Monitoring the "Change" in a tag value.

xC0MMAND0x · Dec 2, 2013

Greetings!

I am wondering what Instruction to use for the following scenario...

I have some PLC Tags that represent the "Status" of a particular machine. The Status values are integers ranging from 1-5 in value; I have all of the setup and working no problems.

Now, I need to setup a Monitoring BOOL as a trigger for some data processing. Essentially I need to have a BOOL that triggers TRUE if the Value of "Status" changes at all.

Is there an instruction geared for monitoring a value or seeing if it changes values?

I suppose I could use a compare register and use the current value of the Status tag to compare, but I figured there should be an easier way.

Thanks!

jkerekes · Dec 2, 2013

What processor? Look at DTR. Or you can program your own. DTR only works on DINTs.

xC0MMAND0x · Dec 2, 2013

jkerekes said:
What processor? Look at DTR. Or you can program your own. DTR only works on DINTs.

Seems like this *should* do what I want, if I convert my data for the machine status into an Array.

I tried this as a test, and I couldn't get the output to latch on.

I have a test array, referencing itself, and I changed the value. I looked at the instruction help.

I just want to compare a tag value to itself, and if it's changed in a scan I need an output to turn true.

widelto · Dec 2, 2013

use a timer, when timer ticks move your tag value to a know place, next time timer ticks compare instant tag value to value you move a moment ago if they are the same do nothing is values are different set a bit.

Ken Roach · Dec 2, 2013

I do it the simple way: compare the current status value to the "old" status value, then copy the current status value to the "old" status value tag. You can do it with latches, or as though it was a one-shot.

jkerekes · Dec 2, 2013

The DTR is a one-shot. The output is true for only one scan.

bernie_carlton · Dec 2, 2013

Ken, in your example the counter would not count up correctly if the status changed on adjacent scans. If that were critical an ADD might be better.

xC0MMAND0x · Dec 2, 2013

bernie_carlton said:
Ken, in your example the counter would not count up correctly if the status changed on adjacent scans. If that were critical an ADD might be better.

Excellent point, Bernie.

I am not using a counter as an output, just a Toggle that loads data into a FIFO register. I tested things out with the logic and it's working perfectly!

As always, thank you to this wonderful community and all of those who helped.

Ken Roach · Dec 2, 2013

Fine point, Bernie... I don't use a counter most of the time. Thanks for pointing that out.

daba · Dec 2, 2013

I usually do this the simple way, which generates an output bit that is true for one scan whenever the value changes. The attached pic shows the code I use.

However, if the value can change again during the same scan, the bit will remain true, so it doesn't indicate every "change-of-state". I would hazard a guess that your machine "state" can't change more than once during a single processor scan

But this begs the question "Is there code in the PLC that writes new values to the machine status?". If there is, why can't that same code trigger a conditional JSR, or even an EVENT instruction to go and do your data processing.

daba · Dec 2, 2013

You also said you wanted to generate a BOOL whenever the value changed, to initiate data-logging.

Do you need the BOOL tag, you can execute the data-logging directly without having to create a new tag....

Gerry · Dec 2, 2013

xC0MMAND0x said:
Seems like this *should* do what I want, if I convert my data for the machine status into an Array.

I tried this as a test, and I couldn't get the output to latch on.

I have a test array, referencing itself, and I changed the value. I looked at the instruction help.

I just want to compare a tag value to itself, and if it's changed in a scan I need an output to turn true.

Your test doesn't work because you have the source and reference set to the same tag. The DTR looks for differences between source and reference, filtered by the mask. The tag you're testing will always be equal to itself.

So, create a separate tag for the reference. The DTR will copy the source to the reference each scan, after comparing. With the mask set to '1', you will only be checking for changes to bit 0 of the source. Set the mask to '-1' to check all bits.

Monitoring the "Change" in a tag value.

xC0MMAND0x

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jkerekes

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xC0MMAND0x

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widelto

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Ken Roach

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jkerekes

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bernie_carlton

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xC0MMAND0x

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Ken Roach

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daba

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daba

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Gerry

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