PLC Trouble

I went to work Saturday to try a few things mentioned. The computer, with RS logix 500, was taken offline. When I say offline I mean going to the drop down menu and clicking go offline. About 5 secs. later the equipment faulted. No faults on SLC 5/04 processor, in program, nowhere. Went back online through drop down menu hit hit reset and restarted equipment. At this point Maintenance decided to change the rack. Will know more on Monday. I am leaning towards the watchdog timer because it seems to be the same amount of time, each time I go offline.

I don't even disconnect the pc. When I am "hooked up" with a cable and online with RS Logix 500 software, we are good. When I go offline from the drop down menu everything stops about 5 secs. later. Go back online, hit reset, everything runs again. What are your thoughts. Today we changed the rack out.

About 5 secs. later the equipment faulted. No faults on SLC 5/04 processor, in program, nowhere.

Went back online through drop down menu, hit reset and restarted equipment.

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Exactly where are you "hitting reset" ? Is that a button on a console somewhere, or a bit that you're toggling in PLC memory ? Are you referring to the "Clear Major Error" button in the RSLogix 500 Status file Errors tab ?

The controller watchdog is meant to detect when the logic goes into an endless loop; The maximum value for that setting is 2.5 seconds. I cannot think of a way that having an RSLogix 500 session online would affect the actual controller watchdog. And a controller watchdog would definitely cause a flashing red RUN LED, indicating a Major Fault.

You've said that it's an SLC-5/04 controller. Are you online via DH+, or via the serial port ?

I'm having trouble rectifying your description of having the controlled system "fault" with your assertion that the SLC controller itself is not entering a fault mode.

Is the machine simply stopping ? Is there a programmed condition that it considers to be a system fault ?

I wonder if the Node Active bit for the RSLinx/RSLogix computer's network interface in the DH+ active node table could have something to do with this. I would expect RSLinx to keep the Node Active bit true, not RSLogix, but I've never investigated that closely.

I believe some else has requested this ,but I will do it again. Post your program it's .RSS file. Zip it first.

DBONN said:

I went to work Saturday to try a few things mentioned. The computer, with RS logix 500, was taken offline. When I say offline I mean going to the drop down menu and clicking go offline. About 5 secs. later the equipment faulted. No faults on SLC 5/04 processor, in program, nowhere. Went back online through drop down menu hit hit reset and restarted equipment.

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Assuming you meant Halted and fault LED. . .

Is the CPU going into Program mode by itself?

This actually has happened to me one time with a 5/04. I could not make the symptom worse or better with anything I did. If you cycled the keyswitch it would run, or if you went online and put it in run mode, even with the key in the run position! I never could prove it, but I am pretty sure that the root cause would have to be a physical intermittent problem with the keyswitch itself.

This program I had written and there were no fault handling routines or anything tricky in the code.

Mine did this about once a week for a few weeks, then it happened twice in the same day. The replacement processor had no problems, and the flaky one did fine in the test rack so I put the dreaded black sharpie X next to the serial number, and left it in the lab at my old job. I had a little pile of PLC parts and drives with my X-clamation of suspicion, awaiting testing. My black X meant "I bet the repair shop reports no problem found...but there is one..."

That CPU is probably still floating around in the R/A repair loop.



Seriously though, don't shut down the plant to look at the sticker for that X, it's more likely that the system bits are set up to auto clear faults, trigger a fault routine which does so, or you have a power supply problem/chassis problem.

Not fixed yet. We were down all weekend and replaced the plc rack. Will keep all posted as to if this helped or not. The one thing that got my attention is someone mentioning a watchdog timer. It seems to be the same amount of time that passes when the equipment stops. I am going to look for some timer that is equal to about 3 seconds. Not sure where to look though.

When I say reset I mean the rest the operators depress to clear faults. I have been told by our local Rockwell Engineers to try different keyswitch positions but it did not work. We akso changed the PLC cpu and that didn't work.

DBONN said:

Not fixed yet. We were down all weekend and replaced the plc rack. Will keep all posted as to if this helped or not. The one thing that got my attention is someone mentioning a watchdog timer. It seems to be the same amount of time that passes when the equipment stops. I am going to look for some timer that is equal to about 3 seconds. Not sure where to look though.

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The watchdog timer is a timer that gets reset by the SLC between scans.
It is typically set to a value of about 100ms. If the PLC can't scan the code in 100ms, the watchdog will time out, and the SLC will fault (red LED), and all the outputs will go to the fault state, and the error code in RSLogix will tell you that while on line looking at the status words.

The watchdog timer should not be set to an unreasonable value. I do have two SLCs with a lot of devicenet that have a 900ms first scan, so I had to bump up the watchdog to a full second, even after removing many of the OEM M file instructions...still, somehow in hardware there is a terrible delay on first scan, and then it runs like a normal SLC in the 20-25ms range. Nearly all the hundreds of other SLCs I have supported use a 100ms watchdog...unless you got weird code, this is a wild goose chase, one any of us could answer if you could post the code or some screenshots of the status view while online.

EDIT: Also, rather than just tell us your action plan, go back and carefully read some of the specific questions asked above and try to answer them. I think that would get you a solution much quicker.

DBONN said:

When I say reset I mean the rest the operators depress to clear faults.

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Awesome, now we are getting somewhere. The operators push a button either a real one or one that is on a PC screen, or one that is on a hardware based HMI (Panelview)?

In either case, the next thing you need to specify is LED status on the CPU when this fault happens.

I have been told by our local Rockwell Engineers to try different keyswitch positions but it did not work. We akso changed the PLC cpu and that didn't work.

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To reset with the keyswitch, put to the program position, wait a couple of seconds, then back the middle, pause two seconds, then to the RUN position and watch for even a flicker of the RUN led.

I am guessing that you are downloading code each time this happens. Your program has (most likely) an un-trapped overflow or other math error. The copy you download has not calculated or accumulated this error

zip it and post it, pleASE!

I am still not sure how keeping the PC online prevents the fault entirely unless you do indeed have weird code.

DBONN, you absolutely need to follow what that "reset button" does.

Maybe the Reset button cycles power to the controller... we have no idea. You absolutely have to figure that out and learn what condition the machine is actually entering.

If I press the reset button the machine resets for a few seconds then stops(Computer offline). If I go back online and press reset machine will run. Changed plc rack over the weekend but same results.

DBONN said:

The computer, with RS logix 500, was taken offline. When I say offline I mean going to the drop down menu and clicking go offline.

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This would be in the RSLogix 500 software? You ARE taking it "offline" and not changing to "PROGRAM"? Offline should have no effect on the PLC, and it's shutting down with the PC turned ON, and going "offline" with RSL500 causes it?

I want to make sure our terminology is correct.

Everything you described is correct. Any ideas?

Check program for System bit Usage. It sounds like the PLC is monitoring The comm port for communication if no Comm's then fault.

I was thinking of the DH+ Active Node status bit, or something else in the Status file, but that would have been part of the original program and not something that developed over the past few weeks.

The Reset sequence gets my attention because DBONN says that he puts RSLogix 500 back online then pushes the Reset button and the machine starts.

From what I understand, if he does not put RSLogix 500 back online but presses the Reset button, the machine does not restart.

If we knew how the Reset button worked, we could focus on something fixable. Is it an ordinary PLC input ? Maybe it's an e-stop Reset. Maybe it's a power cycle button. Maybe it's a cell on a touchscreen that executes a VBA macro that sends a text message that orders pizza.