Siemens CPU 315 2-DP working memory issue

i am facing a strange issue at a site where CPU 315 is linked to the SCADA and from SCADA we are controlling various process basically its a gypsum plant and from there we start and stop various areas of the plant for instance we have grinding area and there are start and stop buttons given at the SCADA.

now sometimes the start button is locked means we press it from the SCADA but it does not starts the process and meanwhile the stop command is active actually we change the color of the buttons to show that the button has been pressed the the sequence has started or is going to stop so the stop button is green and flashing telling that the process is stopping while the process is stopped it should go away and in this way it doesn't allows us to start the sequence again.

before i go further just need to update that the work memory of the CPU has reached 99%

4 times this fault has appeared in various areas of the plant and what we did was go online the PLC delete all the blocks and download all the blocks from the recent backup and restart the PLC and after that it became ok.

now is it the issue of the CPU should i replace it with the a one having higher work memory or should i troubleshoot my logic or the SCADA bits because i have already done that and there is noting un usual that i can see there...

please help and suggest what shoudl be done...

That the work memory is at 99% should not affect the execution of the code in memory.
However, it means it is very difficult to make program changes online. You will have to delete a block before you download the modified block, and that may result in the CPU faulting. So for this reason alone, I would definitely replace it for one with more work memory.

The problem with program not responding to the buttons in the SCADA may be in the PLC code. You have to troubleshoot it online.
Or it may be because the comms is overloaded so that the SCADA is not really able to keep up and the display does not reflect the reality.
Since it is an 315 (or 315-2DP ?) I am guessing that you have the SCADA connected via MPI. Since MPI is very weak, it may well be the reason for comms problems.
So if you are going to replace the CPU anyway, I recommend that you go for a 315-2PN/DP and switch the SCADA connection to ethernet.

Dear Jesper,

thanks you very much for the informative reply if it was an issue in the logic then how did it kept running for 2 years....

i tried to monitor the bits online associated with the buttons and guess what the bit linked to the button was at state "0" while it was displaying state "1" at the SCADA.....

yesterday this happened again and what i did this time was checked the blocks consistency and unfortunately i found one FC with programming error and i removed it but that FC was not being used at all any where in the logic it was just a dummy which i put there during the testing phase and it has been there all the time... but as it had error in it and was not being used any where so i removed it and downloaded the blocks again...

after the download the operator was happy that now the SCADA looks different because he had to set all the parameters again uptill now the plant is running fine but still confused whether to change the CPU or check for the communication issue or troubleshoot the logic..

as i told you the i traced the bit linked to the button that was causing problem in the PLC and at the PLC it was not set while on SCADA it was showing that the bit has been set i also checked for the multiple use of the Bit whether i mistakenly used it somewhere else or no but it wasn't used any where else other then that place....

now the client is asking me that ok we will replace the CPU but are you sure that replacing the CPU will solve the issue and after reading your post which very valid and with technical comments i am again confused whether to replace or check for the comms issue.....

Regards...

Try to check an update cycle for that button in SCADA and increase it. Also check update cycles for other objects in the screen with that button and reduce them as most as possible. May be they take a lot of comm. resources of the CPU and it makes high load.

Oleksandr,

the issue is not with only one specific button you never know which button will start behaving like that.

The fault is moving not staying to a specific button or an area.

qayyum faiz, as I can see, something in SCADA takes a lot of comm. resources of your CPU. Check all you screens, scripts, trends for update cycles and try to find out the reason and who makes the main load. You could try to decrease the update cycles of your objects and see the result. For example some output field has update cycle 1 sec try to reduce it to 2 sec. Or trend system is archiving some tag with 500 ms cycle. Try to archive it by tag change event, etc.

315-2dp cpu

Dear Faiz,

Good day,

What you did for your CPU should solve your problem, you don't need to replace your CPU(Actually replacing the CPU is not the right clue to solve the problem, what you have done is the solution).

Usually having the CPU loaded to up to 99% or even sometimes reaching a "bottle neck" occurs as a result of several program downloads, & what you have done is the right remedy for such a problem.

Now you should focus on solving your SCADA issue which doesn't have any link to your CPU usage.

Good luck,
Hagos