Controllogix ADD or CTU not operating properly

Hey Guys,

I have an installation that has been running for about 3 years and recently some totalizer issues have started creeping up.

I have a 1756-L55 version 15.5 performing simple totalizing of multiple flowmeters. The flow total comes in the form of a pulse, very slowly every 30 seconds or so depending upon the flow meter. I did have the total routine programmed with the pulse contact, pulse OSR, and ADD function with DINT variables, adding one each pulse. Then after would divide by 10 if the pulse in the flow meter was only 0.1m3/hr. I have tried replacing the ADD function with a CTU and the same result. I can put a 100ms timer in parallel with the flow pulse and watch it count up, going something like the following:
337843,337844,337845,336233,336234,337846,337847,337848 etc.

Could this for any reason be due to the fact the TAG.TOTALSTORE (DINT) and subsequent TAG.TOTAL (REAL) is part of a User Defined Data Type?

Any other thoughts, perhaps on DINT data type ADD limitations, CTU?????

A float cannot be incremented by 1 after it reaches 16,777,215 due to IEEE-754 float resolution issues. At that point you loose resolution in the ones digit and adding just one won't work. As the float gets even larger you loose resolution in the ten's digit, then the hundred's digit, etc. This can present a challenge in totalizing.

Even when using a DINT you can run into resolution problems if a floating point number is involved in a calculation because the DINT has to be converted to a float by the processor before the calculation can be performed.

You haven't given us enough info about how you are using the DINTs and REALs to say for sure if this is whats happening, but its one possibility. If you can post the program or part of the program that would help us help you.

some totalizer issues

Click to expand...

You don't actually say what the issues are. Did your system stop totalizing? Is it totalizing incorrectly?

A section of the code and a complete description of the problem is needed.

Let's set aside the hypothesis that ADD and CTU are not working correctly and look for a change in the program, in the HMI or network, or in the electrical devices connected to the controller.

I agree that I would have to see your program code to venture an opinion.

From your description of the sequence of values, it appears as though the TAG.TOTALSTORE tag is decreasing by 1612, then increasing by 1612 a little later.

1612 is a funny number; if it was 1024 or 2048 or 4096 then I'd assume a binary sub-element was being changed via some other mechanism (HMI, message, other logic, etc). But 1612, that's a little weird. That's 064C hex. It's 0000 0110 0100 1100 binary.

The test that I would do is to replace the UDT sub-element Tag.Totalstore with a new tag, called PLCTalk_Tag_Totalstore. If this tag's value has the same behavior, then you have a logic issue that goes right to the ADD or CTU logic. If it doesn't, then the problem is probably related to operations on the UDT or on the HMI.

Just as a quick response to the replies.

A float cannot be incremented by 1 after it reaches 16,777,215 due to IEEE-754 float resolution issues.

"I did have the total routine programmed with the pulse contact, pulse OSR, and ADD function with DINT variables, adding one each pulse."

You don't actually say what the issues are. Did your system stop totalizing? Is it totalizing incorrectly?

"I can put a 100ms timer in parallel with the flow pulse and watch it count up, going something like the following:
337843,337844,337845,336233,336234,337846,337847,3 37848 etc."

Sorry if the issue was not clear, I thought I explained it quite well, I will get a screenshot of the program, but really guys, discrete contact to CTU function....that is all and it still does not totalize. Ken thank you for some other stuff to try, I will try a tag outside of the the UDDT.

Hey guys,

Please see attached link

http://www.filedropper.com/plccounter

Watch around the 340500-340600 mark

Our work firewall won't let me access that.

bernie_carlton said:

Our work firewall won't let me access that.

Click to expand...

Here is the screen shot he posted. It looks like its counting to me?

It's a video showing the counter counting.

Does the video show the 'pulse' turning on and off. And I wonder where the 'System_100_ms_timer' comes from?

bernie_carlton said:

Does the video show the 'pulse' turning on and off. And I wonder where the 'System_100_ms_timer' comes from?

Click to expand...

Yes it does. You can watch the counter increment and see the timer pulse sometimes ( of course the video capture is not fast enough the see all the timers pulses) Probably can't see all the pulses in RSLogix either.

That's correct guys, it is too fast to see in logix. The 100ms timer is just to test the counter and will be removed when I figure out what is wrong. That screen capture is a great example of what is happening. The fourth digit is switching between 6 and 2 as it is in the screen capture. If you watch the video again you will see this around the 600 mark.

Watching the counter increment to right around 340700 the accum value goes back down to about 340200. Very consistent.

I would look for something else writing to the accum value.

Mickey the example shown in the video is with a new tag created from scratch, not using User-defined and not referenced anywhere else, PLC or HMI.

Very change behavior. I am not a controllogix user so not much help there.
I am just reporting what I see for those that can't view the video.

So there is no way to post the file, or e-mail one of us to look at it?