Hello,

I'm struggling with a very basic problem/question. I have some PLC tag linked HMI tags (of course...). The problem, is that when I switch off the PLC (electrically) these tags values are not changing as if the PLC was working!

This is very annoying and not safe. Tags should sign something, zero itself or something...

It is strange that I did not find any topic on this, or maybe I missed something basic thing?

I also tried to use "LED" PLC block to query the PLC state and put it into an HMI tag, but the tag value also "freezes", in other words it stays high even in switched off state of the PLC. But if I restart the RT program, than the HMI tags are refreshed, but only if I restart the program. Forcing update tag did not help.

PS: I need to monitor the on/off state of the PLC. Even the value of a life bit of the PLC would freeze in a HMI tag.

So my basic question: how to display PLC tag driven HMI tags with switched off PLC?

Thanks for reading this.

Bob

Hi Bob,

Firstly, an important distinction. If the PLC goes into stop but remains online, the values will still be held in the PLC registers and the HMI will still correctly read those values. It is possible for the PLC to be in stop and for there to be no obvious indication of that on the HMI.

If the PLC goes offline, be it because it has been turned off altogether, or only because the connection has been lost, the HMI displays hashes ‘#####’ in place of any value read from the PLC as a way of indicating that it does not know what the correct values are.

I’m not sure why you would expect the HMI registers to go to zero if their connection to the PLC is lost. It is far from axiomatic that would be the correct thing to do.

Indicating whether the PLC is online or offline and whether it is in run or in stop is a particular challenge and is a topic I discussed quite recently on another thread:

'TIA Portal V15 Show PLC Status' created by AutoGen_7873714

Hello,

@Ken: thanks, I read your comment. Your solution is basically a HMI Watchdog. I get the concept. But on my panel the shortest schedule time is 1 min. So increasing a tag continuously would involve a VB loop and I want to avoid that.

@Murof: thanks. This kind of solution works. I get 0, 4, 6, 8 values for OFF, STOP, STARTUP, RUN states. Of course this has to be queried manually and 0 the tag before running GetPLCMode.

The only automatic PLC State recognition worked with System Events (140.000 & 1) so far, but it needs 20-30 seconds to recognize the PLC switched off state.

Bob_Z

@Ken: thanks, I read your comment. Your solution is basically a HMI Watchdog. I get the concept. But on my panel the shortest schedule time is 1 min. So increasing a tag continuously would involve a VB loop and I want to avoid that.

I assure you Bob, no VB script is required – there is no VB scripting with a basic panel and it works perfectly with a basic panel. And the one-minute minimum scheduled task with a Comfort panel is only relevant for the business of indicating PLC offline. The run / stop indication does not require the scheduled task.

I should make it clear; the basic technique was outlined in a Siemens FAQ document that I stumbled on through pursuing something on another thread on another topic. The document is very old and is specifically for old HMIs programmed with WinCC Flex. I have had to adapt the technique outlined for modern HMIs programmed with TIA Portal WinCC. But I have tested it with both basic and comfort panels and found it to work very well, and as I said on the other thread, so far, it seems to be more robust than any other technique I have found. Again, I reserve final judgement until I have been using it in earnest for a period.

The run / stop indication on both basic and comfort panels is pretty immediate, and the online / offline indication on Comfort panels may wait up to a minute, but it didn’t seem to be a big deal to me. As I said on the other thread, you can get a basic panel to run the online / offline test on an operator action. You could try to make it a bit more smoke and mirrors by running it on loading the page that indicates the status. Of course, that won’t work if the status changes while the page is displayed.

And even on the one-minute minimum scheduled task, I noticed a contribution by Murof on another thread the other day which mentioned a bit of a trick you can do to generate a timed event more frequently than once a minute. As mentioned by Murof on that thread it has its downside, but it’s worth considering.

Anyway, I can post an attachment of the Siemens FAQ document from which I took the technique so that you can do your own adaptation of the technique, and / or I can attach a copy of my test program so that you can test it for yourself to see whether you think it is any good. Let me know…

Hello Ken,

you're right, there's no VB script for basic panels.

My goal was to get a automatic(!) response (like an alarm) within 2-5sec if the PLC is switched off. As I wrote above, the system event triggers within 20-30sec automatically, and/or I can query manually a value for OFF/RUN/STOP state of the PLC immediately using GetPLCMode. So far this is what I achieved.

Maybe another option would be to use a separate PLC to monitor the others. This device considered to be ON all the time. In this case this PLC could tell which one is in which state. The HMI can show the "monitoring" PLC's tags of course.

If it cost you only to post here the link of that old documentation I would like to go through it.

Thanks.

B

Bob_Z

If it cost you only to post here the link of that old documentation I would like to go through it.

No problem: