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Old April 16th, 2021, 12:16 PM   #1
OWEN.DAVID
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Fuzzy logix when mapping inputs?

We have an input map that executes at the top of every scan.

In most cases we are using the MOV instruction to copy the input data into a human referenceable tag of the same data type. For example,

MOV(IO_AB_Node2:11:I.Ch6Data,AI_Winch_Pressure)

However, in some cases the AI_... tag has been been created as an alias of the same input.



Does this mean that the Alias tag will be updated once by the MOV instruction and then again if/when the data refreshes at input card? Such that a reference to this tag later in the scan could retrieve a different value?

If so, this seems to be nullify the benefit of the input map, i.e. data doesn't change during the scan.

What is best practice for a project using an input map?

*use MOV to copy input data only to a BASE tag

*use MOV to copy input data to an ALIAS tag for some benefit?


Or is it more nuanced than this?
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Old April 16th, 2021, 12:29 PM   #2
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Copying to an Alias tag has no effect. An Alias is just another name for the same memory location.
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Old April 16th, 2021, 12:46 PM   #3
OWEN.DAVID
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So if the IO updates are asynchronous to the scan then an Alias tag value can change during the scan?

Which defeats the typical purpose of the input map routine?


I found a past discussion here which may clarify things

http://www.plctalk.net/qanda/showthread.php?t=90130

Last edited by OWEN.DAVID; April 16th, 2021 at 12:59 PM. Reason: searched for past discussions
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Old April 16th, 2021, 01:03 PM   #4
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Quote:
Originally Posted by OWEN.DAVID View Post
So if the IO updates are asynchronous to the scan then an Alias tag value can change during the scan?

Which defeats the typical purpose of the input map routine?


I found a past discussion here which may clarify things

http://www.plctalk.net/qanda/showthread.php?t=90130
Yes, that is a good thread. The answers to your questions above are yes, and yes.
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Old April 16th, 2021, 02:15 PM   #5
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Thanks for confirming, I have inherited a real onion of a project here!

With a six year history and lots of different layers and code styles. This is largely due to different technicians, undocumented online edits, bug fixes, forces, bug override tags, and non-descriptive work arounds. Hopefully just as many examples of good programming as bad!
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Old April 16th, 2021, 02:27 PM   #6
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Quote:
Originally Posted by OWEN.DAVID View Post
Thanks for confirming, I have inherited a real onion of a project here!

With a six year history and lots of different layers and code styles. This is largely due to different technicians, undocumented online edits, bug fixes, forces, bug override tags, and non-descriptive work arounds. Hopefully just as many examples of good programming as bad!
So the program looks like a control panel after a few years in the field?
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Old April 16th, 2021, 02:41 PM   #7
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Input mapping in Loghix5000 is only necessary when you want to reference an input multiple times within the scan.

So long as a physical input is only read once, then it is actually synchronous to the scan, it's just that all of the inputs don't get updated at the same point in the scan.

We used to get over the asynchronicity by handling ALL inputs with some relevant handling function code.

Valve and pump feedbacks would be handled by AOIs that provided an interface to the code and all alarming functions.

Level probes would be handled by AOIs that did debouncing and alarming etc.

Straightforward vanilla inputs would also be handled by the debounce AOI, with the debounce times set to zero.

... and so on ...

The beauty of using AOI plant handlers is that EVERY similar device will be handled the same way, and of course using the physical inputs as AOI parameters means that they get mapped into the code just once.
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Old April 16th, 2021, 02:51 PM   #8
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So the program looks like a control panel after a few years in the field?
I've seen control panels that looked like that the day after commissioning !!!

Especially the MCC in Athens where they had removed ALL the trunking lids and all the cables had fell out ! That same MCC had a piece of cardboard to stop you touching the live unshielded terminals of the main door-interlocked isolator, which couldn't be used because it had been fitted badly. That same MCC that had NO LABELS on the start/stop push-buttons and auto-manual switches for the various motors. You had to know which was which motor .....

We provided the PLC panel, they had the MCC panel made by Stavros in his shed ....
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