Store system parameters

I am working on my first programming project, and I need to be able to store some of my system parameters (i.e. bit states, and totalizer values) in the event of a power failure.

Then once power is restored, I need to be able to read this information back to the PLC. I am using a CompactLogix controller.

Any help would be appreciated.

Variables, dints & floats etc, will be retained. Bool data types that are acted on by OTE instruction are not retentive. You can make them retentive by using OTL & OTU instructions. Timers should be used with RTOs instead of TONs & TOFs to be retentive.

I didn't realize it was that simple. It looks like I've been working on a problem that wasn't really a problem. I have already been using the RTOS, OTL and OTU in my logic, so it looks like I'm covered.

Thanks for your help.

You're welcome and welcome to the forum.

jholloway,

Congratulations on your first project.
Your posting just says project so i am going to assume a machine of some sort.

Even though Jstolaruk is true in his post, I urge you to use caution! If you use the latch / unlatch commands to retain your outputs, you must consider the following:

1. NFPA 79 - electrical standard for industrial machinery.
2. How is the machine going to operate when you looose power or have an e-stop condition? How will the air cylinders and other like devices operate? What will happen when you restore power? Who or what can be in the machine when power,e-stop is restored.

I have seen one machine programmed using the latch commands operate when the e-stop was reset (the safety guard was off to help get a part out of a machine). The machine picked up where it left off and broke a spring on the part being assembled.
The part of the spring that broke barely missed the plant managers neck. It hit the concrete block wall and made a good sized hole in it. He was not a happy camper.

In short, no machine may restart its operation when power is restored due to a loss of power or an e-stop condition. There are always exceptions to this rule but in general, its true.

You must consider how the machine is designed, its operation sequence, how the pneumatics work.

I don't mean to be such a stick in the mud, but i also don't want someone to get hurt either. 25 years ago, my boss gave me a shot at a project and reviewed my work. He asked me to explain my logic and the reasoning behind it. Then he walked me through the project asking lots of questions and offering a lot of what if situations. Boy did i feel dumb. He was very polite in the way he went through my plans and i am a lot smarter for it.

Please look at everything and ask lots of questions and what if situations.

Good luck,
james

What James said.

You're going to find, as you program machines, that the problem isn't in running them. Running is usually pretty easy. It's the transitions that are hard. Changing states from Off to Start to Run, or Run to Off, or especially E-Stop to Reset. Hint: What happens to all those latched relays when I hit E-Stop? When I press Reset after removing the E-Stop? When I press Start after resetting the E-Stop?

It's also in error handling when an actuator fails, a sensor fails, or the machine is set up for Product 1 but the OPR starts trying to run Product 2.

A really really long time ago, one of my college profs had a catch phrase that has been very applicable to machine programming. He was an ME, but still. It was a design course, and he'd always say "Boys," (hey, I said this was a LONG time ago!) "Boys, you have to ask yourself... How can it fail?"

Turns out that is machine programming in a nutshell. So, as you write the logic, look at it and ask yourself that question. At the least, you have to handle failures or incorrect situations without hurting the people, the machine, the product... usually in that order. At best, you want to provide automatic recovery, or simple manual recovery.

Welcome to the forum, and have fun! The greatest thing about being a controls guy is that your job... is to make it go!

I probably should have given more information in my original post. The project I am working on is a very small water treatment facility (125 gpm max). So the only moving parts are the pumps. We have provided local disconnects for each of the pumps for maintenance etc.

There is more than one set of filters available, and I am using the latched bits to tell the PLC which set of filters should be online.

If the operator needs to service one of the filters, I have provided a lockout that will prevent that filter set from coming online until the lockout has been deactivated.

Thanks again, guys for all the advice and encouragement.

jholloway said:

I probably should have given more information in my original post. The project I am working on is a very small water treatment facility (125 gpm max). So the only moving parts are the pumps. We have provided local disconnects for each of the pumps for maintenance etc.

There is more than one set of filters available, and I am using the latched bits to tell the PLC which set of filters should be online.

If the operator needs to service one of the filters, I have provided a lockout that will prevent that filter set from coming online until the lockout has been deactivated.

Thanks again, guys for all the advice and encouragement.

Click to expand...

No valves??

since you mentioned that.
I hate to bring this up but, waste water treatment plants are
governed by federal law and not just anyone can do a water treatment project. you have to have the licenses to do that type of work. if you are doing the controls and programming, you must work under the engineering manager and show him everything.

Back to your project.
lock outs are fine, but what happens when the latched output
that controls a motor has the soft motor starter turned on and someone then turns off the power at the motor lock out. What happens when that motor is then then turned back on and that soft start motor starter is still on? good by motor starter.

it's questions like this that you need to ask.
another saying goes like this, "if you can think of a situation, you better plan for it - it will happen sooner or later".

One company i know of has a machine that must be manually
reset to home everytime the power goes out. It takes 2-3 DAYS
to do this. I am not joking, its 2-3 days. the machine is that complicated and there are so many different situations to deal with. I cannot tell you who it is due to an agreement i signed.

regards,
james

James Mcquade said:

One company i know of has a machine that must be manually
reset to home everytime the power goes out. It takes 2-3 DAYS
to do this. I am not joking, its 2-3 days. the machine is that complicated and there are so many different situations to deal with. I cannot tell you who it is due to an agreement i signed.

Click to expand...

That's okay, that's some real weak engineering if it takes that long and I wouldn't contract them because of it.

James Mcquade said:

It takes 2-3 DAYS
to do this. I am not joking, its 2-3 days.

Click to expand...

I cannot even imagine a machine that takes that long to get runnong upon a power loss, but if it is so, it might make sense to invest into second power feed or something like that. Think of a blast furnace that cannot be shut down no matter what.