RSLogix/Studio 5000 Capture/Track Previous Value

[ohnedich6]

Good day all,

PLC: RSLogix/Studio 5000 using Ladder Logic

I've been toying around with the idea of capturing the previous value of a tag inside of my PLC, but I cannot seem to get started with the best way of coding this. I was hoping I could get some help with this.

I would like the tag Previous_Tag to be able to capture the value regardless whether the number increases or decreases. Also, the value may change in an inconsistent manner/pattern at times (I.E. 1 ->4, 4->11).

Thank you for your help.

-Nathan

 

[robertmee]

Pretty Basic 101....

Initial Previous Tag to Current Tag on startup (MOV Current Tag Previous Tag)

Then standard logic

If Current Tag <> Previous Tag, MOV Current Tag Previous Tag

 

[gclshortt]

If you have the following parameters:
TAG
THIS_TAG
PREVIOUS_TAG
Compare TAG to THIS_TAG if not equal then using a one shot Move THIS_TAG to PREVIOUS_TAG and TAG to THIS_TAG

Hope this helps.
Regards,

 

[Bullzi]

Is the Tag you want to look at a input value? If so are you buffering your I/O?

Remember that the 5000 family of controllers update their inputs Asynchronous to the controller scan. So the controller could update a input value before you get to your MOV Logic and you may not actually capture the true previous state.

 

[ohnedich6]

Is the Tag you want to look at a input value? If so are you buffering your I/O?

Remember that the 5000 family of controllers update their inputs Asynchronous to the controller scan. So the controller could update a input value before you get to your MOV Logic and you may not actually capture the true previous state.

Bullzi,

Its a memory tag which will eventually be used inside of an AOI.

-Nathan

 

[ohnedich6]

robertmee,

I use the NEQ and MOV to determine if a transmitter's reading has frozen, but for this logic it is performing so fast that by the time I can look at the PREVIOUS_TAG it is already equal to the TAG & THIS_TAG.

gclshortt,

Thanks for the suggestion, but all of the values end up being equal to TAG almost instantly.

-Nathan

 

[ohnedich6]

All,

Scratch the second part of my previous post with regards to gclshortt. I had a destructive rung of logic that I did not notice *face palm*. Once I deleted that it worked beautifully.

Thanks very much for you help.

-Nathan