba should understand their relationship after reading your post. It is how it all executes that I would like to understand better. we could call it learning by doing. Look at it like retentive memory if I do it I will retain it and if I just read it when I power down tonight I will forget it.
I am thinking of using the alias to latch a bit and the real world to unlatch one then use the bit to interment a CTU. then i will use a controller tag to latch a bit and the real world tag to unlatch that bit, that bit will interment another CTU.
Then i will compare the two counts with the number of times the PB is pushed from the panel view. I will update the post with my results after I have done so.
Any suggestions of a better way to prove this?
Thanks, Steve

If you'll look on Youtube you can find a demonstration video by Ron Beaufort where he explains the asynchronous updates really well and physically shows a demonstration on how its different from SLCs and PLC/5s. Maybe that will give you some ideas.

Thanks, Alaric I will have to look it up when I get time
I am planing on using this controler to do test and debug on the bench as I am getting rid of most my non AB e-net stuff. I just got it set up and will try and write some code later for the test. Thanks to all, Steve