What happens when a PLC is reloaded?

[Jim ManleyModeratorKarma: 262Rank: Jedi]

Pretty basic question, right?

I’ve been maintaining an Rockwell/AB based system for a few years now. During all that time, I’ve always been able to do program mods with the controller on-line. Now I need to add a couple of PowerFlex 525 VFDs. These changes have to be made off-line so I know that when I bring the modified program on-line, I’m going to have to download it to the controller. This is a bit terrifying since I have no idea what happens when the controller reloads. It’s even more bothersome that I can’t find any thing on the Rockwell/AB site that describes what happens when a controller reloads and restarts.

Am I being overly cautious here?

Anyone out there that can tell me what to expect when I do this?

[George BarnetteParticipantKarma: 171Rank: Jedi]

Hi, Jim

What basically happens on a download is everything gets downloaded from the program on the laptop in the last state it was saved. This can be a devastating problem for instance on a conveyor system that shifts data from station to station, such  as I work with in automotive. That will take you down several hours and the programmer will be encouraged to find other employment. I did some downloads a few weeks back working on a launch at GM but they use their own system on the PLC interface that automatically saves all the data at a point before the download.

If you’re just using the program for a lot of binary stuff–starts, stops, timers, solenoids etc. it usually won’t be too bad. If you have a lot of data transfer, then do an upload, save the program and the tag and data values when prompted and then immediately shut down the machine. At that point make your changes to the uploaded program which will have the last good data “snapshot” and THEN download. Hope this helps and good luck!!

[Jim ManleyModeratorKarma: 262Rank: Jedi]

George –

Thanks for confirming what I suspected.

We will be shutdown between Christmas and New Years so that will be an opportune time to do the changes.

One question: When you say “shut down the machine” I assume you are referring to the PLC.  Do I need to literally power it off or change the operating mode?

Thanks,

Jim

[George BarnetteParticipantKarma: 171Rank: Jedi]

Jim, hi, no don’t shut down the PLC; it has to be powered up and in program mode. If the machine does a lot of data transfer as I mentioned previously, then the machine needs to be stopped. I suppose I ought to have said “stop” the machine. As an example, one wouldn’t want positions, encoders, barcode values etc to keep indexing/refreshing while you’re downloading, because in a download the processor will automatically go to remote program and stop the process anyway. This is what causes problems later. A controlled stop gives you the best chance to save current data vs. positions.

To give you a little firsthand experience, you can do an experiment if you have a lab or have time (or both).

Save your existing program without any changes to a different name. I use whatever name it is now and then add “1”, “2” etc. Then make your changes and save THAT one to a unique name. Download the changed program. Then, open the first program that you saved and select “go online”. It will either say something like “applying changes” and then open, or it will prompt you for an UPLOAD. When you do the upload, the first saved program will become like the second saved program.

So lastly, your original program is still like it was because you saved that program to different names. Where I’m at now at a supplier, I get the “Upload” prompt all the time because it’s been months since anyone has gone online with the program after changes and no one bothers to save it with the current date and archive the old one. Sorry for the long response. The word “download” gets everybody’s attention in automotive really quick. Hope this helps.

[Jim ManleyModeratorKarma: 262Rank: Jedi]

I have a spare PLC but no lab set up to power it up.  I do have the Logix 5000 Emulator so I followed your instructions with a program I had written as part of a simulator I built.

Just for anyone else who may be searching for how to deal with this, I documented the steps I took and the results I got with the simulator.

1. Started the simulator and open the original program (totalizer).  Simulator is in Remote Run mode.
2. Saved the current program (totalizer) to a new name (totalizerA).
3. Put the simulator in Offline Mode.
4. Modified the saved program (totalizerA) by adding a rung with a NOP instruction.
5. Saved the modified program (totalizerA) to a unique name (totalizer1).
6. Downloaded totalizer1 to the simulator (simulator goes from Offline to Remote Program mode)
7. Put the simulator in Offline mode.
8. Closed the totalizer1 program.
9. Open the original unmodified program (totalizer).
10. Put the simulator in online mode.
11. Simulator went into Remote Program mode and I was prompted to upload/download.  I selected upload.
12. Inspected the program I modified and saw that the modified rung was there.
13. Put simulator into Run Mode.

Thanks again for the help/tutorial.

[George BarnetteParticipantKarma: 171Rank: Jedi]

Hi Jim. Glad that worked for you. Any time I can help I will. All I ask is that you help others when you can. I wish you and your family a great Christmas!!

[Tim SmithParticipantKarma: 18Rank: Padawan]

The biggest thing when uploading/downloading is that if your system has items like encoders and the values are critical, try to bring the system to a full stop before saving the file.  Don’t start the system again until you have made the changes and downloaded the new program into the PLC.  This avoids a headache of wrong values.

[Jim ManleyModeratorKarma: 262Rank: Jedi]

Tim –

Thanks.  Fortunately this system consists mostly of pressure/temperature sensors, a lot of digital I/O for actuated valves, flow meters, pumps, and motors.   Nothing I run on this system uses encoders or indexers.

Jim

[blawrenzParticipantKarma: 402Rank: Jedi]

Other considerations would be unexpected movement of equipment.  Be aware of how your program is written, such as the use of Latch bits, if latch bits are used to drive outputs, which I am NOT recommending, but, if they are just be aware that whatever state they were saved in when the program is reloaded is the state that they will be in when the program is put back into Run mode.  (ie. things could move.)  One way around this is the use of the First Scan bit.  In some logic you will see the use of the First Scan bit that is used to reset/unlatch things to a rest state.  If you need more details let me know.

Brad

 

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